Alkyne oxidation. Alkenes are hydrocarbons that have one double c=c bond in their molecules.

Redox reactions involving organic substances

The tendency of organic compounds to oxidize is associated with the presence of multiple bonds, functional groups, hydrogen atoms at the carbon atom containing the functional group.

Sequential oxidation of organic substances can be represented as the following chain of transformations:

Saturated hydrocarbon → Unsaturated hydrocarbon → Alcohol → Aldehyde (ketone) → Carboxylic acid →CO 2 + H 2 O

The genetic relationship between classes of organic compounds is presented here as a series of redox reactions that ensure the transition from one class of organic compounds to another. It is completed by the products of complete oxidation (combustion) of any of the representatives of the classes of organic compounds.

The dependence of the redox ability of organic matter on its structure:

The increased tendency of organic compounds to oxidize is due to the presence of substances in the molecule:

• multiple bonds(that is why alkenes, alkynes, alkadienes are so easily oxidized);
• certain functional groups, capable of being easily oxidized (--SH, -OH (phenolic and alcohol), - NH 2;
• activated alkyl groups located next to multiple bonds. For example, propene can be oxidized to the unsaturated aldehyde acrolein with atmospheric oxygen in the presence of water vapor on bismuth-molybdenum catalysts.

H 2 C═CH−CH 3 → H 2 C═CH−COH

As well as the oxidation of toluene to benzoic acid with potassium permanganate in an acidic environment.

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

• the presence of hydrogen atoms at a carbon atom containing a functional group.

An example is the reactivity in oxidation reactions of primary, secondary and tertiary alcohols by reactivity to oxidation.

Despite the fact that in the course of any redox reactions, both oxidation and reduction occur, the reactions are classified depending on what happens directly to the organic compound (if it is oxidized, they talk about the oxidation process, if it is reduced, about the reduction process) .

So, in the reaction of ethylene with potassium permanganate, ethylene will be oxidized, and potassium permanganate will be reduced. The reaction is called the oxidation of ethylene.

The use of the concept of "oxidation state" (CO) in organic chemistry is very limited and is realized, first of all, in the formulation of equations for redox reactions. However, taking into account that a more or less constant composition of the reaction products is possible only with complete oxidation (combustion) of organic substances, the expediency of arranging the coefficients in the reactions of incomplete oxidation disappears. For this reason, they usually confine themselves to drawing up a scheme for the transformations of organic compounds.

When studying the comparative characteristics of inorganic and organic compounds, we got acquainted with the use of the oxidation state (s.o.) (in organic chemistry, primarily carbon) and methods for determining it:

1) calculation of the average s.d. carbon in an organic molecule:

-8/3 +1

This approach is justified if all chemical bonds in the organic matter are destroyed during the reaction (combustion, complete decomposition).

2) definition of s.o. each carbon atom:

In this case, the oxidation state of any carbon atom in an organic compound is equal to the algebraic sum of the numbers of all bonds with atoms of more electronegative elements, taken into account with the “+” sign at the carbon atom, and the number of bonds with hydrogen atoms (or another more electropositive element), taken into account with the sign "-" at the carbon atom. In this case, bonds with neighboring carbon atoms are not taken into account.

As the simplest example, let's determine the oxidation state of carbon in a methanol molecule.

The carbon atom is bonded to three hydrogen atoms (these bonds are taken into account with the "-" sign), one bond is with the oxygen atom (it is taken into account with the "+" sign). We get: -3 + 1 = -2. Thus, the oxidation state of carbon in methanol is -2.

The calculated degree of oxidation of carbon, although a conditional value, but it indicates the nature of the shift in the electron density in the molecule, and its change as a result of the reaction indicates an ongoing redox process.

We clarify in which cases it is better to use one or another method.

The processes of oxidation, combustion, halogenation, nitration, dehydrogenation, decomposition are redox processes.

When moving from one class of organic compounds to another Andincrease in the degree of branching of the carbon skeleton molecules of compounds within a separate class the oxidation state of the carbon atom responsible for the reducing ability of the compound changes.

Organic substances whose molecules contain carbon atoms with maximum(- and +) CO values(-4, -3, +2, +3), enter into a complete oxidation-combustion reaction, but resistant to mild and medium strength oxidizers.

Substances whose molecules contain carbon atoms in CO -1; 0; +1, oxidize easily, their reduction abilities are close, so their incomplete oxidation can be achieved by one of the known oxidizing agents of low and medium strength. These substances may show dual nature, acting as an oxidizing agent, just as it is inherent in inorganic substances.

When writing the equations for the reactions of combustion and decomposition of organic substances, it is better to use the average value of s.d. carbon.

For example:

Let's make a complete equation of a chemical reaction by the balance method.

The average value of the oxidation state of carbon in n-butane:

The oxidation state of carbon in carbon monoxide (IV) is +4.

Let's make an electronic balance diagram:

Pay attention to the first half of the electronic balance: the carbon atom in the fractional value of s.d. the denominator is 4, so we calculate the transfer of electrons using this coefficient.

Those. going from -2.5 to +4 corresponds to going 2.5 + 4 = 6.5 units. Because 4 carbon atoms are involved, then 6.5 4 \u003d 26 electrons will be given away in total by butane carbon atoms.

Taking into account the found coefficients, the equation for the chemical reaction of n-butane combustion will look like this:

You can use the method for determining the total charge of carbon atoms in a molecule:

(4 C) -10 …… → (1 C) +4 , taking into account that the number of atoms before the = sign and after must be the same, we equalize (4C) -10 …… →[(1 C) +4 ] 4

Therefore, the transition from -10 to +16 is associated with the loss of 26 electrons.

In other cases, we determine the values ​​of s.d. each carbon atom in the compound, while paying attention to the sequence of substitution of hydrogen atoms at primary, secondary, tertiary carbon atoms:

First, the process of substitution occurs at the tertiary, then at the secondary, and, last of all, at the primary carbon atoms.

Alkenes

Oxidation processes depend on the structure of the alkene and the reaction medium.

1. When alkenes are oxidized with a concentrated solution of potassium permanganate KMnO 4 in an acidic environment (hard oxidation) σ- and π-bonds break with the formation of carboxylic acids, ketones, and carbon monoxide (IV). This reaction is used to determine the position of the double bond.

a) If the double bond is at the end of the molecule (for example, in butene-1), then one of the oxidation products is formic acid, which is easily oxidized to carbon dioxide and water:

b) If in the alkene molecule the carbon atom at the double bond contains two carbon substituents (for example, in the molecule of 2-methylbutene-2), then during its oxidation, the formation of a ketone occurs, since the transformation of such an atom into an atom of the carboxyl group is impossible without breaking the C–C bond, which is relatively stable under these conditions:

c) If the alkene molecule is symmetrical and the double bond is contained in the middle of the molecule, then only one acid is formed during oxidation:

A feature of the oxidation of alkenes, in which the carbon atoms in the double bond contain two carbon radicals, is the formation of two ketones:

2. In neutral or slightly alkaline environments, oxidation is accompanied by the formation of diols (dihydric alcohols) , and hydroxyl groups are attached to those carbon atoms between which there was a double bond:

During this reaction, the violet color of the aqueous solution of KMnO 4 is discolored. Therefore, it is used as qualitative reaction into alkenes (Wagner reaction).

3. Oxidation of alkenes in the presence of palladium salts (Wacker process) leads to the formation aldehydes and ketones:

2CH 2 \u003d CH 2 + O 2 PdCl2/H2O→ 2 CH 3 -CO-H

Homologues are oxidized at the less hydrogenated carbon atom:

CH 3 -CH 2 -CH \u003d CH 2 + 1 / 2O 2 PdCl2/H2O→ CH 3 - CH 2 -CO-CH 3

Alkynes

The oxidation of acetylene and its homologues proceeds depending on the medium in which the process takes place.

but) In an acidic environment, the oxidation process is accompanied by the formation of carboxylic acids:

The reaction is used to determine the structure of alkynes by oxidation products:

In neutral and slightly alkaline media, the oxidation of acetylene is accompanied by the formation of the corresponding oxalates (salts of oxalic acid), and the oxidation of homologues is accompanied by the breaking of the triple bond and the formation of salts of carboxylic acids:

For acetylene:

1) In an acidic environment:

H-C≡C-H KMnO 4, H 2 SO 4 → HOOC-COOH (oxalic acid)

3CH≡CH +8KMnO 4 H 2 O→ 3KOOC-COOK potassium oxalate+8MnO 2 ↓+ 2KOH+ 2H 2 O

Arenas

(benzene and its homologues)

When oxidizing arenes in an acidic medium, one should expect the formation of acids, and in an alkaline medium, salts.

Benzene homologues with one side chain (regardless of its length) are oxidized by a strong oxidizing agent to benzoic acid at the α-carbon atom. Benzene homologues, when heated, are oxidized by potassium permanganate in a neutral medium to form potassium salts of aromatic acids.

5C 6 H 5 -CH 3 + 6KMnO 4 + 9H 2 SO 4 \u003d 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 \u003d 5C 6 H 5 COOH + 5CO 2 + 12MnSO 4 + 6K 2 SO 4 + 28H 2 O,

C 6 H 5 -CH 3 + 2KMnO 4 \u003d C 6 H 5 COOK + 2MnO 2 + KOH + H 2 O.

We emphasize that if there are several side chains in an arene molecule, then in an acidic medium each of them is oxidized at an a-carbon atom to a carboxyl group, resulting in the formation of polybasic aromatic acids:

1) In an acidic environment:

C 6 H 5 -CH 2 -R KMnO 4, H 2 SO 4 → C 6 H 5 -COOH benzoic acid+CO2

2) In a neutral or alkaline environment:

C 6 H 5 -CH 2 -R KMnO4, H2O/(OH)→ C 6 H 5 -COOK + CO 2

3) Oxidation of benzene homologues with potassium permanganate or potassium bichromate when heated:

C 6 H 5 -CH 2 -R KMnO 4, H 2 SO 4, t ˚ C→ C 6 H 5 -COOH benzoic acid+ R-COOH

4) Oxidation of cumene with oxygen in the presence of a catalyst (cumene method for producing phenol):

C 6 H 5 CH(CH 3) 2 O2, H2SO4→ C 6 H 5 -OH phenol + CH 3 -CO-CH 3 acetone

5C 6 H 5 CH(CH 3) 2 + 18KMnO 4 + 27H 2 SO 4 → 5C 6 H 5 COOH + 42H 2 O + 18MnSO 4 + 10CO 2 + K 2 SO 4

C 6 H 5 CH (CH 3) 2 + 6H 2 O - 18'→ C 6 H 5 COOH + 2CO 2 + 18H + | x5

MnO 4 - + 8H + + 5ē→ Mn +2 + 4H 2 O | x18

Attention should be paid to the fact that at mild oxidation of styrene with potassium permanganate KMnO 4 in a neutral or slightly alkaline medium the π-bond breaks, glycol (dihydric alcohol) is formed. As a result of the reaction, the colored solution of potassium permanganate quickly becomes colorless and a brown precipitate of manganese (IV) oxide precipitates.

Oxidation strong oxidizing agent- potassium permanganate in an acidic environment - leads to a complete rupture of the double bond and the formation of carbon dioxide and benzoic acid, the solution becomes colorless.

C 6 H 5 -CH═CH 2 + 2 KMnO 4 + 3 H 2 SO 4 → C 6 H 5 -COOH + CO 2 + K 2 SO 4 + 2 MnSO 4 +4 H 2 O

Alcohols

It should be remembered that:

1) primary alcohols are oxidized to aldehydes:

3CH 3 -CH 2 OH + K 2 Cr 2 O 7 + 4H 2 SO 4 \u003d 3CH 3 -CHO + K 2 SO 4 + Cr 2 (SO 4) 3 + 7H 2 O;

2) secondary alcohols are oxidized to ketones:

3) for tertiary alcohols, the oxidation reaction is not typical.

Tertiary alcohols, in the molecules of which there is no hydrogen atom at the carbon atom containing the OH group, do not oxidize under normal conditions. Under harsh conditions (under the action of strong oxidizing agents and at high temperatures), they can be oxidized to a mixture of low molecular weight carboxylic acids, i.e. destruction of the carbon skeleton.

When methanol is oxidized with an acidified solution of potassium permanganate or potassium dichromate, CO 2 is formed.

Primary alcohols during oxidation, depending on the reaction conditions, can form not only aldehydes, but also acids.

For example, the oxidation of ethanol with potassium dichromate in the cold ends with the formation of acetic acid, and when heated, acetaldehyde:

3CH 3 -CH 2 OH + 2K 2 Cr 2 O 7 + 8H 2 SO 4 \u003d 3CH 3 -COOH + 2K 2 SO 4 + 2Cr 2 (SO 4) 3 + 11H 2 O,

If three or more OH groups are bonded to adjacent carbon atoms, then the middle or middle atoms are converted to formic acid when oxidized with hydrochloric acid.

The oxidation of glycols with potassium permanganate in an acidic medium proceeds similarly to the oxidative cleavage of alkenes and also leads to the formation of acids or ketones, depending on the structure of the initial glycol.

Aldehydes and ketones

Aldehydes are more easily oxidized than alcohols to the corresponding carboxylic acids not only under the action of strong oxidizing agents (air oxygen, acidified solutions of KMnO 4 and K 2 Cr 2 O 7), but also under the action of weak ones (ammonia solution of silver oxide or copper hydroxide (II) ):

5CH 3 -CHO + 2KMnO 4 + 3H 2 SO 4 \u003d 5CH 3 -COOH + 2MnSO 4 + K 2 SO 4 + 3H 2 O,

3CH 3 -CHO + K 2 Cr 2 O 7 + 4H 2 SO 4 \u003d 3CH 3 -COOH + Cr 2 (SO 4) 3 + K 2 SO 4 + 4H 2 O,

CH 3 -CHO + 2OH CH 3 -COONH 4 + 2Ag + 3NH 3 + H 2 O

Special attention!!! Oxidation of methanal with an ammonia solution of silver oxide leads to the formation of ammonium carbonate, and not formic acid:

HCHABOUT+ 4OH = (NH 4) 2 CO 3 + 4Ag + 6NH 3 + 2H 2 O.

To compile the equations of redox reactions, both the electron balance method and the half-reaction method (electron-ion method) are used.

For organic chemistry, it is not the oxidation state of the atom that is important, but the shift in electron density, as a result of which partial charges appear on atoms that are in no way consistent with the values ​​of the oxidation states.

Many higher education institutions include tasks on the selection of coefficients in OVR equations by the ion-electronic method (half-reaction method) in tickets for entrance exams. If the school pays at least some attention to this method, it is mainly in the oxidation of inorganic substances.

Let's try to apply the half-reaction method for the oxidation of sucrose with potassium permanganate in an acidic medium.

The advantage of this method is that there is no need to immediately guess and write down the reaction products. They are fairly easy to determine in the course of the equation. An oxidizing agent in an acidic environment most fully manifests its oxidizing properties, for example, the MnO anion - turns into a Mn 2+ cation, easily oxidized organic compounds are oxidized to CO 2.

We write in the molecular form of the transformation of sucrose:

On the left side, 13 oxygen atoms are missing; to eliminate this contradiction, let's add 13 H 2 O molecules.

The left side now contains 48 hydrogen atoms, they are released as H + cations:

Now we equalize the total charges on the right and left:

The half-reaction scheme is ready. Drawing up a scheme of the second half-reaction usually does not cause difficulties:

Let's combine both schemes:

Task for independent work:

Finish UHR and arrange the coefficients using the electronic balance method or the half-reaction method:

CH 3 -CH \u003d CH-CH 3 + KMnO 4 + H 2 SO 4 →

CH 3 -CH \u003d CH-CH 3 + KMnO 4 + H 2ABOUT →

(CH 3) 2 C \u003d C-CH 3 + KMnO 4 + H 2 SO 4 →

CH 3 -CH 2 -CH \u003d CH 2 + KMnO 4 + H 2 SO 4 →

FROMH 3 -CH 2 -C≡C-CH 3 + KMnO 4 + H 2 SO 4 →

C 6 H 5 -CH 3 + KMnO 4 + H2O →

C 6 H 5 -C 2 H 5 + KMnO 4 + H 2 SO 4 →

C 6 H 5 - CH 3 + KMnO 4 + H 2 SO 4 →

My notes:

Particular attention of students should be paid to the behavior of the oxidizing agent - potassium permanganate KMnO 4 in various environments. This is due to the fact that redox reactions in CMMs occur not only in tasks C1 and C2. In the tasks of SZ, representing a chain of transformations of organic substances, oxidation-reduction equations are not uncommon. At school, the oxidizing agent is often written above the arrow as [O]. The requirement for the performance of such tasks at the USE is the mandatory designation of all starting substances and reaction products with the arrangement of the necessary coefficients.

As already mentioned, the oxidation of organic matter is the introduction of oxygen into its composition and (or) the elimination of hydrogen. Recovery is the reverse process (the introduction of hydrogen and the elimination of oxygen). Given the composition of alkanes (СnH2n+2), we can conclude that they are incapable of participating in reduction reactions, but they can participate in oxidation reactions.

Alkanes are compounds with low degrees of carbon oxidation, and depending on the reaction conditions, they can be oxidized to form various compounds.

At ordinary temperatures, alkanes do not react even with strong oxidizing agents (H2Cr2O7, KMnO4, etc.). When introduced into an open flame, alkanes burn. At the same time, in an excess of oxygen, they are completely oxidized to CO2, where carbon has the highest oxidation state of +4, and water. The combustion of hydrocarbons leads to the breaking of all C-C and C-H bonds and is accompanied by the release of a large amount of heat (exothermic reaction).

It is generally accepted that the mechanism of alkane oxidation includes a radical chain process, since oxygen itself is poorly reactive, in order to tear off a hydrogen atom from an alkane, a particle is needed that will initiate the formation of an alkyl radical that will react with oxygen, giving a peroxy radical. The peroxy radical can then abstract a hydrogen atom from another alkane molecule to form an alkyl hydroperoxide and a radical.

It is possible to oxidize alkanes with atmospheric oxygen at 100-150 ° C in the presence of a catalyst - manganese acetate, this reaction is used in industry. Oxidation occurs when an air current is blown through molten paraffin containing a manganese salt.

Because As a result of the reaction, a mixture of acids is formed, then they are separated from the unreacted paraffin by dissolving in aqueous alkali, and then neutralized with mineral acid.

Directly in industry, this method is used to obtain acetic acid from n-butane:

Alkene oxidation

Alkene oxidation reactions are divided into two groups: 1) reactions in which the carbon skeleton is preserved, 2) reactions of oxidative destruction of the carbon skeleton of the molecule along the double bond.

Oxidation reactions of alkenes with preservation of the carbon skeleton

1. Epoxidation (Prilezhaev reaction)

Acyclic and cyclic alkenes, when interacting with peracids in a non-polar medium, form epoxides (oxiranes).

Also, oxiranes can be obtained by oxidation of alkenes with hydroperoxides in the presence of molybdenum-, tungsten-, vanadium-containing catalysts:

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

2. anti-hydroxylation (hydrolysis of epoxides)

Acid (or alkaline) hydrolysis of epoxides leads to the opening of the oxide cycle with the formation of transdiols.

In the first stage, the protonation of the oxygen atom of the epoxide occurs with the formation of a cyclic oxonium cation, which opens as a result of the nucleophilic attack of the water molecule.

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

3. syn-hydroxylation

One of the oldest methods for the oxidation of alkenes is the Wagner reaction (oxidation with potassium permanganate). Initially, during oxidation, a cyclic permanganate ester is formed, which is hydrolyzed to a vicinal diol:

In addition to the Wagner reaction, there is another method for the syn-hydroxylation of alkenes under the action of osmium (VIII) oxide, which was proposed by Krige. Under the action of osmium tetroxide on an alkene in ether or dioxane, a black precipitate of the cyclic ester of osmic acid is formed - osmate. However, the addition of OsO4 to the multiple bond is markedly accelerated in pyridine. The resulting black precipitate of osmate is easily decomposed by the action of an aqueous solution of sodium hydrosulfite:

Potassium permanganate or osmium(VIII) oxide oxidize the alkene to cis-1,2-diol.

Oxidative cleavage of alkenes

The oxidative cleavage of alkenes includes reactions of their interaction with potassium permanganate in alkaline or sulfuric acid, as well as oxidation with a solution of chromium trioxide in acetic acid or potassium dichromate and sulfuric acid. The end result of such transformations is the splitting of the carbon skeleton at the site of the double bond and the formation of carboxylic acids or ketones.

Monosubstituted alkenes with a terminal double bond are cleaved to a carboxylic acid and carbon dioxide:

If both carbon atoms in the double bond contain only one alkyl group, then a mixture of carboxylic acids is formed:

But if an alkene tetrasubstituted with a double bond is a ketone:

The reaction of ozonolysis of alkenes has acquired a much greater preparative significance. For many decades, this reaction served as the main method for determining the structure of the starting alkene. This reaction is carried out by passing a current of an ozone solution in oxygen, an alkene solution in methylene chloride or ethyl acetate at -80 ... -100 ° C. The mechanism of this reaction was established by Krige:

Ozonides are unstable compounds that decompose with an explosion. There are two ways of decomposition of ozonides - oxidative and reductive.

During hydrolysis, ozonides are split into carbonyl compounds and hydrogen peroxide. Hydrogen peroxide oxidizes aldehydes to carboxylic acids - this is oxidative decomposition:

Much more important is the reductive splitting of ozonides. Ozonolysis products are aldehydes or ketones, depending on the structure of the starting alkene:

In addition to the above methods, there is another method proposed in 1955 by Lemieux:

In the Lemieux method, there are no time-consuming procedures for separating manganese dioxide, since dioxide and manganate are again oxidized with periodate to the permanganate ion. This allows only catalytic amounts of potassium permanganate to be used.

In redox reactions, organic substances more often exhibit the properties of reducing agents, while they 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 that cause an increase in the electron density in the 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:

The growth of nucleophilicity in the series

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

Alkene oxidation

With mild oxidation, alkenes are converted to glycols (dihydric alcohols). The 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 slightly 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, a salt and a 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 \u003d 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 carbon atoms in the double bond contain two carbon radicals, two ketones are formed:

Alkyne oxidation

Alkynes oxidize under slightly more severe conditions than alkenes, so they usually oxidize with the triple bond breaking the carbon chain. As in the case of alkenes, the reducing atoms here are carbon atoms linked by a multiple bond. As a result of the reactions, acids and carbon dioxide are formed. Oxidation can be carried out with 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 +2KOH +2H 2 O

In an acidic environment, oxidation goes 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 fairly harsh conditions. Benzene homologues 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 dichromate or potassium permanganate in an acid 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

Alcohol oxidation

The direct products of the oxidation of primary alcohols are aldehydes, while those of secondary alcohols are ketones.

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 acid medium at the boiling point of the aldehyde. Evaporating, 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, but 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 neutral to potassium oxalate.

5CH 2 (OH) - CH 2 (OH) + 8KMnO 4 + 12H 2 SO 4 → 5HOOC -COOH + 8MnSO 4 + 4K 2 SO 4 + 22H 2 O

3CH 2 (OH) - CH 2 (OH) + 8KMnO 4 → 3KOOC -COOK + 8MnO 2 + 2KOH + 8H 2 O

Oxidation of aldehydes and ketones

Aldehydes are rather 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 give ammonium salts in an ammonia solution (“silver mirror” reaction):

CH 3 CH \u003d O + 2OH → CH 3 COONH 4 + 2Ag + H 2 O + 3NH 3

CH 3 -CH \u003d O + 2Cu (OH) 2 → CH 3 COOH + Cu 2 O + 2H 2 O

Formic aldehyde (formaldehyde) is oxidized, as a rule, to carbon dioxide:

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

3CH 2 O + 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 severe conditions by strong oxidizing agents with the breaking of C-C bonds and give mixtures of acids:

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

HCOOH + HgCl 2 \u003d CO 2 + Hg + 2HCl

HCOOH + Cl 2 \u003d CO 2 + 2HCl

HOOC-COOH + Cl 2 \u003d 2CO 2 + 2HCl

Formic acid, in addition to acidic properties, also exhibits some properties of aldehydes, in particular, reducing. It is then oxidized to carbon dioxide. For example:

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

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

HCOOH →(t)CO + H2O

Catalytic oxidation of alkanes:

Catalytic oxidation of alkenes:

Phenol oxidation:

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

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

For epoxidation, a mechanism has been proposed that is characteristic of concerted processes:

The following peracids are used as epoxidizing agents: perbenzoic, m-chloroperbenzoic, mononaphthalic, 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 reacting 30% or 90% hydrogen peroxide with 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 solution of methanesulfonic acid:

or from acid chlorides and hydrogen peroxide:

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

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

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

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

Alkenes with a double bond conjugated with a carbonyl and carboxyl group or another acceptor substituent are inactive, and for their oxidation it is necessary to use stronger oxidizing agents, such as trifluoroperacetic acid obtained from trifluoroacetic acid anhydride and 90% hydrogen peroxide in methylene chloride. An alternative epoxidation method is to react an alkene with a nitrile and 90% hydrogen peroxide:

The simplest oxirane, ethylene oxide, is produced industrially by the oxidation of 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. The hydrolysis of epoxides is catalyzed by both acids and bases. In both cases, vicinal diols are formed, i.e. glycols. In acid catalysis, in the first stage, the protonation of the oxygen atom of the epoxide occurs with the formation of a cyclic oxonium ion, which opens as a result of the nucleophilic attack of the water molecule:

The key step in ring opening, which determines the rate of the entire process, is the nucleophilic attack of 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 the nucleophilic attack of the bromide ion or another nucleophilic agent. From these positions, 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 acid hydrolysis of the epoxide corresponds in total to the 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, so they are commonly used for 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 an anti-hydroxylation of alkenes.
The third modern method of anti-hydroxylation of alkenes was proposed and developed by C. Prevost (1933). The alkene is heated with iodine and silver benzoate or acetate in anhydrous benzene or CCl4. The trans-addition to the double bond initially leads to the formation of iodoether, in which the iodine is further replaced by the benzoate ion, and 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 for anti-hydroxylation of alkenes. However, for acid-sensitive compounds, this method has obvious advantages over the method of anti-hydroxylation with peracids and subsequent acid hydrolysis of the epoxide.
Some salts and oxides of transition metals in higher oxidation states are effective reagents for double bond syn-hydroxylation. Oxidation of alkenes with potassium permanganate, one of the oldest methods for double bond syn-hydroxylation, 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 0ºС:

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 the double bond, but also cleaving the resulting vicinal diol. In order to avoid further degradation of the glycols whenever possible, the reaction conditions must be carefully controlled. The best results are achieved by hydroxylation of alkenes in a slightly alkaline medium (pH ~ 8) at 0 – 5ºС with a dilute ~ 1% aqueous solution of KMnO4. However, glycol yields are usually low (30 - 60%):

Initially, during the oxidation of alkenes with potassium permanganate, a cyclic ester of permanganic acid is formed, which is immediately hydrolyzed to a vicinal diol:

The cyclic ester of permanganic acid has never been isolated as an intermediate, but its formation follows from experiments with labeled 18O 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 for the syn-hydroxylation of alkenes under the action of osmium (VIII) oxide OsO4 was proposed by R. Krige in 1936. Osmium tetraoxide is a colorless crystalline substance, readily soluble in ether, dioxane, pyridine, and other organic solvents. When osmium tetroxide reacts with alkenes in ether or dioxane, a black precipitate of the osmic acid cyclic ester is formed - osmate, which can be easily isolated individually. The addition of OsO4 to the double bond is markedly accelerated in a 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 the products of syn-hydroxylation of alkenes in this method are much higher than when using permanganate as an oxidizing agent. An important advantage of the Krige method is the absence of products of oxidative cleavage of alkenes, which is characteristic of permanganate oxidation:

Osmium tetroxide is an expensive and hard-to-find reagent, besides it is very toxic. Therefore, osmium(VIII) oxide is used for the synthesis of small amounts of hard-to-reach substances in order to obtain the highest diol yield. To simplify the syn-hydroxylation of alkenes under the action of OsO4, a procedure was developed that allows using only catalytic amounts of this reagent. Hydroxylation is carried out with 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 the syn-hydroxylation of alkenes. Initially, the alkene is converted to trans-iodoacetate by reaction with iodine and silver acetate in acetic acid. Then I replace the halogen with an oxy group when treated with aqueous acetic acid when heated. The last step is the hydrolytic cleavage of the acetate group:

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

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

Alkynes with a non-terminal triple bond serve as a potential source for the synthesis of 1,2-diketones under the action of a suitable oxidizing agent. However, no universal reagent has yet been found that causes the oxidation of a triple carbon–carbon bond to a 1,2-dicarbonyl group. RuO 4 proposed for this purpose, ruthenium (VIII) oxide, is too expensive and often causes further oxidative degradation of 1,2-diketones to carboxylic acids. When disubstituted acetylenes interact with such strong oxidizing agents as potassium permanganate, only in a completely neutral medium at pH 7–8 at 0 С can oxidation be stopped at the stage of formation of -diketone. So, for example, stearolic acid at pH 7.5 is oxidized to α-diketone. In most cases, oxidation is accompanied by the cleavage of the triple bond with the formation of carboxylic acids:

The yield of products of oxidative degradation of alkynes is low, and this reaction does not play a significant role in organic synthesis. It is used solely to prove the structure of the natural acetylenic acid contained in the leaves of tropical plants in Central America. During its oxidative destruction, two acids were isolated - lauric and adipic. This means that the original acid is 6-octadecic acid with a normal carbon skeleton of seventeen carbons:

Much more important is the oxidative coupling of alkynes-1 catalyzed by copper salts (the Glaser–Eglinton reaction). In 1870, Glaser discovered that a suspension of copper (I) acetylenide in alcohol is oxidized by atmospheric oxygen to form 1,3-diynes:

For the oxidation of copper (I) acetylenides, potassium hexacyanoferrate (III) K 3 in DME or DMF is more effective as an oxidizing agent. In 1959, Eglinton proposed a much more convenient modification of the oxidative condensation of alkynes. Alkyne is oxidized with copper(II) acetate in a solution of pyridine at 60–70°C. Eglinton's modification proved to be extremely useful for the synthesis of macrocyclic polyynes from ,-diynes. As an illustration, we present the synthesis of two cyclopolyines during the oxidative condensation of hexadiine-1,5 (F. Sondheimer, 1960):

One of the polyins is a product of cyclotrimerization, the other is a product of cyclotetramerization of the starting gesadiin-1,5. The trimer serves as the initial reagent for the synthesis of aromatic -annulenes (for more details on annules, see Chapter 12). Similarly, under the same conditions of nonadiine-1,8, its dimer is obtained - 1,3,10,12-cyclooctadecatetraene along with trimer, tetramer and pentamer:

To obtain unsymmetrical diynes, condensation of haloacetylenes with alkyne-1 (terminal alkyne) in the presence of copper (I) salts and a primary amine is used (coupling according to Kadio–Chodkevich, 1957):

The initial bromalkynes are obtained by the action of sodium hypobromite on alkynes-1 or from lithium and bromine acetylides:

The organocopper derivative of terminal alkyne is generated directly in the reaction mixture of Cu 2 Cl 2 and alkyne-1.

6.3.4. Triple bond electrophilic addition reactions

Electrophilic addition reactions to the triple bond are among the most typical and important reactions of alkynes. In contrast to the electrophilic addition to alkenes, the synthetic application of this large group of reactions was far ahead of the development of theoretical ideas about its mechanism. However, over the past twenty years, the situation has changed significantly and at present it is one of the rapidly developing areas of physical organic chemistry. The HOMO of the alkyne is located lower than the HOMO of the alkene (Sec. 2), and this circumstance in the vast majority of cases predetermines the lower rate of addition of the electrophilic agent to the alkyne compared to the alkene. Another factor that determines the difference in the reactivity of alkynes and alkenes in electrophilic addition reactions is the relative stability of the intermediates that arise when an electrophilic species is added to triple and double bonds. When an electrophilic particle H + or E + is attached to a double bond, a cyclic or open carbocation is formed (Ch. 5). The addition of H + or E + to a triple bond leads to the formation of an open or cyclic vinyl cation. In a linear open vinyl cation, the central carbon atom is in sp-hybrid state, while vacant R-orbital is orthogonal to the -bond. Insofar as sp-hybrid carbon atom of the vinyl cation has a higher electronegativity compared to sp 2-hybrid atom of the alkyl cation, the vinyl cation should be less stable compared to the alkyl cation:

The data of quantum mechanical calculations, as well as thermodynamic data for the gas phase, obtained using high-pressure mass spectrometry and cyclotron resonance spectroscopy, are in full agreement with these considerations. In table. 6.3 shows thermodynamic data for the formation of a number of carbocations and hydrocarbons, related to the gas phase at 25 С.

From the data presented in Tal. 6.3, it follows that the vinyl cation is 47 kcal/mol less stable than the ethyl cation containing the same number of atoms. The same conclusion can be drawn from the ionization enthalpy in the gas phase CH 3 CH 2 Cl and CH 2 =CHCl:

It is easy to see that the combination of both factors - the higher energy of the vinyl cation and the low-lying HOMO of the alkyne - represents the lower reactivity of alkynes compared to alkenes in electrophilic addition reactions. In table. 6.4 contains comparative data on the addition of halogens, sulfene and selenyl chlorides, trifluoroacetic acid and water to various alkenes and alkynes that do not contain any activating or deactivating functional group.

Table 6.4

Comparative characteristics of alkynes and alkenes

in electrophilic addition reactions

It follows from these data that only the addition of acidic agents and water to triple and double bonds occurs at similar rates. The addition of halogens, sulfenchlorides and a number of other reagents to alkenes proceeds 10 2 - 10 5 times faster than to alkynes. This means that hydrocarbons containing non-conjugated triple and double bonds selectively add these reactants at the double bond, for example:

Data on the comparative hydration of alkynes and alkenes should be treated with caution, since the hydration of alkynes requires catalysis by mercury (II) ions, which is inefficient for the addition of water to the double bond. Therefore, the data on the hydration of the triple and double bonds, strictly speaking, are not comparable.

The addition of halogens, hydrogen halides, sulfen chlorides and other electrophilic agents can be carried out in steps, which can be easily illustrated using the following examples: