4.5.b. Oxidative cleavage of alkenes

During the oxidation of alkenes with an alkaline aqueous solution of potassium permanganate when heated or with a solution of KMnO 4 in aqueous sulfuric acid, as well as during the oxidation of alkenes with a solution of chromium (VI) oxide CrO 3 in acetic acid or potassium dichromate and sulfuric acid, the initially formed glycol undergoes oxidative degradation. The end result is the splitting of the carbon skeleton at the site of the double bond and the formation of ketones and/or carboxylic acids as end products, depending on the substituents on the double bond. If both carbon atoms at the double bond contain only one alkyl group, the final product of exhaustive oxidation will be a mixture of carboxylic acids, the tetrasubstituted alkene at the double bond is oxidized to two ketones. Single-substituted alkenes with a terminal double bond are cleaved to carboxylic acid and carbon dioxide.

Due to the low yields of carboxylic acids and ketones, the reactions of exhaustive oxidation of alkenes in the classical version have not found wide application and were previously used mainly to determine the structure of the initial alkene from the products of destructive oxidation. Currently, the oxidation of alkenes (R-CH=CH-R and R-CH=CH 2) to carboxylic acids (RCOOH) with potassium permanganate or dichromate is carried out under conditions of phase transfer catalysis. The yields of carboxylic acids in this case exceed 90%.

4.5.c. Ozonolysis of alkenes

The reaction of alkenes with ozone is the most important method for the oxidative cleavage of alkenes at the double bond. For many decades, this reaction served as the main method for determining the structure of the initial hydrocarbon, and also found application in the synthesis of various carbonyl compounds. The reaction of alkene with ozone is carried out by passing a current of ~5% mixture of ozone and oxygen into a solution of alkene in methylene chloride or ethyl acetate at -80 0 -100 0 C. The end of the reaction is controlled by a test for free ozone with potassium iodide. The mechanism of this peculiar and complex reaction has been established mainly thanks to the work of R. Krige. The first product of the 1,3-dipolar cycloaddition to the double bond is the so-called molozonide (1,2,3-trioxolane). This adduct is unstable and then spontaneously decomposes with ring opening and the formation of normal ozonide (1,2,4-trioxolane) as the final product.

It is now generally accepted that the transformation of molozonide into ordinary ozonide occurs by the splitting-recombination mechanism. Mollozonide undergoes spontaneous opening of the unstable 1,2,3-trioxolane ring with the formation of a carbonyl compound and a bipolar ion, which then react with each other also according to the 1,3-dipolar cycloaddition scheme.

The given scheme of the rearrangement of molozonide into normal ozonide is confirmed by the fact that if another carbonyl compound is present as an "interceptor" of the bipolar ion in the reaction mixture before the complete formation of the ozonide, then the so-called "mixed ozonide" is formed. For example, in ozonization cis-stilbene in the presence of benzaldehyde labeled with the 18 O isotope, the label is part of the ether, and not the peroxide bridge of the ozonide:

This result is in good agreement with the formation of a mixed ozonide upon recombination of a bipolar ion with labeled benzaldehyde:

Ozonides are highly unstable compounds that decompose explosively. They are not isolated individually, but split under the action of a wide variety of regents. It is necessary to distinguish between reductive and oxidative cleavage. During hydrolysis, ozonides are slowly split into carbonyl compounds and hydrogen peroxide. Hydrogen peroxide oxidizes aldehydes to carboxylic acids. This is the so-called oxidative decomposition of ozonides:

Thus, during the oxidative decomposition of ozonides, carboxylic acids and (or) ketones are formed, depending on the structure of the initial alkene. Air oxygen, hydrogen peroxide, peracids or silver hydroxide can be used as oxidizing agents. Most often in synthetic practice, hydrogen peroxide in acetic or formic acid, as well as hydrogen peroxide in an alkaline medium, is used for this purpose.

In practice, the method of oxidative decomposition of ozonides is mainly used to obtain carboxylic acids.

More important is the reductive cleavage of ozonides. The most commonly used reducing agents are zinc and acetic acid, triphenylphosphine, or dimethyl sulfide. In this case, the end products of ozonolysis are aldehydes or ketones, depending on the structure of the starting alkene.

From the above examples, it can be seen that a tetra-substituted alkene at a double bond forms two ketones during ozonolysis and subsequent reductive decomposition of the ozonide, while a tri-substituted alkene gives a ketone and an aldehyde. A disubstituted symmetrical alkene forms two aldehydes during ozonolysis, and alkenes with a terminal bond form an aldehyde and formaldehyde.

An interesting modification of ozonolysis is the method where sodium borohydride is used as an ozonide reducing agent. In this case, the final reaction products are primary or secondary alcohols formed during the reduction of aldehydes and xtones, respectively.

Ozonolysis of alkenes is a complex, time-consuming and explosive process that requires the use of special equipment. For this reason, other methods have been developed for the oxidative cleavage of alkenes to carbonyl compounds and carboxylic acids, which successfully replace the ozonolysis reaction in synthetic practice.

One of the modern preparative methods for the oxidative destruction of alkenes was proposed in 1955 by R. Lemieux. This method is based on the hydroxylation of alkenes with potassium permanganate, followed by cleavage of vicinal glycol with sodium periodate NaIO 4 at pH ~ 7 8. Periodate itself does not interact with alkene. The products of this two-step oxidative cleavage are ketones or carboxylic acids, since aldehydes are also oxidized to carboxylic acids under these conditions. In the Lemieux method, the laborious problem of separating one of the reaction products, manganese dioxide, does not arise, since both dioxide and manganate are again oxidized with periodate to a permanganate ion. This allows only catalytic amounts of potassium permanganate to be used. Below are some typical examples of the oxidative cleavage of alkenes by the Lemieux method.

Citronellol, an alcohol that is part of rose oil, geranium and lemon oils, is oxidized with a mixture of potassium permanganate and sodium periodate in aqueous acetone at 5 10 0 C to 6-hydroxy-4-methylhexanecarboxylic acid with a quantitative yield.

Another variation of this method uses catalytic amounts of osmium tetroxide instead of potassium permanganate (Lemieux & Johnson 1956). A particular advantage of the combination of OsO 4 and NaIO 4 is that it allows the oxidation to be stopped at the aldehyde stage. Osmium tetroxide adds to the double bond of the alkene to form osmate, which is oxidized by sodium periodate to carbonyl compounds with the regeneration of osmium tetroxide.

Instead of osmium tetroxide, ruthenium tetroxide RuO 4 can also be used. Lemieux-Johnson oxidative degradation of alkenes leads to the same products as ozonolysis with reductive cleavage of ozonides.

In terms characteristic of modern organic chemistry, this means that the combination of OsO 4 -NaIO 4 is synthetic equivalent ozonolysis of alkenes followed by reductive cleavage. Similarly, the oxidation of alkenes with a mixture of permanganate and periodate is the synthetic equivalent of ozonolysis with oxidative degradation of ozonides.

Thus, the oxidation of alkenes is not only a set of preparative methods for obtaining alcohols, epoxides, diols, aldehydes, ketones, and carboxylic acids; it is also one of the possible ways to establish the structure of the starting alkene. So, according to the result of the oxidative degradation of the alkene, one can determine the position of the double bond in the molecule, while the stereochemical result syn- or anti- hydroxylation of an alkene makes it possible to draw a conclusion about its geometry.

Description of the presentation REDOX REACTIONS INVOLVING ORGANIC SUBSTANCES on slides

REDOX REACTIONS WITH THE PARTICIPATION OF ORGANIC SUBSTANCES Kochuleva L.R., Chemistry teacher, Lyceum No. 9, Orenburg

In organic chemistry, oxidation is defined as a process in which, as a result of the transformation of a functional group, a compound passes from one category to a higher one: alkene alcohol aldehyde (ketone) carboxylic acid. Most oxidation reactions involve the introduction of an oxygen atom into the molecule or the formation of a double bond with an already existing oxygen atom due to the loss of hydrogen atoms.

OXIDIZERS For the oxidation of organic substances, compounds of transition metals, oxygen, ozone, peroxides, and compounds of sulfur, selenium, iodine, nitrogen, and others are usually used. Of the oxidizing agents based on transition metals, chromium (VI) and manganese (VII), (VI) and (IV) compounds are mainly used. The most common chromium (VI) compounds are a solution of potassium dichromate K 2 Cr 2 O 7 in sulfuric acid, a solution of chromium trioxide Cr. O 3 in dilute sulfuric acid.

OXIDIZERS During the oxidation of organic substances, chromium (VI) in any medium is reduced to chromium (III), however, oxidation in an alkaline medium in organic chemistry does not find practical application. Potassium permanganate KMn. O 4 in different environments exhibits different oxidizing properties, while the strength of the oxidizing agent increases in an acidic environment. Potassium manganate K 2 Mn. O 4 and manganese (IV) oxide Mn. O 2 exhibit oxidizing properties only in an acidic environment

ALKENES Depending on the nature of the oxidizing agent and the reaction conditions, various products are formed: dihydric alcohols, aldehydes, ketones, carboxylic acids When oxidized with an aqueous solution of KMn. O 4 at room temperature, the π-bond breaks and dihydric alcohols are formed (Wagner reaction): Discoloration of the potassium permanganate solution - a qualitative reaction for a multiple bond

ALKENES Oxidation of alkenes with a concentrated solution of potassium permanganate KMn. O 4 or potassium dichromate K 2 Cr 2 O 7 in an acidic medium is accompanied by a rupture of not only π-, but also σ-bonds Reaction products - carboxylic acids and ketones (depending on the structure of the alkene) Using this reaction, the products of alkene oxidation can be determined the position of the double bond in its molecule:

ALKENES 5 CH 3 -CH \u003d CH-CH 3 +8 KMn. O 4 +12 H 2 SO 4 → 10 CH 3 COOH +8 Mn. SO 4+4 K 2 SO 4+12 H 2 O 5 CH 3 –CH=CH-CH 2 -CH 3 +8 KMn. O 4 +12 H 2 SO 4 → 5 CH 3 COOH +5 CH 3 CH 2 COOH +8 Mn. SO 4 +4 K 2 SO 4 +12 H 2 O CH 3 -CH 2 -CH \u003d CH 2 +2 KMn. O 4 +3 H 2 SO 4 → CH 3 CH 2 COOH + CO 2 +2 Mn. SO 4 + K 2 SO 4 +4 H 2 O

ALKENES Branched alkenes containing a hydrocarbon radical at the carbon atom connected by a double bond form a mixture of carboxylic acid and ketone upon oxidation:

ALKENES 5 CH 3 -CH \u003d C-CH 3 + 6 KMn. O 4 +9 H 2 SO 4 → │ CH 3 5 CH 3 COOH + 5 O \u003d C-CH 3 + 6 Mn. SO 4 + 3 K 2 SO 4+ │ CH 3 9 H 2 O

ALKENES Branched alkenes containing hydrocarbon radicals at both carbon atoms connected by a double bond form a mixture of ketones upon oxidation:

ALKENES 5 CH 3 -C=C-CH 3 + 4 KMn. O 4 +6 H 2 SO 4 → │ │ CH 3 10 O \u003d C-CH 3 + 4 Mn. SO 4 + 2 K 2 SO 4 + 6 H 2 O │ CH

ALKENES As a result of the catalytic oxidation of alkenes with atmospheric oxygen, epoxides are obtained: Under harsh conditions, when burned in air, alkenes, like other hydrocarbons, burn to form carbon dioxide and water: C 2 H 4 + 3 O 2 → 2 CO 2 + 2 H 2 O

ALKADIENES CH 2 =CH−CH=CH 2 There are two terminal double bonds in the oxidized molecule, therefore, two molecules of carbon dioxide are formed. The carbon skeleton is not branched, therefore, when the 2nd and 3rd carbon atoms are oxidized, carboxyl groups CH 2 \u003d CH - CH \u003d CH 2 + 4 KMn are formed. O 4 + 6 H 2 SO 4 → 2 CO 2 + HCOO−COOH + 4 Mn. SO 4 +2 K 2 SO 4 + 8 H 2 O

ALKYNES Alkynes are easily oxidized by potassium permanganate and potassium dichromate at the site of a multiple bond When alkynes are treated with an aqueous solution of KMn. O 4 it becomes discolored (qualitative reaction to a multiple bond) When acetylene reacts with an aqueous solution of potassium permanganate, a salt of oxalic acid (potassium oxalate) is formed:

ALKYNES Acetylene can be oxidized with potassium permanganate in a neutral medium to potassium oxalate: 3 CH≡CH +8 KMn. O 4 → 3 KOOC–COOK +8 Mn. O 2 +2 KOH +2 H 2 O In an acidic environment, oxidation goes to oxalic acid or carbon dioxide: 5 CH≡CH +8 KMn. O 4 +12 H 2 SO 4 → 5 HOOC - COOH + 8 Mn. SO 4 +4 K 2 SO 4 +12 H 2 O CH≡CH + 2 KMn. O 4 +3 H 2 SO 4 \u003d 2 CO 2 + 2 Mn. SO 4 + 4 H 2 O + K 2 SO

ALKYNES Oxidation with potassium permanganates in an acidic medium when heated is accompanied by a break in the carbon chain at the site of the triple bond and leads to the formation of acids: Oxidation of alkynes containing a triple bond at the extreme carbon atom is accompanied under these conditions by the formation of carboxylic acid and CO 2:

ALKYNES CH 3 C≡CCH 2 CH 3 + K 2 Cr 2 O 7 + 4 H 2 SO 4 → CH 3 COOH + CH 3 CH 2 COOH + Cr 2 (SO 4) 3 + K 2 SO 4 + 3 H 2 O 3 CH 3 C≡CH+4 K 2 Cr 2 O 7 +16 H 2 SO 4 →CH 3 COOH+3 CO 2++ 4 Cr 2(SO 4)3 + 4 K 2 SO 4 +16 H 2 O CH 3C≡CH+8KMn. O 4+11 KOH →CH 3 COOK + K 2 CO 3 + 8 K 2 Mn. O 4 +6 H 2 O

Cycloalkanes and Cycloalkenes Under the action of strong oxidizing agents (KMn. O 4 , K 2 Cr 2 O 7 , etc.), cycloalkanes and cycloalkenes form dibasic carboxylic acids with the same number of carbon atoms: 5 C 6 H 12 + 8 KMn. O 4 + 12 H 2 SO 4 → 5 HOOC (CH 2) 4 COOH + 4 K 2 SO 4 + 8 Mn. SO 4 +12 H 2 O

ARENES Benzene Resistant to oxidizing agents at room temperature Does not react with aqueous solutions of potassium permanganate, potassium dichromate and other oxidizing agents Can be oxidized with ozone to form dialdehyde:

ARENES Benzene homologues Oxidize relatively easily. The side chain undergoes oxidation, in toluene - the methyl group. Mild oxidizing agents (Mn. O 2) oxidize the methyl group to the aldehyde group: C 6 H 5 CH 3+2 Mn. O 2+H 2 SO 4→C 6 H 5 CHO+2 Mn. SO 4+3 H 2 O

ARENA Stronger oxidizers - KMn. O 4 in an acidic medium or a chromium mixture, when heated, oxidizes the methyl group to a carboxyl group: In a neutral or slightly alkaline medium, not benzoic acid itself is formed, but its salt, potassium benzoate:

ARENE In acid medium 5 C 6 H 5 CH 3 +6 KMn. O 4 +9 H 2 SO 4 → 5 C 6 H 5 COOH + 6 Mn. SO 4 +3 K 2 SO 4 + 14 H 2 O In a neutral environment C 6 H 5 CH 3 +2 KMn. O 4 \u003d C 6 H 5 COOK + 2 Mn. O 2 + KOH + H 2 O In an alkaline environment C 6 H 5 CH 2 CH 3 + 4 KMn. O 4 \u003d C 6 H 5 COOK + K 2 CO 3 + 2 H 2 O + 4 Mn. O2 + KOH

ARENES Under the action of strong oxidizing agents (KMn. O 4 in an acid medium or a chromium mixture), the side chains are oxidized regardless of the structure: the carbon atom directly attached to the benzene ring to a carboxyl group, the remaining carbon atoms in the side chain to CO 2 Oxidation of any homologue benzene with one side chain under the action of KMn. O 4 in an acidic environment or a chromium mixture leads to the formation of benzoic acid:

ARENES Benzene homologues containing several side chains form the corresponding polybasic aromatic acids upon oxidation:

ARENES In a neutral or slightly alkaline medium, oxidation with potassium permanganate produces a carboxylic acid salt and potassium carbonate:

ARENA 5 C 6 H 5 -C 2 H 5 + 12 KMn. O 4 + 18 H 2 SO 4 -> 5 C 6 H 5 -COOH + 5 CO 2 + 12 Mn. SO 4 + 6 K 2 SO 4 + 28 H 2 O C 6 H 5 -C 2 H 5 +4 KMn. O 4 → C 6 H 5 -COOK + K 2 CO 3 + KOH +4 Mn. O 2 +2 H 2 O 5 C 6 H 5 -CH (CH 3) 2 + 18 KMn. O 4 + 27 H 2 SO 4 ---> 5 C 6 H 5 -COOH + 10 CO 2 + 18 Mn. SO 4 + 9 K 2 SO 4 + 42 H 2 O 5 CH 3 -C 6 H 4 -CH 3 +12 KMn. O 4 +18 H 2 SO 4 → 5 C 6 H 4 (COOH) 2 +12 Mn. SO 4 +6 K 2 SO 4 + 28 H 2 O CH 3 -C 6 H 4 -CH 3 + 4 KMn. O 4 → C 6 H 4(COOK)2 +4 Mn. O 2 +2 KOH + 2 H 2 O

STYRENE Oxidation of styrene (vinylbenzene) with a solution of potassium permanganate in an acidic and neutral medium: 3 C 6 H 5 -CH═CH 2 + 2 KMn. O 4 + 4 H 2 O → 3 C 6 H 5 -CH -CH 2 + 2 Mn. O 2 + 2 KOH ı ı OH OH Oxidation with a strong oxidizing agent—potassium permanganate in an acidic medium—results in the complete cleavage 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 KMn. O 4 + 3 H 2 SO 4 → C 6 H 5 -COOH + CO 2 + K 2 SO 4 + 2 Mn. SO 4 +4 H 2 O

ALCOHOLS The most suitable oxidizing agents for primary and secondary alcohols are: KMn. O 4 chromium mixture. Primary alcohols, except methanol, are oxidized to aldehydes or carboxylic acids:

ALCOHOLS Methanol is oxidized to CO 2: Ethanol under the action of Cl 2 is oxidized to acetaldehyde: Secondary alcohols are oxidized to ketones:

ALCOHOLS Dihydric alcohol, ethylene glycol HOCH 2 -CH 2 OH, when heated in an acidic medium with a solution of KMn. O 4 or K 2 Cr 2 O 7 is easily oxidized to oxalic acid, and in neutral to potassium oxalate. 5 CH 2 (OH) - CH 2 (OH) + 8 KMn. O 4 +12 H 2 SO 4 → 5 HOOC - COOH + 8 Mn. SO 4 +4 K 2 SO 4 +22 H 2 O 3 CH 2 (OH) - CH 2 (OH) + 8 KMn. O 4 → 3 KOOC–COOK +8 Mn. O 2 +2 KOH +8 H 2 O

PHENOLS They are easily oxidized due to the presence of a hydroxo group connected to the benzene ring. Phenol is oxidized by hydrogen peroxide in the presence of a catalyst to diatomic phenol pyrocatechol, and when oxidized with a chromium mixture, to para-benzoquinone:

ALDEHYDES AND KETONES Aldehydes are easily oxidized, while the aldehyde group is oxidized to a carboxyl group: 3 CH 3 CHO + 2 KMn. O 4 + 3 H 2 O → 2 CH 3 COOK + CH 3 COOH + 2 Mn. O 2 + H 2 O 3 CH 3 CH \u003d O + K 2 Cr 2 O 7 + 4 H 2 SO 4 \u003d 3 CH 3 COOH + Cr 2 (SO 4) 3 + 7 H 2 O Methanal is oxidized to CO 2:

ALDEHYDES AND KETONES Qualitative reactions to aldehydes: oxidation with copper (II) hydroxide "silver mirror" reaction Salt, not acid!

ALDEHYDES AND KETONES Ketones are oxidized with difficulty, weak oxidizing agents do not act on them. Under the action of strong oxidizing agents, C-C bonds are broken on both sides of the carbonyl group to form a mixture of acids (or ketones) with a smaller number of carbon atoms than in the original compound:

ALDEHYDES AND KETONES In the case of an asymmetric ketone structure, oxidation is predominantly carried out from the side of the less hydrogenated carbon atom at the carbonyl group (Popov-Wagner rule). Based on the oxidation products of the ketone, its structure can be established:

FORMIC ACID Among the saturated monobasic acids, only formic acid is easily oxidized. This is due to the fact that in formic acid, in addition to the carboxyl group, an aldehyde group can also be isolated. 5 NUN + 2 KMn. O 4 + 3 H 2 SO 4 → 2 Mn. SO 4 + K 2 SO 4 + 5 CO 2 + 8 H 2 O Formic acid reacts with an ammonia solution of silver oxide and copper (II) hydroxide HCOOH + 2OH → 2 Ag + (NH 4) 2 CO 3 + 2 NH 3 + H 2 O HCOOH + 2 Cu(OH) 2 CO 2 + Cu 2 O↓+ 3 H 2 O In addition, formic acid is oxidized by chlorine: HCOOH + Cl 2 → CO 2 + 2 HCl

UNSATURATED CARBOXIC ACIDS Easily oxidized with an aqueous solution of KMn. O 4 in a weakly alkaline medium with the formation of dihydroxy acids and their salts: In an acidic medium, the carbon skeleton breaks at the site of the C=C double bond with the formation of a mixture of acids:

OXALIC ACID Easily oxidized by KMn. O 4 in an acidic environment when heated to CO 2 (permanganatometry method): When heated, it undergoes decarboxylation (disproportionation reaction): In the presence of concentrated H 2 SO 4, when heated, oxalic acid and its salts (oxalates) disproportionate:

We write down the reaction equations: 1) CH 3 CH 2 CH 2 CH 3 2) 3) 4) 5) 16.32% (36.68%, 23.82%) Pt, to X 3 X 2 Pt, to. KMn. O 4 KOH X 4 heptane KOH, to benzene. X 1 Fe, HCl. HNO 3 H 2 SO 4 CH 3 + 4 H 2 CH 3 + 6 KMn. O 4 + 7 KOHCOOK + 6 K 2 Mn. O 4 + 5 H 2 O COOK + KOH+ K 2 CO 3 to NO 2 + H 2 O+ HNO 3 H 2 SO 4 N H 3 C l + 3 F e C l 2 + 2 H 2 ON O 2 + 3 F e + 7 H C l

18. Redox reactions (continued 2)

18.9. OVR involving organic substances

In OVR organic substances with inorganic organic substances are most often reducing agents. So, when organic matter burns in excess of oxygen, carbon dioxide and water are always formed. Reactions are more difficult when less active oxidizing agents are used. In this section, only the reactions of representatives of the most important classes of organic substances with some inorganic oxidizing agents are considered.

Alkenes. 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:

C 2 H 4 + 2KMnO 4 + 2H 2 O CH 2 OH–CH 2 OH + 2MnO 2 + 2KOH (cooling)

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 (heating)

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 (heating)

3) CH 3 CH \u003d CHCH 2 CH 3 + 6KMnO 4 + 10KOH CH 3 COOK + C 2 H 5 COOK + 6H 2 O + 6K 2 MnO 4 (heating)

4) CH 3 CH \u003d CH 2 + 10KMnO 4 + 13KOH CH 3 COOK + K 2 CO 3 + 8H 2 O + 10K 2 MnO 4 (heating)

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

Alkynes. Alkynes begin to 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 alkanes, the reducing atoms here are carbon atoms linked in this case by a triple 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 (heating)

Sometimes it is possible to isolate intermediate oxidation products. Depending on the position of the triple bond in the molecule, these are either diketones (R 1 –CO–CO–R 2) or aldoketones (R–CO–CHO).

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

3C 2 H 2 + 8KMnO 4 \u003d 3K 2 C 2 O 4 + 2H 2 O + 8MnO 2 + 2KOH

In an acidic environment, oxidation goes to carbon dioxide:

C 2 H 2 + 2KMnO 4 + 3H 2 SO 4 \u003d 2CO 2 + 2MnSO 4 + 4H 2 O + K 2 SO 4

Benzene homologues. 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 \u003d C 6 H 5 COOK + 2MnO 2 + KOH + H 2 O (at boiling)

C 6 H 5 CH 2 CH 3 + 4KMnO 4 \u003d C 6 H 5 COOK + K 2 CO 3 + 2H 2 O + 4MnO 2 + KOH (when heated)

Oxidation of these substances with dichromate or potassium permanganate in an acidic environment leads to the formation of benzoic acid.

Alcohols. 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 \u003d 3CH 3 CHO + K 2 SO 4 + Cr 2 (SO 4) 3 + 7H 2 O (heating)

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. Tertiary alcohols are not oxidized under these conditions, but methyl alcohol is oxidized to carbon dioxide. All reactions take place when heated.

Dihydric alcohol, ethylene glycol HOCH 2 -CH 2 OH, when heated in an acid medium with a solution of KMnO 4 or K 2 Cr 2 O 7, is easily oxidized to carbon dioxide and water, but sometimes it is possible to isolate intermediate products (HOCH 2 -COOH, HOOC- COOH, etc.).

Aldehydes. 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. All reactions take place when heated:

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

Formaldehyde with an excess of oxidizing agent is oxidized to carbon dioxide.

18.10. Comparison of the redox activity of various substances

From the definitions of the concepts "oxidizing atom" and "reducing atom" it follows that atoms in the highest oxidation state have only oxidizing properties. On the contrary, only reducing properties are possessed by atoms in the lowest oxidation state. Atoms in intermediate oxidation states can be both oxidizing and reducing agents.

However, based only on the degree of oxidation, it is impossible to unambiguously assess the redox properties of substances. As an example, consider the connections of the elements of the VA group. Nitrogen(V) and antimony(V) compounds are more or less strong oxidizers, bismuth(V) compounds are very strong oxidizers, and phosphorus(V) compounds have practically no oxidizing properties. In this and other similar cases, it matters how much a given oxidation state is characteristic of a given element, that is, how stable compounds containing atoms of a given element in this oxidation state are.

Any OVR proceeds in the direction of the formation of a weaker oxidizing agent and a weaker reducing agent. In the general case, the possibility of any OVR, as well as any other reaction, can be determined by the sign of the change in the Gibbs energy. In addition, to quantify the redox activity of substances, the electrochemical characteristics of oxidizing agents and reducing agents (standard potentials of redox pairs) are used. Based on these quantitative characteristics, it is possible to build series of redox activity of various substances. The series of metal stresses known to you is built in this way. This series makes it possible to compare the reduction properties of metals in aqueous solutions under standard conditions ( With= 1 mol/l, T= 298.15 K), as well as the oxidizing properties of simple aquacations. If ions (oxidizing agents) are placed in the top line of this series, and metal atoms (reducing agents) are placed in the bottom line, then the left side of this series (up to hydrogen) will look like this:

In this series, the oxidizing properties of ions (top line) increase from left to right, while the reducing properties of metals (bottom line), on the contrary, increase from right to left.

Taking into account the differences in redox activity in different media, it is possible to construct similar series for oxidizing agents. So, for reactions in an acidic medium (pH = 0), a "continuation" of a series of metal activity is obtained in the direction of enhancing the oxidizing properties

As in the activity series of metals, in this series the oxidizing properties of oxidizing agents (top row) increase from left to right. But, using this series, it is possible to compare the reducing activity of reducing agents (bottom line) only if their oxidized form coincides with that given in the top line; in this case it amplifies from right to left.

Let's look at a few examples. To find out if this redox is possible, we will use the general rule that determines the direction of the redox reactions (reactions proceed in the direction of the formation of a weaker oxidizing agent and a weaker reducing agent).

1. Can magnesium reduce cobalt from a CoSO 4 solution?
Magnesium is a stronger reducing agent than cobalt, and Co 2 ions are stronger oxidizing agents than Mg 2 ions, therefore, it is possible.
2. Can a solution of FeCl 3 oxidize copper to CuCl 2 in an acidic environment?
Since Fe 3B ions are stronger oxidizing agents than Cu 2 ions, and copper is a stronger reducing agent than Fe 2 ions, it is possible.
3. Is it possible, by blowing oxygen through a solution of FeCl 2 acidified with hydrochloric acid, to obtain a solution of FeCl 3?
It would seem not, since in our series oxygen is to the left of Fe 3 ions and is a weaker oxidizing agent than these ions. But in an aqueous solution, oxygen is almost never reduced to H 2 O 2, in this case it is reduced to H 2 O and occupies a place between Br 2 and MnO 2. Therefore, such a reaction is possible, however, it proceeds rather slowly (why?).
4. Is it possible to oxidize H 2 O 2 in an acidic environment with potassium permanganate?
In this case, H 2 O 2 is a reducing agent and a reducing agent is stronger than Mn 2B ions, and MnO 4 ions are oxidizing agents stronger than oxygen formed from peroxide. Therefore, it is possible.

A similar series constructed for OVR in an alkaline medium looks like this:

Unlike the "acid" series, this series cannot be used in conjunction with the metal activity series.

Electron-ion balance method (half-reaction method), intermolecular OVR, intramolecular OVR, OVR dismutation (disproportionation, self-oxidation-self-healing), OVR switching, passivation.

1. Using the method of electron-ion balance, make up the equations of the reactions that occur when a solution of a) H 2 S (S, more precisely, S 8 ) is added to a solution of potassium permanganate acidified with sulfuric acid; b) KHS; c) K 2 S; d) H 2 SO 3; e) KHSO 3 ; e) K 2 SO 3 ; g) HNO 2 ; g) KNO 2 ; i) KI (I 2 ); j) FeSO 4 ; k) C 2 H 5 OH (CH 3 COOH); l) CH 3 CHO; m) (COOH) 2 (CO 2 ); n) K 2 C 2 O 4 . Here and below, where necessary, oxidation products are indicated in curly brackets.
2. Make up the equations of the reactions that occur when the following gases are passed through a solution of potassium permanganate acidified with sulfuric acid: a) C 2 H 2 (CO 2 ); b) C 2 H 4 (CO 2 ); c) C 3 H 4 (propyne) (CO 2 and CH 3 COOH); d) C 3 H 6 ; e) CH 4 ; e) HCHO.
3. The same, but the reducing agent solution is added to the neutral potassium permanganate solution: a) KHS; b) K 2 S; c) KHSO 3 ; d) K 2 SO 3; e) KNO 2 ; e) KI.
4. The same, but potassium hydroxide solution was previously added to the potassium permanganate solution: a) K 2 S (K 2 SO 4 ); b) K 2 SO 3; c) KNO 2 ; d) KI (KIO 3 ).
5. Make equations for the following reactions occurring in solution: a) KMnO 4 + H 2 S ...;
   b) KMnO 4 + HCl ...;
   c) KMnO 4 + HBr ...;
   d) KMnO 4 + HI ...
6. Write the following OVR equations for manganese dioxide:
7. Solutions of the following substances are added to a solution of potassium dichromate acidified with sulfuric acid: a) KHS; b) K 2 S; c) HNO 2 ; d) KNO 2 ; e) KI; e) FeSO 4 ; g) CH 3 CH 2 CHO; i) H 2 SO 3 ; j) KHSO 3 ; k) K 2 SO 3. Write the equations for the ongoing reactions.
8. The same, but the following gases are passed through the solution: a) H 2 S; b) SO2.
9. Solutions of a) K 2 S (K 2 SO 4 ) are added to a potassium chromate solution containing potassium hydroxide; b) K 2 SO 3; c) KNO 2 ; d) KI (KIO 3 ). Write the equations for the ongoing reactions.
10. A solution of potassium hydroxide was added to a solution of chromium(III) chloride until the initially formed precipitate was dissolved, and then bromine water was added. Write the equations for the ongoing reactions.
11. The same, but at the last stage, a solution of potassium persulfate K 2 S 2 O 8 was added, which was reduced during the reaction to sulfate.
12. Write the equations for the reactions taking place in the solution:

a) CrCl 2 + FeCl 3; b) CrSO 4 + FeCl 3; c) CrSO 4 + H 2 SO 4 + O 2;

d) CrSO 4 + H 2 SO 4 + MnO 2; e) CrSO 4 + H 2 SO 4 + KMnO 4.

13. Write equations for the reactions that take place between solid chromium trioxide and the following substances: a) C; b) CO; c) S (SO 2 ); d) H 2 S; e) NH 3 ; e) C 2 H 5 OH (CO 2 and H 2 O); g) CH 3 COCH 3 .
14. Make up the equations of the reactions that occur when the following substances are added to concentrated nitric acid: a) S (H 2 SO 4 ); b) P 4 ((HPO 3) 4 ); c) graphite; d) Se; e) I 2 (HIO 3 ); f) Ag; g) Cu; i) Pb; j) KF; k) FeO; l) FeS; m) MgO; o) MgS; p) Fe(OH) 2 ; c) P 2 O 3 ; m) As 2 O 3 (H 3 AsO 4 ); y) As 2 S 3; f) Fe(NO 3) 2; x) P 4 O 10 ; c) Cu 2 S.
15. The same, but with the passage of the following gases: a) CO; b) H 2 S; c) N 2 O; d) NH3; e) NO; e) H 2 Se; g) HI.
16. The reactions will proceed in the same way or differently in the following cases: a) a piece of magnesium was placed in a high test tube two-thirds filled with concentrated nitric acid; b) a drop of concentrated nitric acid was placed on the surface of a magnesium plate? Write reaction equations.
17. What is the difference between the reaction of concentrated nitric acid with hydrosulfide acid and with gaseous hydrogen sulfide? Write reaction equations.
18. Will OVR proceed in the same way when anhydrous crystalline sodium sulfide and its 0.1 M solution are added to a concentrated solution of nitric acid?
19. A mixture of the following substances was treated with concentrated nitric acid: Cu, Fe, Zn, Si and Cr. Write the equations for the ongoing reactions.
20. Make up the equations of the reactions that occur when the following substances are added to dilute nitric acid: a) I 2 ; b) Mg; c) Al; d) Fe; e) FeO; f) FeS; g) Fe (OH) 2; i) Fe(OH) 3 ; j) MnS; k) Cu 2 S; l) CuS; m) CuO; n) Na 2 S cr; p) Na 2 S p; c) P 4 O 10 .
21. What processes will take place when a) ammonia, b) hydrogen sulfide, c) carbon dioxide is passed through a dilute solution of nitric acid?
22. Make up the equations of the reactions that occur when the following substances are added to concentrated sulfuric acid: a) Ag; b) Cu; c) graphite; d) HCOOH; e) C 6 H 12 O 6; f) NaCl cr; g) C 2 H 5 OH.
23. When hydrogen sulfide is passed through cold concentrated sulfuric acid, S and SO 2 are formed, hot concentrated H 2 SO 4 oxidizes sulfur to SO 2. Write reaction equations. How will the reaction proceed between hot concentrated H 2 SO 4 and hydrogen sulfide?
24. Why is hydrogen chloride obtained by treating crystalline sodium chloride with concentrated sulfuric acid, while hydrogen bromide and hydrogen iodine are not obtained in this way?
25. Make up the equations of the reactions that take place during the interaction of dilute sulfuric acid with a) Zn, b) Al, c) Fe, d) chromium in the absence of oxygen, e) chromium in air.
26. Make up the reaction equations that characterize the redox properties of hydrogen peroxide:

In which of these reactions is hydrogen peroxide an oxidizing agent and in which is it a reducing agent?

27. What reactions occur when the following substances are heated: a) (NH 4) 2 CrO 4; b) NaNO 3 ; c) CaCO 3 ; d) Al(NO 3) 3; e) Pb(NO 3) 3 ; f) AgNO 3 ; g) Hg (NO 3) 2; i) Cu(NO 3) 2 ; j) CuO; l) NaClO 4 ; l) Ca(ClO 4) 2; m) Fe(NO 3) 2; n) PCl 5 ; p) MnCl 4 ; c) H 2 C 2 O 4 ; m) LiNO 3 ; s) HgO; f) Ca(NO 3) 2; x) Fe(OH) 3 ; c) CuCl 2 ; h) KClO 3 ; w) KClO 2 ; w) CrO 3 ?
28. When hot solutions of ammonium chloride and potassium nitrate are drained, a reaction occurs, accompanied by gas evolution. Write an equation for this reaction.
29. Make up the equations of the reactions that occur when a) chlorine is passed through a cold solution of sodium hydroxide, b) bromine vapor. The same, but through a hot solution.
30. When interacting with a hot concentrated solution of potassium hydroxide, selenium undergoes dismutation to the nearest stable oxidation states (–II and +IV). Write an equation for this OVR.
31. Under the same conditions, sulfur undergoes a similar dismutation, but the excess sulfur reacts with sulfite ions to form thiosulfate ions S 2 O 3 2 . Write the equations for the ongoing reactions. ;
32. Make up the equations for the electrolysis reactions of a) a copper nitrate solution with a silver anode, b) a lead nitrate solution with a copper anode.

Experience 1. Oxidizing properties of potassium permanganate in an acidic environment. To 3-4 drops of a solution of potassium permanganate add an equal volume of a dilute solution of sulfuric acid, and then a solution of sodium sulfite until discoloration. Write an equation for the reaction. Experience 2.Oxidizing properties of potassium permanganate in a neutral medium. Add 5-6 drops of sodium sulfite solution to 3-4 drops of potassium permanganate solution. What substance was isolated in the form of a precipitate? Experience 3. Oxidizing properties of potassium permanganate in an alkaline medium. Add 10 drops of concentrated sodium hydroxide solution and 2 drops of sodium sulfite solution to 3-4 drops of potassium permanganate solution. The solution should turn green. Experience 4. Oxidizing properties of potassium dichromate in an acidic environment. Acidify 6 drops of potassium dichromate solution with 4 drops of dilute sulfuric acid solution and add sodium sulfite solution until the color of the mixture changes. Experience 5. Oxidizing properties of dilute sulfuric acid. Place a zinc granule in one test tube, and a piece of copper tape in the other. Add 8-10 drops of dilute sulfuric acid solution to both tubes. Compare what is happening. EXPERIENCE IN A FAN CABINET! Experience 6. Oxidizing properties of concentrated sulfuric acid. Similar to experiment 5, but add concentrated sulfuric acid solution. A minute after the start of the release of gaseous reaction products, insert strips of filter paper moistened with solutions of potassium permanganate and copper sulfate into the test tubes. Explain what is happening. EXPERIENCE IN A FAN CABINET! Experience 7. Oxidizing properties of dilute nitric acid. Similar to experiment 5, but add a dilute nitric acid solution. Observe the color change of the gaseous reaction products. EXPERIENCE IN A FAN CABINET! Experience 8. Oxidizing properties of concentrated nitric acid. Place a piece of copper tape in a test tube and add 10 drops of concentrated nitric acid solution. Gently heat until the metal is completely dissolved. EXPERIENCE IN A FAN CABINET! Experience 9. Oxidizing properties of potassium nitrite. To 5-6 drops of a solution of potassium nitrite add an equal volume of a dilute solution of sulfuric acid and 5 drops of a solution of potassium iodide. What substances are formed? Experience 10. Reducing properties of potassium nitrite. To 5-6 drops of a solution of potassium permanganate add an equal volume of a dilute solution of sulfuric acid and a solution of potassium nitrite until the mixture is completely discolored. Experience 11.Thermal decomposition of copper nitrate. Place one microspatula of copper nitrate trihydrate in a test tube, fix it in a rack and gently heat it with an open flame. Observe dehydration and subsequent salt decomposition. EXPERIENCE IN A FAN CABINET! Experience 12.Thermal decomposition of lead nitrate. Carry out similarly to experiment 11, placing lead nitrate in a test tube. EXPERIENCE IN A FAN CABINET! What is the difference between the processes occurring during the decomposition of these salts?

Drawing up equations of redox reactions involving organic substances

IN In connection with the introduction of the Unified State Examination (USE) as the only form of final certification of secondary school graduates and the transition of high school to specialized education, the preparation of high school students for the most “expensive” tasks in terms of points of part “C” of the USE test in chemistry is becoming increasingly important. Despite the fact that the five tasks of part “C” are considered different: the chemical properties of inorganic substances, the chains of transformations of organic compounds, computational tasks, all of them are to some extent related to redox reactions (ORRs). If the basic knowledge of the OVR theory is mastered, then it is possible to correctly complete the first and second tasks in full, and the third - partially. In our opinion, a significant part of the success in the implementation of part "C" lies precisely in this. Experience shows that if, studying inorganic chemistry, students cope well enough with the tasks of writing OVR equations, then similar tasks in organic chemistry cause great difficulties for them. Therefore, throughout the study of the entire course of organic chemistry in specialized classes, we try to develop in high school students the skills of compiling OVR equations.

When studying the comparative characteristics of inorganic and organic compounds, we introduce students to 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 a molecule of organic matter;

2) definition of s.d. every carbon atom.

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

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P When studying the topic “Alkanes”, we show that the processes of oxidation, combustion, halogenation, nitration, dehydrogenation, and decomposition are redox processes. 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:

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

In other cases, when studying the topic “Alkanes”, we determine the values ​​of s.d. each carbon atom in the compound, while drawing students' attention to the sequence of substitution of hydrogen atoms at primary, secondary, tertiary carbon atoms:

Thus, we bring students to the conclusion that at the beginning the process of substitution occurs at the tertiary, then at the secondary, and, last of all, at the primary carbon atoms.

P When studying the topic “Alkenes”, we consider oxidation processes depending on the structure of the alkene and the reaction medium.

When alkenes are oxidized with a concentrated solution of potassium permanganate KMnO 4 in an acidic medium (hard oxidation), - and - 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.

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:

We emphasize that 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 a ketone is formed, since the transformation of such an atom into an atom of the carboxyl group is impossible without breaking C–C bond, relatively stable under these conditions:

We clarify that 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:

We report that 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:

Considering the oxidation of alkenes in neutral or slightly alkaline media, we focus the attention of high school students on the fact that under such conditions, 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:

IN In a similar way, we consider the oxidation of acetylene and its homologues, depending on the medium in which the process takes place. So, we clarify that 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:

IN All rules are worked out with students on specific examples, which leads to a better assimilation of theoretical material. Therefore, when studying the oxidation of arenes in various media, students can independently make assumptions that in an acidic medium one should expect the formation of acids, and in an alkaline medium, salts. The teacher will only have to clarify which reaction products are formed depending on the structure of the corresponding arena.

We show by examples that benzene homologues with one side chain (regardless of its length) are oxidized by a strong oxidizing agent to benzoic acid at the -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:

P The acquired skills in compiling OVR equations for hydrocarbons allow them to be used in the study of the “Oxygen-containing compounds” section.

So, when studying the topic “Alcohols”, students independently compose the equations for the oxidation of alcohols, using the following rules:

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.

In order to prepare for the exam, it is advisable for the teacher to give additional information to these properties, which will undoubtedly be useful for students.

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,

3CH 3 -CH 2 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.

Let us remind students again about the influence of the environment on the products of alcohol oxidation reactions, namely: a hot neutral solution of KMnO 4 oxidizes methanol to potassium carbonate, and the remaining alcohols to salts of the corresponding carboxylic acids:

When studying the topic “Aldehydes and ketones”, we focus students' attention on the fact that aldehydes are more easily oxidized than alcohols into 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 and under the influence of weak (ammonia solution of silver oxide or copper (II) hydroxide):

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.

We pay special attention to the oxidation of methanal with an ammonia solution of silver oxide, since in this case, ammonium carbonate is formed, and not formic acid:

HCHO + 4OH \u003d (NH 4) 2 CO 3 + 4Ag + 6NH 3 + 2H 2 O.

As our long-term experience shows, the proposed method of teaching high school students how to write OVR equations with the participation of organic substances increases their final USE result in chemistry by several points.

Select the main carbon chain in the molecule. First, it must be the longest. Secondly, if there are two or more chains of the same length, then the most branched one is selected from them. For example, in a molecule there are 2 chains with the same number (7) of C atoms (highlighted in color):

In case (a), the chain has 1 substituent, and in case (b), it has 2. Therefore, option (b) should be chosen.

1. Number the carbon atoms in the main chain so that the C atoms associated with the substituents receive the lowest possible numbers. Therefore, the numbering starts from the end of the chain closest to the branch. For example:

Name all radicals (substituents), indicating in front the numbers indicating their location in the main chain. If there are several identical substituents, then for each of them a number (location) is written separated by a comma, and their number is indicated by prefixes di-, three-, tetra-, penta- etc. (For example, 2,2-dimethyl or 2,3,3,5-tetramethyl).

The names of all substituents are arranged in alphabetical order (as established by the latest IUPAC rules).

Name the main chain of carbon atoms, i.e. the corresponding normal alkane.

Thus, in the name of a branched alkane, the root + suffix is ​​the name of a normal alkane (Greek numeral + suffix "an"), prefixes are numbers and names of hydrocarbon radicals. Name construction example:

Chem. St. alkanesCracking of alkanes. Cracking is a process of thermal decomposition of hydrocarbons, which is based on the reactions of splitting the carbon chain of large molecules with the formation of compounds with a shorter chain. Isomerization of alkanes Alkanes of normal structure under the influence of catalysts and when heated are able to turn into branched alkanes without changing the composition of the molecules, i.e. enter into isomerization reactions. These reactions involve alkanes whose molecules contain at least 4 carbon atoms. For example, the isomerization of n-pentane to isopentane (2-methylbutane) occurs at 100°C in the presence of an aluminum chloride catalyst:

The starting material and the product of the isomerization reaction have the same molecular formulas and are structural isomers (carbon skeleton isomerism).

Dehydrogenation of alkanes

When alkanes are heated in the presence of catalysts (Pt, Pd, Ni, Fe, Cr 2 O 3 , Fe 2 O 3 , ZnO), their catalytic dehydrogenation– splitting off of hydrogen atoms due to the breaking of C-H bonds.

The structure of the dehydrogenation products depends on the reaction conditions and the length of the main chain in the starting alkane molecule.

1. Lower alkanes containing from 2 to 4 carbon atoms in the chain, when heated over a Ni-catalyst, split off hydrogen from neighboring carbon atoms and turn into alkenes:

Along with butene-2 this reaction produces butene-1 CH 2 \u003d CH-CH 2 -CH 3. In the presence of a Cr 2 O 3 /Al 2 O 3 catalyst at 450-650 С from n-butane is also received butadiene-1,3 CH 2 =CH-CH=CH 2 .

2. Alkanes containing more than 4 carbon atoms in the main chain are used to obtain cyclical connections. At the same time, it happens dehydrocyclization- dehydrogenation reaction, which leads to the closure of the chain into a stable cycle.

If the main chain of an alkane molecule contains 5 (but not more) carbon atoms ( n-pentane and its alkyl derivatives), then when heated over a Pt catalyst, hydrogen atoms are split off from the terminal atoms of the carbon chain, and a five-membered cycle is formed (cyclopentane or its derivatives):

Alkanes with a main chain of 6 or more carbon atoms also enter into the dehydrocyclization reaction, but always form a 6-membered cycle (cyclohexane and its derivatives). Under the reaction conditions, this cycle undergoes further dehydrogenation and turns into an energetically more stable benzene cycle of an aromatic hydrocarbon (arene). For example:

These reactions underlie the process reforming– processing of petroleum products in order to obtain arenes ( aromatization saturated hydrocarbons) and hydrogen. transformation n- alkanes in arenas leads to improved knock resistance of gasoline.