Carbon atoms in organic compounds are tetravalent and can be in three different states of hybridization (Table 22.1).

Table 22.1

Hybridization of carbon atoms

In the formation of organic compounds, a special role is played by the ability of carbon atoms to combine with each other to form chains, branched chains and cycles. C-C bonds are much stronger than bonds between other identical atoms, which explains the stability of carbon structures:

The carbon atoms linked together are called the carbon skeleton of the molecule.

The spatial configuration of carbon structures is determined by the hybridization of carbon atoms. With ^-hybridization of all atoms, zigzag chains are formed. In the case of the formation of a cycle, the carbon atoms deviate from the planar arrangement. Examples are shown in the diagrams:

If there are carbon atoms in the 5p state in the zigzag chain 2 hybridization, then there are areas with a planar arrangement of atoms. In the presence of carbon atoms in the state of sp hybridization, linear sections of the chain appear.

The terminal carbon atoms, called primary, have three valences for the addition of other atoms and atomic groups: H, OH, Cl, NH 2 etc. Non-terminal atoms bonded to two carbon atoms are calledsecondary. Two more atoms join them. There are no primary carbon atoms in the carbon cycle. If there is a branching in the circuit, tertiary carbon atom. It has only one valency left to attach other atoms. Finally, in the chain of carbon atoms, two branches can appear at one atom. Such an atom is called Quaternary; it is bonded only to carbon atoms:

In a molecule with one carbon atom, this atom is calledisolated.

Depending on the type of hybridization, carbon forms single (a) and multiple - double (a + l) and triple (a + 2n) bonds. n-bonds can occur not only between carbon atoms, but also with atoms attached to carbon. A special type of chemical bond is conjugate a double bond that occurs when there are more than two atoms in the carbon chain in the state of 5p 2 hybridization (see Fig. 6.26). It follows from the figure that unpaired electrons in non-hybrid p-orbitals can form bonds between any adjacent carbon atoms, and this leads to delocalization of the p-bond along the entire chain of p 2 carbon atoms. In chemical reactions, the presence of an n-bond can manifest itself between atoms 1 And 2, then between the atoms 2 and 3 etc.

Compounds in which there are multiple bonds and, respectively,sp2-and sp carbon atoms are calledunsaturated. If these are hydrocarbons, then the hydrogen content in them is less than the maximum possible. These compounds exhibit an increased reactivity, since the electron cloud of the n-bond is concentrated on two sides of the C atoms and therefore quite easily shifts from one of the two atoms to the other under the influence of the reagent molecules.

The most important classes of organic compounds, in addition to carbon and hydrogen, may contain oxygen, nitrogen, halogens, and sulfur. Of these elements, hydrogen has a lower electronegativity than carbon, while the rest have a higher one. The covalent bonds of carbon with them are to some extent polar, and the atoms have partial electric charges ± 8 :

The polarity of the bonds affects the reactivity of the compounds.

Carbon atoms have the ability to form stable bonds with several different atoms at once. This leads to many combinations rarely seen in the world. organic chemistry. Compare carbon and aluminum. The latter forms four halides (AIF3, A1C1 3 , A1Br 3 , AP 3) and a hydride A1H 3 . Carbon, on the other hand, can give many molecules with the simultaneous presence of various halogens, as well as hydrogen and other carbon atoms: CH 3 C1, CH 2 C1 2, CH 2 ClBr, CHFClBr, CH 3 CHS1Br, etc. This is also one of the reasons for the diversity of organic compounds.

The structural formulas of molecules are widely used in organic chemistry. Structural formulas can be depicted with varying degrees of specification and approximation to the real structure. Consider several varieties of formulas depicting a propane molecule.

In polyatomic molecules of organic compounds, continuous rotation of atomic groups around axes coinciding with the direction of single C-C bonds is possible (for short, they say: rotation around C-C connections). In the simplest case of ethane C 2 H 6, two groups of CH 3 rotate one relative to the other almost freely, like two wheels freely put on an axle:

Molecules with carbon chains of four or more atoms in the process of internal rotation bend like a caterpillar, creating all kinds of conformations(mutual positions) of atoms both in the volume and on the plane. A chain of five carbon atoms has three planar conformations:

Bulk transition conformations arise between the three planar conformations. The horseshoe conformation is favorable for the formation of a cyclic structure.

In organic molecules, there are separate parts (fragments) that differ in composition. Branches consisting of carbon and hydrogen can be attached to the main carbon chain or cycle, called hydrocarbon radicals. The simplest radicals already encountered in the text are methyl -CH 3 and ethyl -C 2 H 5 . The fourth bond of the radical is represented by a dash or a dot (CH 3). The remaining elements, except for carbon and hydrogen, in the molecules of organic compounds are considered as functional groups. This term is related to chemical reactions go predominantly with the participation of these groups. In the organic compounds CH 3 COOP and C 2 H 5 NH 2 already encountered in the text, there are functional groups -COOH (carboxyl) with acidic properties and -NH 2 (amino group) with basic properties.

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1. Hybridization of carbon atomic orbitals

atomic orbital is a function that describes the density of the electron cloud at each point in space around the nucleus of an atom. An electron cloud is a region of space in which an electron can be found with a high probability.

To harmonize the electronic structure of the carbon atom and the valency of this element, the concepts of excitation of the carbon atom are used. In the normal (unexcited) state, the carbon atom has two unpaired 2 R 2 electrons.

In an excited state (when energy is absorbed) one of 2 s 2-electrons can pass to free R-orbital. Then four unpaired electrons appear in the carbon atom. At the second energy level except 2 s-there are three orbitals 2 R-orbitals. These 2 R-orbitals have an ellipsoidal shape, similar to dumbbells, and are oriented in space at an angle of 90 ° to each other. 2 R-Orbitals denote 2 R X, 2R y and 2 R z according to the axes along which these orbitals are located.

When chemical bonds are formed, the electron orbitals acquire the same shape.

So, in saturated hydrocarbons, one s-orbital and three R-orbitals of a carbon atom to form four identical (hybrid) sR 3-orbitals:

This - sR 3 - hybridization.

Hybridization- alignment (mixing) of atomic orbitals ( s And R) with the formation of new atomic orbitals, called hybrid orbitals.

TETRAHEDRON (angles = 109°28?

sR 2 -Hybridization- mixing one s- and two R-orbitals. As a result, three hybrid sR 2 -orbitals.

These sR 2 -orbitals are located in the same plane (with axes X, at) and are directed to the vertices of the triangle with an angle between the orbitals of 120°.

unhybridized R-orbital is perpendicular to the plane of the three hybrid sR 2 orbitals (oriented along the axis z).

Upper half R-orbitals are above the plane, the lower half is below the plane.

Type sR 2-carbon hybridization occurs in compounds with a double bond:

C=C, C=O, C=N.

Moreover, only one of the bonds between two atoms (for example, C=C) can be a bond. (The other bonding orbitals of the atom point in opposite directions.)

The second bond is formed as a result of the overlap of non-hybrid R-orbitals on both sides of the line connecting the nuclei of atoms.

Covalent bond formed by lateral overlap R-orbitals of neighboring carbon atoms is called pi( R)-bond .

sR-Hybridization s- and one R sR-orbitals. sR- Orbitals are located on the same line (at an angle of 180 °) and directed in opposite directions from the nucleus of the carbon atom. Two R at-connections. On the image sR-orbitals are shown along the axis y, and the unhybridized two R-orbitals- along the axes X And z.

The triple carbon-carbon bond C?C consists of a y-bond that occurs when overlapping sp-hybrid orbitals, and two p-bonds.

2. Reactions of electrophilic substitution of hydrogen atoms in the benzene series

1. Halogenation reaction. The halogenation reaction of the benzene ring is carried out in the presence of catalysts (most often iron or aluminum halides). The role of the catalyst is to form a highly polarized complex with a halogen: FORMULA. The leftmost chlorine atom in the complex becomes electron-unsaturated as a result of the polarization of the Cl - Cl bond and is capable of interacting with nucleophilic reagents (in this case, with benzene):

e - the complex splits off a proton and turns into a substitution product (chlorobenzene). The proton interacts with - with the regeneration of aluminum chloride, while forming hydrogen chloride:

In the case of an excess of halogen, di- and polyhalo-substituted ones can be obtained, up to the complete replacement of all hydrogen atoms in benzene.

Direct iodination in the aromatic nucleus cannot be carried out due to the low reactivity of iodine. Direct fluorination of aromatic hydrocarbons proceeds so vigorously that a complex mixture of products is formed, in which the target fluorine derivatives are contained in small amounts. Depending on the conditions for the halogenation reaction of alkylbenzenes, the halogen can replace hydrogen atoms in benzene ring(“in the cold” in the presence of Lewis acids) or in the side chain (when heated or exposed to light). In the latter case, the reaction proceeds according to the free radical mechanism, similar to the substitution mechanism in alkanes.

2. Nitration reaction. Benzene reacts slowly with concentrated nitric acid. The nitration rate increases significantly if the nitration reaction is carried out with a mixture of concentrated nitric and sulfuric acids (usually in a ratio of 1:2); this mixture is called nitrating.

The process occurs due to the fact that sulfuric acid, as a stronger one, protonates nitric acid, and the resulting protonated particle decomposes into water and an active electrophilic reagent, the nitronium cation (nitronium cation).

The nitration reaction of benzene is an electrophilic substitution reaction and is ionic in nature. First, the formation of a p-complex occurs as a result of the interaction of the electrons of the benzene ring with a positively charged particle of the nitronium cation.

Then there is a transition of the p-complex to the y-complex. In this case, two p-electrons out of six go to the formation covalent bond C-NO2+. The remaining four electrons are distributed among the five carbon atoms of the benzene ring. A y-complex is formed in the form of an unstable carbocation.

The unstable y-complex under the influence of the HSO4- ion loses a proton with the formation of the aromatic structure of nitrobenzene.

3. Sulfonation reaction. To introduce a sulfo group into the benzene ring, a fuming liquid is used. sulfuric acid, i.e., containing an excess of sulfuric anhydride (SO3). The electrophilic species is SO3. The mechanism of sulfonation of aromatic compounds includes the following steps:

4. Friedel-Crafts alkylation reaction. The role of the catalyst (usually AlCl3) in this process is to enhance the polarization of the haloalkyl with the formation of a positively charged species, which enters into an electrophilic substitution reaction: FORMULA

3. Anthracene: structure and basic chemical properties

Anthracene - a compound whose molecule consists of three aromatic rings lying in the same plane. It is obtained from the anthracene fraction of coal tar, boiling at 300-350°C. In laboratory practice, anthracene can be obtained

a) according to the Friedel-Crafts reaction:

b) according to the Fittig reaction:

The most active positions in the anthracene molecule are the ninth and tenth positions, which are under the influence of the two extreme rings. Anthracene easily enters into addition reactions according to these provisions:

Under the action of oxidizing agents, anthracene easily forms anthraquinone, which is widely used for the synthesis of dyes:

4. Conjugated dienes and methods for their synthesis

Diene hydrocarbons (dienes) are unsaturated hydrocarbons having two double bonds of the general formula СnH2n-2.

The two double bonds in a hydrocarbon molecule can be arranged in different ways. If they are concentrated at one carbon atom, they are called cumulated: -C \u003d C \u003d C- If two double bonds are separated by one single bond, they are called conjugated: -C \u003d C - C \u003d C- If double bonds are separated by two or more simple ones bonds, they are called isolated: -C=C- (CH2)n - C=C-

5. Orientation rules in the benzene ring

When studying substitution reactions in the benzene ring, it was found that if it already contains any substituent, then, depending on its nature, the second one enters a certain position. Thus, each substituent on the benzene ring exhibits a specific guiding or orienting effect. The position of the newly introduced substituent is also influenced by the nature of the substituent itself, i.e., whether the active reagent has an electrophilic or nucleophilic nature. All substitutes, by the nature of their guiding action in are divided into two groups.

Substituents of the first kind send the input group to the ortho and para positions:

Substituents of this kind include the following groups, arranged in descending order of their orienting strength: N(CH3)2, NH2, OH, CH3 and other alkyls, as well as Cl, Br, I.

Substituents of the second kind V electrophilic substitution reactions direct input groups to the meta position. Substituents of this kind include the following groups: - NO2, - C N, - SO3H, - CHO, - COOH.

6. The nature of the double bond and the chemical properties of ethylene compounds

According to modern concepts, the two bonds connecting two unsaturated carbon atoms are not the same: one of them is a y-bond, and the other is a p-bond. The latter bond is less strong and "breaks" during addition reactions.

The nonequivalence of two bonds in unsaturated compounds is indicated, in particular, by a comparison of the energy of formation of single and double bonds. The energy of formation of a single bond is 340 kJ/mol (about 82 kcal/mol), and a double bond is 615 kJ/mol (about 147 kcal/mol). Naturally, less energy is expended to break the p-bond than to break the y-bond. Thus, the fragility of a double bond is explained by the fact that one of the two bonds that form a double bond has a different electronic structure than ordinary -bonds, and has less strength.

Names of olefins usually produced from the name of the corresponding saturated hydrocarbons, but the ending - en replaced by the ending - ilene According to international nomenclature, instead of ending - ilene olefins are given a shorter ending - en.

isomerism olefins depends on the isomerism of the chain of carbon atoms, i.e. on whether the chain is straight or branched, and on the position of the double bond in the chain. There is also a third reason for the isomerism of olefins: a different arrangement of atoms and atomic groups in space, i.e., stereoisomerism. Isomerism, depending on the different arrangement of atoms and atomic groups in space, is calledspatial isomerism , orstereoisomerism .

Geometric , orcis- Andtrans isomerism , is a kind of spatial isomerism depending on the different location atoms with respect to the plane of the double bond.

To indicate the place of a double bond (as well as branches in the chain), according to the international IUPAC nomenclature, the carbon atoms of the longest chain are numbered, starting from the end to which the double bond is closest. Thus, the two straight chain isomers of butylene would be called butene-1 and butene-2:

1. Hydrogenation reaction. Unsaturated hydrocarbons readily add hydrogen to the double bond in the presence of catalysts 67 (Pt, Pd, Ni). With a Pt or Pd catalyst, the reaction proceeds at 20 ... 100 ° C, with Ni - at higher temperatures:

2. Halogenation reaction. Alkenes under normal conditions add halogens, especially chlorine and bromine. As a result, dihalo derivatives of alkanes are formed containing halogens at neighboring carbon atoms, the so-called vicinal dihaloalkanes: CH

3CH=CH2 + Cl2>CH3CHClCH2Cl

3. Reaction of addition of hydrogen halides. Hydrohalogenation

4. Reaction of hydration of alkenes. Under normal conditions, alkenes do not react with water. But in the presence of catalysts, under heating and pressure, they add water and form alcohols:

5. Sulfuric acid addition reaction. The interaction of alkenes with sulfuric acid proceeds similarly to the addition of hydrogen halides. As a result, acid esters of sulfuric acid are formed:

6. Alkene alkylation reaction. Catalytic addition of alkanes with a tertiary carbon atom to alkenes is possible (catalysts - H2SO4, HF, AlCl3 and BF3):

7. Alkene oxidation reaction. Alkenes are easily oxidized. Depending on the oxidation conditions, various products are formed. When burned in air, alkenes are converted into carbon dioxide and water: CH2=CH2 + 3O2> 2CO2 + 2H2O.

When alkenes interact with atmospheric oxygen in the presence of a silver catalyst, organic oxides are formed:

Acyl hydroperoxides act similarly on ethylene (Prilezhaev reaction):

One of the most characteristic oxidation reactions is the interaction of alkenes with a slightly alkaline solution of potassium permanganate KMnO4 with the formation of dihydric alcohols - glycols (Wagner reaction). The reaction proceeds in the cold as follows:

Concentrated solutions of oxidizing agents (potassium permanganate in an acidic environment, chromic acid, Nitric acid) break the alkene molecule at the double bond with the formation of ketones and acids:

8. Alkene ozonation reaction. It is also widely used to establish the structure of alkenes:

9. Substitution reactions. Alkenes under certain conditions are also capable of substitution reactions. So, at high-temperature (500 ... 550 ° C) chlorination of alkenes, hydrogen is replaced in the allyl position:

10. Alkene polymerization reaction

CH2 = CH2 > (-CH2 - CH2 -) n turns out polyethylene

11. Isomerization reaction. At high temperatures or in the presence of catalysts, alkenes are able to isomerize, either changing the structure of the carbon skeleton or moving the double bond:

7. Naphthalene and its structure. Hückel's rule

Hydrocarbons of the naphthalene series are the main aromatic hydrocarbon coal tar. There are a large number of polycyclic aromatic compounds in which the benzene rings have common orthocarbon atoms. The most important of them are naphthalene, anthracene and phenanthrene. In anthracene, the rings are connected linearly, while in phenanthrene, at an angle, unlike the benzene molecule, not all bonds in the naphthalene core have the same length:

Hückel's rule : Aromatic is a planar monocyclic conjugated system containing (4n + 2) p-electrons (where n = 0,1,2...).

Thus, planar cyclic conjugated systems containing 2, 6,10, 14, etc. will be aromatic. p-electrons.

8. Alkynes and sp-hybridization of the carbon atom. Methods for obtaining alkynes

Hydrocarbons of the acetylene series have the general formula

WITH n H2 n-2

The first simplest hydrocarbon of this series is C2H2 acetylene. The structural formula of acetylene, like other hydrocarbons of this series, contains a triple bond:

N - S? S - N.

sR-Hybridization- this is mixing (alignment in form and energy) of one s- and one R-orbitals with the formation of two hybrid sR-orbitals. sR- Orbitals are located on the same line (at an angle of 180 °) and directed in opposite directions from the nucleus of the carbon atom.

Two R-orbitals remain unhybridized. They are located mutually perpendicular to the directions at-connections.

On the image sR-orbitals are shown along the axis y, and the unhybridized two R-orbitals- along the axes X And z.

A triple carbon-carbon bond С?С consists of a y-bond arising from the overlapping of sp-hybrid orbitals and two p-bonds.

Calcium carbide is produced on an industrial scale by heating coal in electric furnaces with quicklime at a temperature of about 2500 ° C according to the reaction

CaO + 3C > CaC2 + CO.

If water acts on calcium carbide, then it rapidly decomposes with the release of gas - acetylene:

A newer industrial method for producing acetylene is the pyrolysis of hydrocarbons, in particular methane, which at 1400 ° C gives a mixture of acetylene with hydrogen:

2CH4> H-C=C-H + 3H2.

1. Dehydrohalogenation of vicinal dihaloalkanes

2. Reaction of sodium acetylenides with primary alkyl halides:

3. Dehalogenation of vicinal tetrahaloalkanes:

9. Production methods and chemicalproperties of alcohols

Alcohols are derivatives of hydrocarbons in which one or more hydrogen atoms are replaced by the corresponding number of hydroxyl groups (-OH).

General formula of alcohols

where R is an alkyl or substituted alkyl group.

The nature of the radical R, to which the hydroxyl group is associated, determines the limit or unsaturation of alcohols, and the number of hydroxyl groups determines its atomicity: alcohols are monoatomic, diatomic, trihydric and polyhydric.

Getting: 1. Hydration of alkenes

2. Enzymatic hydrolysis of carbohydrates. Enzymatic hydrolysis of sugars under the action of yeast is the oldest synthetic chemical process- is still of great importance for the production of ethyl alcohol.

When using starch as a starting material, in addition to ethyl alcohol, fusel oil is also formed (in smaller quantities), which is a mixture of primary alcohols, mainly isopentyl, isopropyl and isobutyl.

3. Synthesis of methyl alcohol:

4. Reaction of hydroboration-oxidation of alkenes:

5. Syntheses of alcohols using the Grignard reagent:

Properties: The chemical properties of alcohols are determined both by the structure of the alkyl radical and by the reactive hydroxyl group. Reactions involving the hydroxyl group can proceed either with the breaking of the C-OH bond (360 kJ/mol) or with the breaking O-N connections(429 kJ/mol) A. Breaking the C-OH bond

1. Reaction with hydrogen halides:

ROH + HX >RX + H2O.

The reactivity decreases in the series: HI > HBr > HCl

2. Reaction with phosphorus trihalides:

3. Dehydration of alcohols in the presence of water-removing agents:

B. Disconnection HE

4. Reaction of alcohols with metals(Na, K, Mg, Al)

5. Formation of ethers:

Esterification reaction

6. Oxidation reactions When alcohols are oxidized with a chromium mixture or KMnO4 in a solution of sulfuric acid, the composition of the products depends on the nature of the carbon atom (primary, secondary or tertiary) to which the hydroxyl group is attached: primary alcohols form aldehydes, secondary alcohols form ketones.

9. Alkadienes and methods for their preparation

Diene hydrocarbons (dienes) are unsaturated hydrocarbons having two double bonds of the general formula

The two double bonds in a hydrocarbon molecule can be arranged in different ways.

If they are concentrated at one carbon atom, they are called cumulated:

If two double bonds are separated by one single bond, they are called conjugated:

If double bonds are separated by two or more simple bonds, then they are called isolated: -C=C- (CH2)n - C=C-

Dienes are usually obtained by the same methods as simple alkenes. For example, the most important diene, 1,3-butadiene (used to make synthetic rubber), is produced in the United States by dehydrogenating butane:

In the USSR, the industrial synthesis of butadiene-1,3 was used according to the method of S.V. Lebedev (1933) from ethyl alcohol at 400...500 °C over MgO-ZnO catalyst:

The reaction includes the following steps: alcohol dehydrogenation to aldehyde, aldol condensation of acetaldehyde, aldol reduction to butanediol-1,3, and finally alcohol dehydration:

10. Electronegativity of elements and types of chemical bonds

Electronegativity (h) (relative electronegativity) is a fundamental chemical property of an atom, a quantitative characteristic of the ability of an atom in a molecule to displace common electron pairs towards itself, that is, the ability of atoms to pull electrons of other atoms towards themselves.

The highest degree of electronegativity is in halogens and strong oxidizing agents (p-elements of group VII, O, Kr, Xe), and the lowest is in active metals(s-elements of group I).

Ionic. The electronic configuration of an inert gas for any atom can be formed due to the transfer of electrons: the atoms of one of the elements give up electrons, which pass to the atoms of another element.

In this case, a so-called ionic (electrovalent, heteropolar) bond is formed between these atoms.

This type of bond occurs between atoms of elements that have significantly different electronegativity (for example, between a typical metal and a typical non-metal).

covalent bond. When interacting atoms that are equal (atoms of the same element) or close in electronegativity, electron transfer does not occur. The electronic configuration of an inert gas for such atoms is formed as a result of the generalization of two, four or six electrons by interacting atoms. Each of the shared pairs of electrons forms one covalent (homeopolar) bond:

A covalent bond is the most common type of bond in organic chemistry. She is strong enough.

A covalent bond and, accordingly, a molecule can be non-polar when both bonded atoms have the same electron affinity (eg H:H). It can be polar when an electron pair is pulled towards it due to the greater electron affinity of one of the atoms:

With this method, the designations + and - mean that the atom with the sign has an excess electron density, and the electron density on the atom with the + sign is slightly lower compared to isolated atoms.

Donor-acceptor bond. When atoms that have lone electron pairs interact with a proton or another atom that lacks two electrons to form an octet (doublet), the lone electron pair becomes common and forms a new covalent bond between these atoms.

In this case, an atom that donates electrons is called a donor, and an atom that accepts electrons is called an acceptor:

chemical covalent benzene naphthalene

In the emerging ammonium ion, the formed covalent bond differs from the bonds that existed in the ammonia molecule, only in the way of formation, in physical and chemical properties all four N-H bonds absolutely identical.

Semipolar connection. This type of donor-acceptor bond is often found in molecules of organic compounds (for example, in nitro compounds, sulfoxides, etc.).

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Most organic compounds have a molecular structure. Atoms in substances with a molecular type of structure always form only covalent bonds with each other, which is also observed in the case of organic compounds. Recall that a covalent bond is such a type of bond between atoms, which is realized due to the fact that atoms socialize part of their outer electrons in order to acquire the electronic configuration of a noble gas.

By the number of socialized electron pairs, covalent bonds in organic matter can be divided into single, double and triple. These types of connections are indicated in the graphic formula, respectively, by one, two or three lines:

The multiplicity of the bond leads to a decrease in its length, so a single C-C bond has a length of 0.154 nm, a double C=C bond - 0.134 nm, a triple C≡C bond - 0.120 nm.

Types of bonds according to the way the orbitals overlap

As is known, orbitals can have different shapes, for example, s-orbitals are spherical, and p-dumbbells are different shape. For this reason, bonds can also differ in the way electron orbitals overlap:

ϭ-bonds - are formed when the orbitals overlap in such a way that the region of their overlap is intersected by a line connecting the nuclei. Examples of ϭ-bonds:

π-bonds - are formed when the orbitals overlap, in two areas - above and below the line connecting the nuclei of atoms. Examples of π bonds:

How to know when there are π- and ϭ-bonds in a molecule?

With a covalent type of bond, there is always a ϭ-bond between any two atoms, and it has a π-bond only in the case of multiple (double, triple) bonds. Wherein:

• Single bond - always a ϭ-bond
• A double bond always consists of one ϭ- and one π-bond
• A triple bond is always formed by one ϭ and two π bonds.

Let us indicate these types of bonds in the propinoic acid molecule:

Hybridization of carbon atom orbitals

Orbital hybridization is a process in which orbitals that originally have different shapes and energies are mixed, forming in return the same number of hybrid orbitals, equal in shape and energy.

For example, when mixing one s- and three p- four orbitals are formed sp 3-hybrid orbitals:

In the case of carbon atoms, hybridization always takes part s- orbital, and the number p-orbitals that can take part in hybridization varies from one to three p- orbitals.

How to determine the type of hybridization of a carbon atom in an organic molecule?

Depending on how many other atoms a carbon atom is bonded to, it is either in the state sp 3, or in the state sp 2, or in the state sp- hybridization:

Let's practice determining the type of hybridization of carbon atoms using the example of the following organic molecule:

The first carbon atom is bonded to two other atoms (1H and 1C), so it is in the state sp-hybridization.

• The second carbon atom is bonded to two atoms - sp-hybridization
• The third carbon atom is bonded to four other atoms (two C and two H) - sp 3-hybridization
• The fourth carbon atom is bonded to three other atoms (2O and 1C) - sp 2-hybridization.

Radical. Functional group

The term "radical" most often means a hydrocarbon radical, which is the remainder of a molecule of any hydrocarbon without one hydrogen atom.

The name of the hydrocarbon radical is formed based on the name of the corresponding hydrocarbon by replacing the suffix –en to suffix –silt .

Functional group - a structural fragment of an organic molecule (a certain group of atoms), which is responsible for its specific chemical properties.

Depending on which of the functional groups in the molecule of the substance is the eldest, the compound is assigned to one or another class.

R is the designation of a hydrocarbon substituent (radical).

Radicals can contain multiple bonds, which can also be considered as functional groups, since multiple bonds contribute to the chemical properties of the substance.

If an organic molecule contains two or more functional groups, such compounds are called polyfunctional.

This lesson will help you get an idea about the topic "Covalent bond in organic compounds". You will remember the nature of chemical bonds. Learn about how a covalent bond is formed, which is the basis of this bond. This lesson also discusses the principle of building Lewis formulas, talks about the characteristics of a covalent bond (polarity, length and strength), explains A. Butlerov's theory, talks about what an inductive effect is.

Topic: Introduction to organic chemistry

Lesson: Covalent bond in organic compounds.

Communication properties (polarity, length, energy, directivity)

The chemical bond is mainly electrostatic in nature. For example, a hydrogen molecule is formed from two atoms, because it is energetically favorable for two electrons to be in the field of attraction of two nuclei (protons). This state, in the form of an H 2 molecule, has less energy than two separate hydrogen atoms.

Most organic substances contain .

For education covalent bond between two atoms, each atom usually provides common use over one electron.

The simplified model uses the two-electron approximation, i.e. all molecules are built on the basis of the summation of two electronic bonds characteristic of the hydrogen molecule.

From the point of view of the law of interaction of electric charges (Coulomb's law), electrons cannot approach each other due to the enormous forces of electrostatic repulsion. But according to the laws quantum mechanics, electrons with opposite spins interact with each other and form an electron pair.

If a covalent bond is denoted as a pair of electrons, we get another form of writing the formula of a substance - an electronic formula or Lewis formula

(Amer. J. Lewis, 1916). Rice. 1.

Rice. 1. Lewis formulas

In organic molecules, there are not only single bonds, but also double and triple. In the Lewis formulas, they are denoted, respectively, by two or three pairs of electrons. Rice. 2

Rice. 2. Designation of double and triple bonds

Rice. 3. Covalent non-polar bond

An important characteristic of a covalent bond is its polarity. A bond between identical atoms, for example, in a hydrogen molecule or between carbon atoms in an ethane molecule non-polar - in it, the electrons equally belong to both atoms. See Fig. 3.

Rice. 4. Covalent polar bond

If the covalent bond is formed by different atoms, then the electrons in it are shifted to a more electronegative atom. For example, in the hydrogen chloride molecule, the electrons are shifted to the chlorine atom. Small partial charges arise on atoms, which are denoted by d+ and d-. Rice. 4.

How more difference between the electronegativity of atoms, the more polar the bond.

The mutual influence of atoms in a molecule leads to the fact that a displacement of bond electrons can occur, even if they are between identical atoms.

For example, in 1,1,1-trifluoroethane CH 3 CF 3, electronegative fluorine atoms “pull” the electron density from the carbon atom onto themselves. Often this is indicated by an arrow instead of a valence dash.

As a result, the carbon atom bound to the fluorine atoms has a lack of electron density, and it pulls valence electrons towards itself. Such a shift in the electron density along the bond chain is calledsubstituent inductive effect. Rice. 5.

Rice. 5. Electron density shift in 1,1,1-trifluoroethane

Bond length and strength

Important characteristics of a covalent bond are its length and strength. The length of most covalent bonds is from 1 * 10 -10 m to 2 * 10 -10 m, or from 1 to 2 in angstroms (1 A \u003d 1 * 10 -10 m).

The strength of a bond is the energy it takes to break that bond. Typically, a break of 1 mol or 6.023 * 10 23 bonds is given. See table. 1.

At one time it was thought that molecules could be represented by structural formulas lying in the plane of the paper, and these formulas reflect, almost reflect, the true structure of the molecule. But around the middle of the 19th century, it turned out that this was not the case. For the first time, as I said in previous lessons, I came to this conclusion when I was still a student of Van't Hoff. And he did this on the basis of the experiments of the outstanding French biologist and chemist Pasteur.

The fact is that Pasteur studied the salts of tartaric acid. And you can say he was lucky. crystallizing mixed salt tartaric acid, he discovered under a microscope that he obtained, in general, a set of exactly the same, very pretty crystals. But these crystals can easily be divided into two groups that are in no way compatible with each other, namely: all crystals are divided into two parts, one of which is a mirror image of the other.

So the optical, or mirror, was first discovered. Pasteur was able to manually separate these crystals with tweezers under a microscope and found that all chemical properties were practically the same. Only one, rather, physical property does not coincide, namely: solutions of one type of crystals and a mirror of another type of crystals differently rotated the plane of polarization of light passing through them.

Rice. 6. Models of the methane molecule

In order to explain the results of Pasteur's experiments, van't Hoff assumed that the carbon atom is always in a non-planar environment, and this non-planar environment has neither a center nor a plane of symmetry. Then the carbon atom, connected to 4 other different fragments of the molecule, which are not identical to each other, must have mirror symmetry. It was then that van't Hoff suggested the tetrahedral structure of the carbon atom. Optical isomerism followed from this assumption. As a result, it was possible to explain the spatial structure of organic compounds. Rice. 6.

But scientists are faced with another mystery that has not been solved so far. The fact is that in nature, organic compounds that are actually formed in organic living matter, as a rule, contain left-handed, meaning the plane of polarization of transmitted light, amino acids and right-handed sugars. While in any organic synthesis a mixture of such isomers is necessarily obtained.

The reason for this selectivity of living nature is still not clear. But this does not prevent scientists from continuing to synthesize more and more new organic compounds and study their properties.

The formulas drawn on the plane do not reflect the spatial arrangement of atoms relative to each other. However, the tetrahedral structure of the carbon atom in molecules with single bonds leads to the existence of optical isomerism

Summing up the lesson

You have gained an understanding of the topic "Covalent Bond in Organic Compounds". You remembered the nature of chemical bonds. We learned about how a covalent bond is formed, which is the basis of this bond. Considered the principle of constructing Lewis formulas. We learned about the characteristics of a covalent bond (polarity, length and strength), what is the inductive effect.

Bibliography

1. Rudzitis G.E. Chemistry. Basics general chemistry. Grade 10: textbook for educational institutions: a basic level of/ G. E. Rudzitis, F.G. Feldman. - 14th edition. - M.: Education, 2012.

2. Chemistry. Grade 10. Profile level: studies. for general education institutions / V.V. Eremin, N.E. Kuzmenko, V.V. Lunin and others - M.: Drofa, 2008. - 463 p.

3. Chemistry. Grade 11. Profile level: textbook. for general education institutions / V.V. Eremin, N.E. Kuzmenko, V.V. Lunin and others - M.: Drofa, 2010. - 462 p.

4. Khomchenko G.P., Khomchenko I.G. Collection of problems in chemistry for those entering the universities. - 4th ed. - M.: RIA "New Wave": Publisher Umerenkov, 2012. - 278 p.

Homework

1. No. 12, 15 (p. 11) Rudzitis G.E., Feldman F.G. Chemistry: Organic Chemistry. Grade 10: textbook for educational institutions: basic level / G. E. Rudzitis, F.G. Feldman. - 14th edition. - M.: Education, 2012.

2. Compose the structural and electronic formulas of ethane C 2 H 6, ethene C 2 H 4, propyne C 3 H 8.

3. Give examples from inorganic chemistry, showing that the atoms in the molecule affect each other and their properties change.

Organic chemistry is of exceptionally important cognitive and national economic importance.

Natural organic substances and their transformations underlie the phenomena of Life. Therefore, organic chemistry is the chemical foundation of biological chemistry and molecular biology- sciences that study the processes occurring in the cells of organisms at the molecular level. Research in this area allows a deeper understanding of the essence of wildlife phenomena.

Many synthetic organic compounds are produced by industry for use in a variety of industries - these are petroleum products, fuel for various engines, polymeric materials (rubbers, plastics, fibers, films, varnishes, adhesives, etc.), surfactants, dyes , plant protection products, medications, flavoring and perfumery substances, etc. Without knowledge of the basics of organic chemistry modern man unable to competently use all these products of civilization.

Raw sources of organic compounds are oil and natural gas, hard and brown coal, oil shale, peat, agricultural and forestry products.

The criterion for dividing compounds into organic and inorganic is their elemental composition.

Organic compounds are chemical substances containing carbon, for example:

CH 3 -CN, CH 3 -CH 2 -OH, CS 2, CH 3 COOH, CH 3 -NH 2, CH 3 -NO 2, CH 3 -COOC 2 H 5 .

Organic compounds differ from inorganic compounds in a number of characteristic features:

Almost all organic substances burn or are easily destroyed when heated with oxidizing agents, releasing CO 2 (on this basis, it is possible to establish that the substance under study belongs to organic compounds);

· in the molecules of organic compounds, carbon can be combined with almost any element of the periodic system;

Organic molecules may contain a sequence of carbon atoms connected in chains (open or closed);

Molecules of most organic compounds do not dissociate into sufficiently stable ions;

reactions of organic compounds proceed much more slowly and in most cases do not reach the end;

The phenomenon is widespread among organic compounds isomerism ;

· organic substances have lower phase transition temperatures (bp, mp).

Organic compounds are much larger than inorganic ones.

The main provisions of Butlerov's theory of chemical structure

1. Atoms in molecules are connected to each other in a certain sequence according to their valencies. The sequence of interatomic bonds in a molecule is called its chemical structure and is reflected by one structural formula (structural formula).

2. Chemical structure can be installed chemical methods. (Currently modern physical methods are also used).

3. The properties of substances depend on their chemical structure.

4. By the properties of a given substance, you can determine the structure of its molecule, and by the structure of the molecule, you can predict the properties.

5. Atoms and groups of atoms in a molecule mutually influence each other.

Already from the moment when researchers learned to determine the elemental composition of compounds, it was noticed that often compounds with the same elemental composition have completely different chemical and physical properties. The identification of the causes of such behavior stimulated the creation of a theory of the structure of organic compounds. For the first time such a theory was formulated by A.M. Butlerov.

Butlerov's theory was the scientific foundation of organic chemistry and contributed to its rapid development. Based on the provisions of the theory, A.M. Butlerov explained the phenomenon isomerism , predicted the existence of various isomers and obtained some of them for the first time.

The structure of the carbon atom

Obviously, all reactions that organic molecules enter into are associated with the structure of the carbon atom of a particular molecule and the rearrangement of its outer valence orbitals in the process of transformations.

In the unexcited state, the carbon atom has 2 electrons in the s-orbitals of the second sublevel (2s-orbitals), as well as 2 electrons in two (out of a total of 3) p-orbitals of the 2nd sublevel (2p x - and 2p y -orbitals):

Thus, in the outer orbitals, carbon has 4 electrons capable of forming bonds. According to the theory, the forms of s - and p-orbitals describe the probability of finding an electron relative to the nucleus of an atom. Unhybridized s - and p-orbitals have the shape of a sphere and a uniform "dumbbell" and are arranged in space according to the scheme below:

When compounds are formed from atomic carbon (or in the composition of carbon compounds), a change in the shape and arrangement in space relative to the nucleus of an atom of the outer orbitals of carbon occurs, called hybridization . Schematically, hybridization can be represented as follows:

Of the four unhybridized atomic s - and p-orbitals having different shapes, as a result sp 3 hybridizations (which means changing one s- and three R-orbitals) are obtained four equivalent in energy and form hybridized e molecular orbitals shaped like a distorted dumbbell.

To ensure minimal steric hindrance and mutual repulsion, these four equivalent orbitals are located in space at equal distances from each other, directed towards the vertices of the tetrahedron (the carbon atom nucleus is located in the center of the tetrahedron), and the spatial angles between the orbitals are about 109 ° 28 ':

In this state, four bonds as a result of overlapping orbitals can be formed without hindrance. In such hybridization, carbon is present (exclusively) in the composition of alkanes, cycloalkanes and alcohols.

Thus, for example, the ethane molecule looks like (yellow spheres show hydrogen atoms, more precisely, their s-orbitals):

The bond between carbon atoms is formed by overlapping hybridized orbitals. Such connections are called s- bonds (sigma bonds). Around s- bonds, the rotation of fragments of the molecule is possible.

Hybridization - a change in the shape and location in space relative to the nucleus of an atom of its outer electronic orbitals, during the formation of bonds with other atoms. Another definition: hybridization - mixing of orbitals , as a result of which their alignment in form and energy occurs.

A carbon atom that has a multiple bond (alkenes -C \u003d C -, carbonyl compounds\u003e C \u003d O, carboxylic acids and their derivatives -COOH, -COOR, etc.) has a different hybridization (sp 2), respectively, the shape and location in space of the outer orbitals:

In the state of sp 2 hybridization at carbon, there are only 3 hybridized orbitals (obtained from one s - and two p-orbitals), which are located in the same plane at an angle of 120 ° between them, and the fourth ( unhybridized) the p-orbital is perpendicular to this plane. The double bond is formed as a result overlapping unhybridized orbitals between adjacent carbon atoms (or between carbon and oxygen), the figure shows an ethylene (ethene) molecule:

Bonds formed by overlapping unhybridized p orbitals are called p- connections. Thus, a multiple (double) bond in the ethene molecule is formed by one sigma and one pi bond.

Rotation of molecular fragments around p- for obvious reasons, at normal temperature it is impossible (additional energy is needed to break the overlapping p-orbitals), this causes the presence of spatial (geometric) isomers in alkenes, subject to some additional conditions, which will be discussed below.

On the image unhybridized p-orbitals are at a distance - spaced artificially, for better perception, although in reality they "touch" each other, overlapping from above and below, but forming only one additional bond.

Carbon with a triple bond (in alkynes and nitriles) is in the state sp hybridizations :

A pair of hybridized orbitals is located in a line, at an angle of 180° and oppositely directed. Two unhybridized p-orbitals, according to the principle of minimum repulsion and to minimize steric hindrance, are located perpendicular to this line and at an angle of 90 ° between them. The triple bond in alkynes is formed as a result of the overlap of hybridized orbitals (one s- bond) and two unhybridized p-orbitals of neighboring carbon atoms (two p-bonds). So, for example, the model of an acetylene molecule (ethyne) looks like:

As a result of reactions, carbon is able to both change and maintain the state of its hybridization.

Types of bonds in molecules of organic substances

The predominant type of bond in the molecules of organic compounds is a covalent bond. A pair of bond electrons is divided between atoms in approximately equal degree, if characterizing C-C or C-H bonds. This is caused by approximately equal electron affinity ( electronegativity) C and H atoms.

In the case when carbon is bonded to a more electronegative atom (halogens, oxygen, nitrogen), the bond can be largely polarized, and partial positive (on carbon) and negative (on halogen, oxygen, nitrogen) charges can form on the atoms. However, the degree of ionicity of such a bond is minimal.

Due to the non-polarity of the C-C and C-H bonds, the predominant way to break it is homolytic, when a pair of electrons is divided equally between atoms. With this bond breaking, uncharged, but very reactive particles with unpaired electrons, called radicals, are formed. For alkanes, reactions with the intermediate formation of radicals are very characteristic. Such transformations are initiated by the introduction from outside of energy sufficient to break the bond (heating) or compounds that initiate the formation of radicals upon weak heating or ultraviolet irradiation (peroxides, halogens, azo compounds, chemical initiators that generate radicals as a result of a chemical reaction). By and large, open ring alkanes and cycloalkanes are chemically relatively inert.

In contrast, alkenes are much more reactive. The reason for this is the unsaturation (multiple bond) and the availability of loose electron density of overlapping p-orbitals p- bonds for the action of electrophilic reagents (compounds with empty outer orbitals or electron-deficient connections). As a result, the double bond disappears and electrophiles are added. The reactions proceed with intermediate formation of positively charged intermediates (carbocations) or radicals.

Another group of reactions is associated with the polarization of the carbon-halogen, oxygen or nitrogen bond. These reactions have a more complex mechanism and depend on the structure of the substrate, reagent, and reaction conditions (solvent, catalyst, etc.).

There are more complex types of reactions ( cycloaddition or the Diels–Alder reaction ), the detailed mechanism of which has not yet been studied in all its subtleties.

Types of reactions in organic chemistry

Thus, only a few types of reactions that organic compounds enter into can be distinguished:

1) reactions substitution when one atom (or grouping of atoms) is replaced by another atom (or grouping of atoms). The carbon skeleton remains unchanged. Reactions proceed through a preliminary rupture of the bond, followed by the formation of a new one;

2) reactions accession . They are characteristic of compounds having unsaturation (multiple bonds), as a result of which the addition of other molecules (hydrogen, water, halogens, oxygen, hydrogen halides, etc.) is possible;

3) reactions splitting off (elimination), when molecules (water, ammonia, halogens, hydrogen halides, hydrogen, CO, CO 2, etc.) are split off from a molecule of an organic compound. Such reactions are often named according to the type of cleaved off molecule, respectively, dehydration, deamination, dehalogenation, dehydrohalogenation, dehydrogenation, decarbonylation, decarboxylation etc.;

4) reactions condensation when the carbon skeleton of the molecule is enlarged;

5) cracking (or splitting) reactions, as a result of which the carbon skeleton is split into smaller molecules;

6) reactions oxidation , accompanied by the removal of hydrogen molecules (a special case of the cleavage reaction), or with the simultaneous introduction of oxygen molecules (the transformation of alcohols into aldehydes and ketones and, further, into acids);

7) reactions isomerization (or rearrangement of the carbon skeleton or cycles)

8) reactions polymerization , as a result of which long unbranched polymer molecules are obtained from small molecules (monomers). In living nature, there are examples of the formation of branched polymer molecules, in which organic molecules of monosaccharides (carbohydrates) act as structural units.

Classification of organic compounds

Despite the variety of organic compounds, the basis of their molecules are chains and rings formed from carbon atoms. Compounds containing only carbon and hydrogen are called hydrocarbons. In this case, part of the valencies of carbon is spent on the formation of bonds with neighboring carbon atoms, and free valences bind carbon with hydrogen, oxygen, nitrogen, sulfur, and, much less often, with other atoms of the periodic system. Very often, such a "skeleton" of carbon atoms is preserved as a result of chemical transformations undergone by a molecule of an organic compound, which greatly facilitates the prediction of the composition of products. Often reactions are limited to the replacement of one or more hydrogen atoms with another element or group of atoms (otherwise called a grouping or functional group ), resulting in an organic compound of a different class. Depending on the grouping that replaced one of the hydrogen atoms in the molecule of an organic compound as a result of the reaction, classes of organic compounds are distinguished.

Often, as a result of the reaction, one functional group is replaced by another, while maintaining the carbon skeleton. However, numerous reactions are also known that are accompanied by a change in the carbon skeleton of the molecule.

Table

Some functional groups of organic compounds

Homology and homology series

homologues - organic compounds (of the same class, see above), differing in one or more methylene groups (units -C H 2 -). Alkanes have homologues, for example, methane, ethane, propane, butane, etc., in which the number of carbon atoms changes by one (or by the same number of methylene units).

Homologs of aromatic compounds are benzene, toluene, xylenes, mesitylene, ethylbenzene and others alkyl-substituted benzenes. These compounds, according to the gross formula, also differ by one or more methylene units (-CH 2 -). Accordingly, methanol, propanol and ethanol, acetone and methyl ethyl ketone, acetic and propionic acids, etc. are homologues.

Isomerism of organic compounds

The structure formula (structural formula) describes the order of connection of atoms in a molecule, i.e. her chemical structure. chemical bonds in the structural formula are represented by dashes. The bond between hydrogen and other atoms is usually not indicated (such formulas are called abbreviated structural formulas).

Structural formulas differ from molecular (gross) formulas, which show only which elements and in what ratio are included in the composition of the substance (i.e., the qualitative and quantitative elemental composition), but do not reflect the order of binding atoms. For example, n-butane and isobutane have the same molecular formula C 4 H 10, but a different sequence of links.

Thus, the difference in substances is due not only to different qualitative and quantitative elemental composition, but also to different chemical structures, which can only be reflected in structural formulas. Even before the creation of the theory of structure, substances of the same elemental composition, but with different properties, were known. Such substances were called isomers, and the phenomenon itself isomerism. At the heart of isomerism, as shown by A.M. Butlerov, lies difference in structure molecules that are made up of the same set of atoms. Thus, isomerism is the phenomenon of the existence of compounds that have the same qualitative and quantitative composition, but a different structure and, consequently, different properties.

For example, when a molecule contains 4 carbon atoms and 10 hydrogen atoms, two isomeric compounds can exist:

Depending on the nature of the differences in the structure of isomers, there are structural And spatial isomerism.

Structural isomerism

Structural isomers - compounds of the same qualitative and quantitative composition, differing in the order of binding atoms, i.e. chemical structure.

For example, the composition C 4 H 8 corresponds to 5 structural isomers:

Among organic compounds, the existence of a colossal number of only structural isomers is theoretically possible. Thus, among alkanes containing only carbon and hydrogen atoms, the number of possible isomers increases exponentially with the increase in the number of carbon atoms. If for a compound of composition C 4 H 10 only two isomers are possible, then for pentanes C 5 H 12 the number of such isomers increases to three, C 6 H 14 has 5 isomers, C 7 H 16 - 9 isomers, C 8 H 18 - 18 isomers, C 9 H 20 - 35 isomers, and for the compound pentacosane C 25 H 52, theoretically, the existence of no less than 36,797,588 isomers is possible.

In the example above, the following isomers can be distinguished:

- double bond positions (butene-1 and butene-2);

- carbon skeleton (butenes-1 and -2 and isobutylene);

- cycle sizes (cyclobutane and methylcyclopropane);

- interclass isomers (alkenes and cycloalkanes).

Interclass isomers are, for example, ethanol and dimethyl ether, which have the same gross formula C 2 H 6 O, but completely different structures and belong to different classes. They differ not only in chemical properties (the more inert dimethyl ether does not react with metallic sodium, unlike ethanol), but also in physical ones. Ethanol is a liquid at normal temperature, while dimethyl ether is a gas.

Cyclic and acyclic organic compounds

It can be seen that among the structural isomers of organic compounds there may be molecules containing in their composition cycles built from carbon atoms different number (and often more than one such cycle in the composition of the molecule). On this basis, distinguish ali cyclic compounds (containing cycles, or simply cyclic compounds) and A cyclic compounds (not containing cycles, but built exclusively from chains of carbon atoms, often branched).

Carbocyclic compounds contain only carbon atoms in the cycle. They are divided into two groups that differ significantly in chemical properties: aliphatic cyclic (abbreviated alicyclic) And aromatic connections.

Heterocyclic compounds contain in the cycle, in addition to carbon atoms, one or more atoms of other elements - heteroatoms(from Greek. heteros- other, different) - oxygen, nitrogen, sulfur, etc.

Spatial isomerism

Spatial isomers (geometric isomers, stereoisomers) with the same composition and the same chemical structure, they differ in the spatial arrangement of atoms in the molecule.

Spatial isomers are optical (mirror) and cis-trans- isomers. In the example shown above, butene-2, which exists in nature in the form qi With - And trance- butenes-2:

Spatial isomerism appears, in particular, when carbon has four different substituents:

If any two of them are interchanged, another spatial isomer of the same composition is obtained. The physicochemical properties of such isomers differ significantly. Compounds of this type are distinguished by their ability to rotate the plane of polarized light passed through the solution of such compounds by a certain amount. In this case, one isomer rotates the plane of polarized light in one direction, and its isomer in the opposite direction. As a result of such optical effects this type of isomerism is called optical isomerism .

For more information on optical isomerism, see the section on oxygen-containing and nitrogen-containing organic compounds.

Optical isomerism is a special case of spatial isomerism. Optical isomers are called molecules that differ in the spatial arrangement of groups and atoms, having the same composition and the same order of atomic bonding. Solutions of such compounds are capable of rotating the plane of polarized light transmitted through them by a certain angle.

1.3.3. Nomenclature of organic compounds

Due to the presence of a huge number of organic compounds, the system of their designation (naming) is of great importance in such a way that by name it is easy to establish its structure (chemical structure), and, accordingly, all chemical and physical properties. Thus, the name should reflect the chemical structure of the organic compound as accurately as possible, including the possibility of identifying structural and geometric isomers. To date, three types of nomenclature of organic compounds have developed:

1. trivial ;

2. rational ;

3. systematic (or substitute, or nomenclature IUPAC ).

The presence of trivial names is associated with history. Previously, researchers often gave names to compounds according to the source of their isolation or according to some organoleptic, physicochemical properties. Trivial names are in circulation sometimes with the same rights (if not more often) than systematic names. So, for example, the name acetic acid, formic acid, lactose, urea and many other names still exist.

Rational nomenclature

This type of nomenclature has become widespread as a result of the fact that some compounds can be named as a kind of parent compound, from which they differ by substituents. An example might be neopentane ("new pentane"), a hydrocarbon of the alkane class of composition C 5 H 12. The name "neopentane" is considered trivial, and says absolutely nothing about its structure. According to the nomenclature of the second type, this hydrocarbon can be called tetramethylmethane. Name tetramethylmethane is already much more informative in terms of information about the structure of the molecule. One can imagine a methane molecule with all four hydrogen atoms replaced by methyl groups.

Systematic the same name for neopentane is the name 2,2-dimethylpropane , drawn up according to the rules developed by the International Union of Pure and Applied Chemistry (IUPAC - International Union of Pure and Applied Chemistry). The structural formula of neopentane is given below:

A detailed consideration of the rules for naming organic compounds will be done by us later, when considering individual classes of organic compounds, since each case has its own characteristics.

The replacement of hydrogen atoms in alkane molecules by any heteroatom (halogen, nitrogen, sulfur, oxygen, etc.) or group causes a redistribution of electron density. The nature of this phenomenon is different. It depends on the properties of the heteroatom (its electronegativity) and on the type of bonds through which this influence propagates.

Inductive effect

If the influence of the deputy is transferred with the participation s- bonds, then there is a gradual change in the electronic state of the bonds. This polarization is called inductive effect (I) , is represented by an arrow in the direction of electron density shift. The electron density always shifts towards a MORE ELECTRO-NEGATIVE atom or group of atoms:

CH 3 -CH 2 --> Cl,

HO CH 2 -CH 2 -->Cl ,

CH 3 -CH 2 --> COOH,

CH 3 -CH 2 --> NO 2 etc.

The inductive effect is due to the tendency of an atom or a group of atoms to supply or pull electron density onto itself, and therefore it can be positive or negative. A negative inductive effect is exhibited by elements that are more electronegative than carbon, i.e. halogens, oxygen, nitrogen and others, as well as groups with a positive charge on the element associated with carbon. The negative inductive effect decreases from right to left in a period and from top to bottom in a group of the periodic table:

F > O > N,

F > Cl > Br > J.

In the case of fully charged substituents, the negative inductive effect increases with increasing electronegativity of the carbon-bonded atom:

>O + - >> N +< .

In the case of complex substituents, the negative inductive effect is determined by the nature of the atoms that make up the substituent. In addition, the inductive effect depends on the nature of the hybridization of atoms. Thus, the electronegativity of carbon atoms depends on the hybridization of electron orbitals and changes in the following direction:

sp3< sp2 < sp .

The positive inductive effect is exhibited by elements less electronegative than carbon; groups with a total negative charge; alkyl groups. +I-effect decreases in the series:

(CH 3 ) 3 С -\u003e (CH 3) 2 CH-\u003e CH 3 -CH 2 -\u003e CH 3 -\u003e H-.

The inductive effect of a substituent decays rapidly as the chain length increases.

Table

Summary table of substituents and their electronic effects

mesomeric effect

The presence of a substituent with a free pair of electrons or a vacant p-orbital attached to a system containing p-electrons leads to the possibility of mixing the p-orbitals of the substituent (occupied or vacant) with p-orbitals and redistributing the electron density in compounds. Such an effect is called mesomeric .

The shift of the electron density is usually insignificant and the bond lengths practically do not change. An insignificant shift in the electron density is judged from the dipole moments, which are small even in the case of large conjugation effects on the extreme atoms of the conjugated system.

The mesomeric effect is represented by a curved arrow pointing towards the electron density shift. The electron density always shifts to the side more electronegative atom located at the edge of the structure and connected to the rest of the structure multiple bond:

Depending on the direction of displacement of the electron cloud, the mesomeric effect can be positive (+M), an atom or when a grouping of atoms transfers electrons to the pi system:

and negative (- M), when the grouping of atoms pulls electrons out of the pi system:

The positive mesomeric effect (+M) decreases with an increase in the electronegativity of an atom carrying a lone pair of electrons, due to a decrease in the tendency to give it away, and also with an increase in the volume of the atom. The positive mesomeric effect of halogens changes in the following direction:

F > Cl > B > J (+M effect).

Groups with lone pairs of electrons on an atom attached to a conjugated group have a positive mesomeric effect. pi-system:

- NH 2 ( NHR, NR2) > OH ( OR) > X (halogen) (+M-effect).

The positive mesomeric effect is reduced if the atom is bonded to an electron acceptor group:

-NH 2 > -NH-CO-CH 3 .

The negative mesomeric effect increases with an increase in the electronegativity of the atom and reaches its maximum values ​​if the acceptor atom carries a charge:

>C=O + H >> >C=O.

A decrease in the negative mesomeric effect is observed if the acceptor group is conjugated with the donor group:

-CO-O - << - СО -NH2< -CO-OR < -CO-H(R) << -CO- CO- < -CO-X ( halogen ) (- M-effect).

Table

Hyperconjugation or hyperconjugation

An effect similar to the positive mesomeric effect occurs when a hydrogen at a multiple bond is replaced by an alkyl group. This effect is directed towards the multiple bond and is called hyperconjugation(superconjugation):

The effect resembles a positive mesomeric one, since it donates electrons to the conjugate p- system:

Superconjugation decreases in the sequence:

CH 3 > CH 3 -CH 2 > (CH 3) 2 CH > (CH 3) 3 C.

To show the effect hyperconjugation it is necessary to have at least one hydrogen atom at the carbon atom adjacent to the p-system. The tert-butyl group does not exhibit this effect, and therefore its mesomeric effect is zero.

Table

Summary table of substituents and their electronic effects