Mild oxidation of alkenes. Alkenes are hydrocarbons with one double c = c bond in their molecules. Attachment of carbenes and carbenoids

In redox reactions, organic substances more often exhibit the properties of reducing agents, and themselves are oxidized. The ease of oxidation of organic compounds depends on the availability of electrons when interacting with an oxidizing agent. All known factors causing an increase in the electron density in molecules of organic compounds (for example, positive inductive and mesomeric effects) will increase their ability to oxidize and vice versa.

The tendency of organic compounds to oxidize increases with the growth of their nucleophilicity, which corresponds to the following rows:

Increase in nucleophilicity in a row

Consider redox reactions members of the most important classes organic matter with some inorganic oxidizing agents.

Alkenes oxidation

Mild oxidation converts alkenes to glycols (dihydric alcohols). Reducing atoms in these reactions are carbon atoms linked by a double bond.

The reaction with a solution of potassium permanganate proceeds in a neutral or weakly alkaline medium as follows:

3C 2 H 4 + 2KMnO 4 + 4H 2 O → 3CH 2 OH – CH 2 OH + 2MnO 2 + 2KOH

Under more severe conditions, oxidation leads to the breaking of the carbon chain at the double bond and the formation of two acids (in a strongly alkaline medium - two salts) or an acid and carbon dioxide (in a strongly alkaline medium - salt and carbonate):

1) 5CH 3 CH = CHCH 2 CH 3 + 8KMnO 4 + 12H 2 SO 4 → 5CH 3 COOH + 5C 2 H 5 COOH + 8MnSO 4 + 4K 2 SO 4 + 17H 2 O

2) 5CH 3 CH = CH 2 + 10KMnO 4 + 15H 2 SO 4 → 5CH 3 COOH + 5CO 2 + 10MnSO 4 + 5K 2 SO 4 + 20H 2 O

3) CH 3 CH = CHCH 2 CH 3 + 8KMnO 4 + 10KOH → CH 3 COOK + C 2 H 5 COOK + 6H 2 O + 8K 2 MnO 4

4) CH 3 CH = CH 2 + 10KMnO 4 + 13KOH → CH 3 COOK + K 2 CO 3 + 8H 2 O + 10K 2 MnO 4

Potassium dichromate in a sulfuric acid medium oxidizes alkenes similarly to reactions 1 and 2.

During the oxidation of alkenes, in which the carbon atoms at the double bond contain two carbon radicals, two ketones are formed:

Oxidation of alkynes

Alkines oxidize under somewhat more severe conditions than alkenes, so they usually oxidize with a triple bond breaking of the carbon chain. As in the case of alkenes, the reducing atoms are here multiple carbon atoms. The reactions produce acids and carbon dioxide. Oxidation can be carried out with potassium permanganate or potassium dichromate in an acidic environment, for example:

5CH 3 C≡CH + 8KMnO 4 + 12H 2 SO 4 → 5CH 3 COOH + 5CO 2 + 8MnSO 4 + 4K 2 SO 4 + 12H 2 O

Acetylene can be oxidized with potassium permanganate in a neutral medium to potassium oxalate:

3CH≡CH + 8KMnO 4 → 3KOOC –COOK + 8MnO 2 + 2KON + 2H 2 O

In an acidic environment, oxidation proceeds to oxalic acid or carbon dioxide:

5CH≡CH + 8KMnO 4 + 12H 2 SO 4 → 5HOOC –COOH + 8MnSO 4 + 4K 2 SO 4 + 12H 2 O
CH≡CH + 2KMnO 4 + 3H 2 SO 4 → 2CO 2 + 2MnSO 4 + 4H 2 O + K 2 SO 4

Oxidation of benzene homologues

Benzene does not oxidize even under rather harsh conditions. Homologues of benzene can be oxidized with a solution of potassium permanganate in a neutral medium to potassium benzoate:

C 6 H 5 CH 3 + 2KMnO 4 → C 6 H 5 COOK + 2MnO 2 + KOH + H 2 O

C 6 H 5 CH 2 CH 3 + 4KMnO 4 → C 6 H 5 COOK + K 2 CO 3 + 2H 2 O + 4MnO 2 + KOH

Oxidation of benzene homologues with potassium dichromate or permanganate in an acidic medium leads to the formation of benzoic acid.

5C 6 H 5 CH 3 + 6KMnO 4 +9 H 2 SO 4 → 5C 6 H 5 COOH + 6MnSO 4 + 3K 2 SO 4 + 14H 2 O

5C 6 H 5 –C 2 H 5 + 12KMnO 4 + 18H 2 SO 4 → 5C 6 H 5 COOH + 5CO 2 + 12MnSO 4 + 6K 2 SO 4 + 28H 2 O

Oxidation of alcohols

Aldehydes are the direct oxidation product of primary alcohols, and ketones are secondary ones.

The aldehydes formed during the oxidation of alcohols are easily oxidized to acids; therefore, aldehydes from primary alcohols are obtained by oxidation with potassium dichromate in an acidic medium at the boiling point of the aldehyde. Evaporation, aldehydes do not have time to oxidize.

3C 2 H 5 OH + K 2 Cr 2 O 7 + 4H 2 SO 4 → 3CH 3 CHO + K 2 SO 4 + Cr 2 (SO 4) 3 + 7H 2 O

With an excess of an oxidizing agent (KMnO 4, K 2 Cr 2 O 7) in any medium, primary alcohols are oxidized to carboxylic acids or their salts, and secondary alcohols - to ketones.

5C 2 H 5 OH + 4KMnO 4 + 6H 2 SO 4 → 5CH 3 COOH + 4MnSO 4 + 2K 2 SO 4 + 11H 2 O

3CH 3 –CH 2 OH + 2K 2 Cr 2 O 7 + 8H 2 SO 4 → 3CH 3 –COOH + 2K 2 SO 4 + 2Cr 2 (SO 4) 3 + 11H 2 O

Tertiary alcohols are not oxidized under these conditions, and methyl alcohol is oxidized to carbon dioxide.

Dihydric alcohol, ethylene glycol HOCH 2 –CH 2 OH, when heated in an acidic medium with a solution of KMnO 4 or K 2 Cr 2 O 7 is easily oxidized to oxalic acid, and in a neutral one - to potassium oxalate.

5СН 2 (ОН) - СН 2 (ОН) + 8КMnO 4 + 12H 2 SO 4 → 5HOOC –COOH + 8MnSO 4 + 4K 2 SO 4 + 22Н 2 О

3СН 2 (ОН) - СН 2 (ОН) + 8КMnO 4 → 3KOOC –COOK + 8MnO 2 + 2КОН + 8Н 2 О

Oxidation of aldehydes and ketones

Aldehydes are quite strong reducing agents, and therefore are easily oxidized by various oxidizing agents, for example: KMnO 4, K 2 Cr 2 O 7, OH, Cu (OH) 2. All reactions take place when heated:

3CH 3 CHO + 2KMnO 4 → CH 3 COOH + 2CH 3 COOK + 2MnO 2 + H 2 O

3CH 3 CHO + K 2 Cr 2 O 7 + 4H 2 SO 4 → 3CH 3 COOH + Cr 2 (SO 4) 3 + 7H 2 O

CH 3 CHO + 2KMnO 4 + 3KOH → CH 3 COOK + 2K 2 MnO 4 + 2H 2 O

5CH 3 CHO + 2KMnO 4 + 3H 2 SO 4 → 5CH 3 COOH + 2MnSO 4 + K 2 SO 4 + 3H 2 O

CH 3 CHO + Br 2 + 3NaOH → CH 3 COONa + 2NaBr + 2H 2 O

silver mirror reaction

With an ammonia solution of silver oxide, aldehydes are oxidized to carboxylic acids, which in an ammonia solution give ammonium salts (the "silver mirror" reaction):

CH 3 CH = O + 2OH → CH 3 COONH 4 + 2Ag + H 2 O + 3NH 3

CH 3 –CH = O + 2Cu (OH) 2 → CH 3 COOH + Cu 2 O + 2H 2 O

Formic aldehyde (formaldehyde) is usually oxidized to carbon dioxide:

5HCOH + 4KMnO 4 (hut) + 6H 2 SO 4 → 4MnSO 4 + 2K 2 SO 4 + 5CO 2 + 11H 2 O

3СН 2 О + 2K 2 Cr 2 O 7 + 8H 2 SO 4 → 3CO 2 + 2K 2 SO 4 + 2Cr 2 (SO 4) 3 + 11H 2 O

HCHO + 4OH → (NH 4) 2 CO 3 + 4Ag ↓ + 2H 2 O + 6NH 3

HCOH + 4Cu (OH) 2 → CO 2 + 2Cu 2 O ↓ + 5H 2 O

Ketones are oxidized under harsh conditions by strong oxidizing agents with rupture C-C links and give a mixture of acids:

Carboxylic acids. Among acids, formic and oxalic have strong reducing properties, which are oxidized to carbon dioxide.

HCOOH + HgCl 2 = CO 2 + Hg + 2HCl

HCOOH + Cl 2 = CO 2 + 2HCl

HOOC-COOH + Cl 2 = 2CO 2 + 2HCl

Formic acid apart from acidic properties, it also exhibits some properties of aldehydes, in particular, reducing ones. In doing so, it is oxidized to carbon dioxide. For example:

2KMnO4 + 5HCOOH + 3H2SO4 → K2SO4 + 2MnSO4 + 5CO2 + 8H2O

When heated with strong dehydrating agents (H2SO4 (conc.) Or P4O10) decomposes:

HCOOH → (t) CO + H2O

Catalytic oxidation of alkanes:

Catalytic oxidation of alkenes:

Oxidation of phenols:

Alkenes - these are hydrocarbons, in the molecules of which there is ONE double C = C bond.

Alkenes nomenclature: suffix appears in the name -EN.

The first member of the homologous series is C2H4 (ethene).

For the simplest alkenes, historically established names are also used:

Ethylene (ethene),

Propylene (propene),

The nomenclature often uses the following monovalent alkene radicals:

Types of isomerism of alkenes:

1. Carbon skeleton isomerism:(starting with C4H8 - butene and 2-methylpropene)

2. Isomerism of the position of the multiple bond:(starting from C4H8): butene-1 and butene-2.

3. Interclass isomerism: with cycloalkanes(starting with propene):

C4H8 - butene and cyclobutane.

4. Spatial isomerism of alkenes:

Due to the fact that free rotation around the double bond is impossible, it becomes possible cis-trans isomerism.

Alkenes having two carbon atoms in a double bond various alternates, can exist in the form of two isomers, differing in the arrangement of substituents relative to the plane of the π-bond:

Chemical properties of alkenes.

Alkenes are characterized by:

· double bond addition reactions,

· oxidation reactions,

· substitution reactions in the "side chain".

OBTAINING ALKENS

ALCADIENES.

These are hydrocarbons containing two double bonds. The first member of the series is C3H4 (propadiene or allene). The suffix appears in the name - DIEN .

Types of double bonds in dienes:

Isomerism of dienes

Electronic structure of conjugated dienes.

Molecule of butadiene-1,3 CH2 = CH-CH = CH2 contains four carbon atoms in sp2 - hybridized and has a flat structure.

π-Electrons of double bonds form a single π-electron cloud (conjugate system ) and are delocalized between all carbon atoms.

The multiplicity of bonds (the number of common electron pairs) between carbon atoms has an intermediate value: there are no purely single and purely double bonds. The structure of butadiene more accurately reflects the formula with delocalized "one and a half" bonds.

CHEMICAL PROPERTIES OF CONJUGATED ALKADIENES.

OBTAINING ALCADIENES.

In tasks of the C3 USE category, special difficulties are caused by the oxidation reactions of organic substances with potassium permanganate KMnO 4 in an acidic medium, proceeding with the rupture of the carbon chain. For example, the oxidation reaction of propene, proceeding according to the equation:

CH 3 – CH = CH 2 + KMnO4 + H 2 SO 4 → CH 3 COOH + CO 2 + MnSO 4 + K 2 SO 4 + H 2 O.

To arrange the coefficients in complex equations of redox reactions like this one, the standard technique suggests drawing up an electronic balance, but after another attempt it becomes obvious that this is not enough. The root of the problem here lies in the fact that the coefficient in front of the oxidizer, taken from electronic balance must be replaced. This article offers two ways to select the correct ratio in front of the oxidizer in order to finally equalize all the elements. Substitution method to replace the coefficient in front of the oxidizer is more suitable for those who are able to count for a long time and painstakingly, since the arrangement of the coefficients in this way can be lengthy (in this example, it took 4 attempts). The substitution method is used in conjunction with the "TABLE" method, which is also discussed in detail in this article. Algebraic way allows no less simple and reliable, but much faster replacement of the coefficient in front of the oxidizer KMnO 4 in comparison with the substitution method, however, it has a narrower scope. The "algebraic" method can only be used to replace the coefficient in front of the oxidizer. KMnO 4 in the equations of the reactions of oxidation of organic substances proceeding with the rupture of the carbon chain.

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On the subject: methodological developments, presentations and notes

Arrangement of coefficients in chemical equations

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Oxidation of alkenes (acyclic and cyclic) when interacting with peracids (peracids) in a non-polar, indifferent medium is accompanied by the formation of alkenes oxides - epoxides, therefore the reaction itself is called the epoxidation reaction.

According to the modern IUPAC nomenclature, a three-membered cycle with one oxygen atom is called oxirane.
Epoxidation of alkenes should be considered as a synchronous, coordinated process, in which ionic intermediates such as the hydroxyl cation OH + are not involved. Epoxidation of alkenes is a process of syn-addition of one oxygen atom at a double bond with complete retention of the configuration of substituents at a double bond:

For epoxidation, a mechanism was proposed that is characteristic of coordinated processes:

Peracids are used as epoxidizing agents: perbenzoic, m-chloroperbenzoic, mononadphthalic, peracetic, pertrifluoroacetic and performic. Aromatic peracids are used as individual reagents, while aliphatic peracids - CH3CO3H, CF3CO3H, and HCO3H - are not isolated individually and are used immediately after their formation by the interaction of 30 or 90% hydrogen peroxide and the corresponding carboxylic acid. Perbenzoic and meta-chloroperbenzoic acids are currently obtained by oxidation of benzoic and meta-chlorobenzoic acids, respectively, with 70% hydrogen peroxide in a methanesulfonic acid solution:

or from acid chlorides and hydrogen peroxide:

Mononadphthalic acid is obtained by a similar method from phthalic anhydride and 30% hydrogen peroxide in aqueous alkali:

Initially, perbenzoic or mononadphthalic acids were used to obtain oxiranes (epoxides):

The method using mononadphthalic acid is especially convenient. Mononadphthalic acid is readily soluble in ether, while one of the reaction products (phthalic acid) is completely insoluble in ether, and the progress of the reaction can be easily judged by the amount of crystalline phthalic acid released.
Currently, meta-chloroperbenzoic acid is most often used for epoxidation. Unlike other peracids, it is stable during storage for a long time (up to 1 year) and absolutely safe during handling. The yields of oxiranes obtained by oxidation of acyclic and cyclic alkenes with meta-chloroperbenzoic acid in methylene chloride solution are usually very high.

Peracids are often generated directly in a reaction mixture of 90% hydrogen peroxide and carboxylic acid in methylene chloride:

Alkenes with a double bond conjugated to a carbonyl and carboxyl group or other acceptor substituent are inactive, and stronger oxidizing agents such as trifluoroacetic acid, obtained from trifluoroacetic anhydride and 90% hydrogen peroxide in methylene chloride, must be used for their oxidation. An alternative method of epoxidation is the interaction of an alkene with nitrile and 90% hydrogen peroxide:

The simplest oxirane, ethylene oxide, is produced industrially by oxidizing ethylene with oxygen in the presence of silver as a catalyst:

The three-membered ring of oxiranes is easily opened under the action of a wide variety of nucleophilic reagents. These reactions will be discussed in detail in Chapter 11 on acyclic and cyclic ethers. Here, only the hydrolysis of epoxides will be considered. Hydrolysis of epoxides is catalyzed by both acids and bases. In both cases, vicinal diols are formed, i.e. glycols. With acid catalysis, in the first stage, the protonation of the epoxide oxygen atom occurs with the formation of a cyclic oxonium ion, which opens as a result of a nucleophilic attack of a water molecule:

The key stage in ring opening, which determines the rate of the entire process, is the nucleophilic attack by water on the protonated form of the epoxide. From the point of view of the mechanism, this process is similar to the opening of the bromonium ion during a nucleophilic attack by the bromide ion or other nucleophilic agent. From this point of view, the stereochemical result should be the formation of trans-glycols during the cleavage of cyclic epoxides. Indeed, in the acid-catalyzed hydrolysis of cyclohexene oxide or cyclopentene oxide, only trans-1,2-diols are formed:

Thus, the two-stage process of alkene epoxidation followed by acidic hydrolysis of epoxide in total corresponds to the reaction of anti-hydroxylation of alkenes.
Both stages of anti-hydroxylation of alkenes can be combined if the alkene is treated with aqueous 30-70% hydrogen peroxide in formic or trifluoroacetic acid. Both of these acids are strong enough to cause epoxy ring opening and are therefore commonly used for the anti-hydroxylation of alkenes, for example:

Base-catalyzed epoxy ring opening also leads to the formation of trans-glycols:

Therefore, the two-stage process of epoxidation of alkenes followed by alkaline hydrolysis of epoxides is also a reaction of anti-hydroxylation of alkenes.
Third modern method anti-hydroxylation of alkenes was proposed and developed by K. Prevost (1933). The alkene is heated with iodine and silver benzoate or acetate in anhydrous benzene or CCl4. trans-addition to the double bond initially leads to the formation of an iodine ester, in which iodine is further replaced by a benzoate ion, and a glycol dibenzoate is obtained:

The Prevost reaction in an anhydrous medium leads to the formation of the same diol as the epoxidation of alkenes followed by hydrolysis:

Thus, the Prevost reaction is a more expensive modification of other methods of anti-hydroxylation of alkenes. However, for compounds sensitive to the action of acids, this method has obvious advantages before the method of anti-hydroxylation using peracids and subsequent acidic hydrolysis of the epoxide.
Some salts and oxides of transition metals of higher oxidation states are effective reagents for syn-hydroxylation of the double bond. The oxidation of alkenes with potassium permanganate, one of the oldest methods for syn-hydroxylation of a double bond, continues to be widely used despite its inherent limitations. cis-1,2-Cyclohexanediol was first obtained by V.V. Markovnikov back in 1878 by hydroxylation of cyclohexene with an aqueous solution of potassium permanganate at 0 ° C:

This method was further developed in the works of the Russian scientist E.E., Wagner, therefore syn-hydroxylation under the action of an aqueous solution of potassium permanganate is called the Wagner reaction. Potassium permanganate is a strong oxidizing agent capable of not only hydroxylating a double bond, but also cleaving the vicinal diol formed. To avoid further degradation of the glycols as much as possible, the reaction conditions must be carefully controlled. The best results are achieved when alkenes are hydroxylated in a slightly alkaline medium (pH ~ 8) at 0 - 5 ° C with a ~ 1% diluted aqueous solution of KMnO4. Nevertheless, the yields of glycols are usually low (30-60%):

Initially, when alkenes are oxidized with potassium permanganate, a cyclic ester of manganese acid is formed, which is immediately hydrolyzed to a vicinal diol:

The cyclic ester of manganese acid has never been isolated as an intermediate, but its formation follows from experiments with 18O-labeled potassium permanganate. K. Weiberg et al. (1957) showed that both oxygen atoms in glycol are labeled during the oxidation of the KMn18O4 alkene. This means that both oxygen atoms are transferred from the oxidizing agent and not from the solvent - water, which is in good agreement with the proposed mechanism.
Another method of syn-hydroxylation of alkenes under the action of osmium (VIII) oxide OsO4 was proposed by R. Kriege in 1936. Osmium tetroxide is a colorless crystalline substance readily soluble in ether, dioxane, pyridine and other organic solvents. When osmium tetroxide interacts with alkenes in ether or dioxane, a black precipitate of cyclic ester of osmium acid is formed - osmate, which can be easily isolated individually. The addition of OsO4 to the double bond is noticeably accelerated in pyridine solution. The decomposition of osmates to vicinal diols is achieved by the action of an aqueous solution of sodium hydrosulfite or hydrogen sulfide:

The yields of products of syn-hydroxylation of alkenes in this method are significantly higher than when using permanganate as an oxidizing agent. An important advantage of the Kriege method is the absence of products of the oxidative degradation of alkenes, which is characteristic of permanganate oxidation:

Osmium tetroxide is an expensive and difficult to obtain reagent, and it is also very toxic. Therefore, osmium (VIII) oxide is used for the synthesis of small amounts of hard-to-find substances in order to obtain the highest diol yield. To simplify the syn-hydroxylation of alkenes under the action of OsO4, a technique was developed that makes it possible to use only catalytic amounts of this reagent. Hydroxylation is carried out using hydrogen peroxide in the presence of OsO4, for example:

It is interesting to note that higher oxides of other transition metals (V2O5, WO3, MoO3, etc.) catalyze the anti-hydroxylation of alkenes.
R. Woodward in 1958 proposed an alternative three-stage method for syn-hydroxylation of alkenes. The alkene is initially converted to trans iodoacetate by reaction with iodine and silver acetate in acetic acid. Then I replace the halogen with an oxygroup when treating with aqueous acetic acid when heated. The last stage consists in the hydrolytic cleavage of the acetate group:

In conclusion of this section, we present the stereochemical relations between the alkene of the cis or trans configuration and the configuration of the resulting vicinal glycol, which can be the cis or trans isomer, the erythro or threo form, the meso or d, l form, depending on from substituents in alkene:

Similar stereochemical relationships are observed in other reactions of syn- or anti-addition of hydrogen, hydrogen halides, water, halogens, boron hydrides and other reagents at a multiple bond.

Oxidation of alkenes with potassium permanganate in an alkaline medium upon heating (severe conditions) leads to the destruction of their carbon skeleton at the site of the double bond. In this case, depending on the number of alkyl groups bonded to the vinyl fragment, two carboxylic acids, an acid and a ketone, or two ketones can be obtained:

Exercise 11. What product is formed during the oxidation of cyclohexene (a) with a dilute solution of potassium permanganate in the cold and (b) with a concentrated solution of potassium permanganate, followed by acidification.

Exercise 12. What products are formed from 1,2-dimethylcyclohexene during its (a) catalytic hydrogenation, (b) oxidation with a dilute solution of potassium permanganate in the cold, (c) ozonation followed by reductive cleavage.

6.5. Oxidation of ethylene to acetaldehyde

Oxidation of ethylene with atmospheric oxygen in the presence of palladium (II) and copper (II) chlorides leads to the formation of acetaldehyde ( Wacker process):

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ethanal (acetaldehyde)

6.6. Ethylene chloroxidation

Vinyl chloride is obtained by ethylene chloroxidation:

6.7. Oxidative ammonolysis

Oxidation of hydrocarbons with atmospheric oxygen in the presence of ammonia leads to the conversion of the methyl group to the cyano group. This oxidation is called oxidative ammonolysis. Acrylonitrile is obtained by oxidative ammonolysis of propylene.

acrylonitrile

Hydrocyanic acid is obtained by oxidative ammonolysis of methane:

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7. Hydroformylation of alkenes (Oxosynthesis)

At a temperature from 30 to 250 o C and a pressure of 100-400 atm. in the presence of dicobaltoctacarbonyl, alkenes add hydrogen and carbon monoxide to form aldehydes. Usually a mixture of isomers is obtained:

Mechanism:

1. Cleavage of the ligand

2. Attachment of ethylene

3. Introduction of ethylene

4. Attachment of the ligand

5. Implementation of CO

6. Oxidative addition of hydrogen

7. Reductive elimination of propanal

8. Attachment of carbenes and carbenoids

In recent years, much attention in organic chemistry has been paid to compounds of divalent carbon - carbenes. Most of the carbenes are unstable and react with other compounds immediately after their formation.

8.1. Carbene structure

Unsubstituted carbene: CH 2, also called methylene, can be in singlet or triplet form. In the singlet form of carbene, two non-bonding electrons with paired spins are in the same orbital, while in the triplet form, two unpaired electrons with parallel spins are in two orbitals of the same energy. Different electronic configurations of singlet and triplet carbenes are reflected both in the different geometry of these particles and in different chemical activities. The divalent carbon atom of the singlet carbene is in the sp 2 -hybrid state, both electrons are located in the sp 2 -hybrid orbital (HOMO), and the p-orbital (LUMO) is free. Triplet carbene is characterized by sp-hybridization of divalent carbon; in this case, two unpaired electrons are located on two p-orbitals, i.e., the triplet carbene is a biradical. The angle H - C - H for singlet methylene, according to the spectral data, is equal to 102-105 0, and for triplet methylene, this angle increases to 135,140 o. This corresponds to the higher stability of triplet methylene. According to the data of quantum mechanical calculations, triplet methylene is actually 10 kcal / mol more stable than singlet methylene.

The substituents, however, cause a change in the relative stability of these two forms of carbenes. For dialkylcarbenes, the triplet form is also more stable than the singlet form, but for digalocarbenes : CHal 2, and other carbenes with substituents containing a lone pair of electrons, the ground state is singlet. The C1-C-C1 bond angle for dichlorocarbene, equal to 106 o, is in good agreement with the singlet form. The higher stability of the singlet form of digalocarbenes in comparison with the triplet form is apparently due to its stabilization due to the lone pair of electrons of the heteroatom

Such stabilization of the triplet form of digalocarbenes is impossible. According to the data of quantum mechanical calculations, the energy of the singlet - triplet transition for dichlorocarbene is 13.5 Kcal / mol.

A. Dichlorocarbene

For the generation of digalocarbenes, methods have been developed based on the reaction of the elimination of hydrogen halide from trihalomethanes under the action of strong foundations... This method was historically the first to generate the first carbene, dichlorocarbene, as an intermediate (J. Hein 1950). When interacting with strong bases from chloroform (pKa of chloroform is ~ 16), bromoform (pKa = 9) and other trihalomethanes, an anion is formed, which is stabilized by the elimination of the halide ion with the formation of dihalocarbene. The action of strong bases on chloroform gives dichlorocarbene:

dichlorocarbene

Organolithium compounds can also be used as a base in an indifferent aprotic medium. Then below -100 0 С it is possible to fix the formation of trichloromethyllithium as an intermediate.

Using strong bases such as RLi, carbenes can be generated from 1,1-dihalo derivatives

In recent years, for the generation of digalocarbens instead of n-butyllithium is widely used as a base of sodium bis (trimethylsilyl) amide.

This produces a chemically inert silazane [bis (trimethylsilyl) amide]. Sodium bis (trimethylsilyl) amide, in contrast to n-butyllithium, can be isolated dry in an inert atmosphere. In practice, its ether solutions are more often used, which can be stored at room temperature for a long time.

Dichlorocarbene can also be generated by thermal decarboxylation of dry sodium trichloroacetate:

One of the most accessible modern methods for the generation of dichlorocarbene from chloroform under the action of sodium hydroxide under conditions of phase transfer catalysis will be discussed in detail later.

Dichlorocarbene combines with alkenes to give dichlorocyclopropanes. The addition occurs stereospecifically - the configuration of the initial alkene is retained in the reaction product, cyclopropane:

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trance-2-butene trance-1,2-dimethyl-3,3-

dichlorocyclopropane

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cis-2-butene qiwith-1,2-dimethyl-3,3-

dichlorocyclopropane

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7,7-dichlorobicycloheptane

When reducing 1,1-dihalocyclopropanes under the action of lithium in mpem-butyl alcohol, zinc in acetic acid or sodium in liquid ammonia, both halogen atoms are replaced by hydrogen. This is one of the general methods for the preparation of cyclopropane derivatives.

bicycloheptane

Control. eleven. Complete the reactions:

(Z) -3-methyl-2-pentene methylenecyclohexane

Answer

B. Methylene

Methylene can be obtained by decomposition of diazomethane. Diazomethane is a relatively unstable substance that decomposes on exposure to nitrogen and methylene.

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diazomethane

Methylene: CH 2 upon photolysis of diazomethane is formed in a less stable singlet form. Singlet methylene under the reaction conditions rapidly loses energy as a result of collisions with diazomethane or nitrogen molecules and turns into a more stable triplet methylene.

Singlet carbene is characterized by synchronous addition to the double bond of the alkene with complete retention of the geometry at the double bond (β-cycloaddition reaction). The addition of the singlet form of carbene at the double bond is thus strictly stereospecific.

B. Simmons reactionSmith

An efficient and experimentally very simple method for the conversion of alkenes into cyclopropane derivatives is based on the reaction of alkenes with methylene iodide and an alloy of zinc and copper. This reaction was discovered in 1958 by Simmons and Smith and immediately gained wide popularity in the synthesis of cyclopropane derivatives. The active species in this reaction is not carbene : CH 2, and carbenoid - iodomethylzinc iodide IZnCH 2 I, formed by the interaction of methylene iodide and zinc-copper pair.

diiodomethane iodomethylzinciodide

(Simmons-Smith reagent)

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The reaction proceeds according to the following mechanism:

The Simmons-Smith reaction is a very convenient method for converting alkenes to cyclopropanes.

Control. 12. Complete the reactions:

Answer

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methylenecyclopentane spiroheptane

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styrene cyclopropylbenzene