In redox reactions, organic substances more often they exhibit the properties of reducing agents, and themselves are oxidized. The ease of oxidation of organic compounds depends on the availability of electrons when interacting with the oxidizing agent. All known factors that cause an increase in electron density in molecules of organic compounds (for example, positive inductive and mesomeric effects) will increase their ability to oxidize and vice versa.
The tendency of organic compounds to oxidize increases with their nucleophilicity, which corresponds to the following rows:
Increase in nucleophilicity in the series
Let's consider redox reactions representatives of the most important classes organic matter with some inorganic oxidizing agents.
Oxidation of alkenes
During 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 occurs 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 along double bond and the formation of two acids (in a strongly alkaline environment - two salts) or acid and carbon dioxide (in a strongly alkaline environment - 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=CH 2 + 10KMnO 4 + 13KOH → CH 3 COOK + K 2 CO 3 + 8H 2 O + 10K 2 MnO 4
Potassium dichromate in a sulfuric acid medium oxidizes alkenes similarly to reactions 1 and 2.
During the oxidation of alkenes, in which the carbon atoms at the double bond contain two carbon radicals, two ketones are formed:
Alkyne oxidation
Alkynes oxidize under slightly more severe conditions than alkenes, so they usually oxidize by breaking the carbon chain at the triple bond. As in the case of alkenes, the reducing atoms here are carbon atoms connected by a multiple bond. As a result of the reactions, acids and carbon dioxide are formed. Oxidation can be carried out with potassium permanganate or 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 environment to potassium oxalate:
3CH≡CH +8KMnO 4 → 3KOOC –COOK +8MnO 2 +2KOH +2H 2 O
In an acidic environment, oxidation proceeds to oxalic acid or carbon dioxide:
5CH≡CH +8KMnO 4 +12H 2 SO 4 → 5HOOC –COOH +8MnSO 4 +4K 2 SO 4 +12H 2 O
CH≡CH + 2KMnO 4 +3H 2 SO 4 → 2CO 2 + 2MnSO 4 + 4H 2 O + K 2 SO 4
Oxidation of benzene homologues
Benzene does not oxidize even under fairly harsh conditions. Benzene homologues can be oxidized with a solution of potassium permanganate in a neutral environment to potassium benzoate:
C 6 H 5 CH 3 + 2KMnO 4 → C 6 H 5 COOK + 2MnO 2 + KOH + H 2 O
C 6 H 5 CH 2 CH 3 + 4KMnO 4 → C 6 H 5 COOK + K 2 CO 3 + 2H 2 O + 4MnO 2 + KOH
Oxidation of benzene homologues with potassium dichromate or permanganate in an acidic environment leads to the formation of benzoic acid.
5C 6 H 5 CH 3 +6KMnO 4 +9 H 2 SO 4 → 5C 6 H 5 COOH+6MnSO 4 +3K 2 SO 4 + 14H 2 O
5C 6 H 5 –C 2 H 5 + 12KMnO 4 + 18H 2 SO 4 → 5C 6 H 5 COOH + 5CO 2 + 12MnSO 4 + 6K 2 SO 4 + 28H 2 O
Oxidation of alcohols
The direct oxidation product of primary alcohols is aldehydes, and the oxidation products of secondary alcohols are ketones.
Aldehydes formed during the oxidation of alcohols are easily oxidized to acids, therefore aldehydes from primary alcohols are obtained by oxidation with potassium dichromate in an acidic medium at the boiling point of the aldehyde. When aldehydes evaporate, they 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 oxidizing agent (KMnO 4, K 2 Cr 2 O 7) in any environment, primary alcohols are oxidized to carboxylic acids or their salts, and secondary ones - 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 do not oxidize 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 environment with a solution of KMnO 4 or K 2 Cr 2 O 7, is easily oxidized to oxalic acid, and in a neutral environment to potassium oxalate.
5CH 2 (OH) – CH 2 (OH) + 8КMnO 4 +12H 2 SO 4 → 5HOOC –COOH +8MnSO 4 +4К 2 SO 4 +22Н 2 О
3CH 2 (OH) – CH 2 (OH) + 8KMnO 4 → 3KOOC –COOK +8MnO 2 +2KOH +8H 2 O
Oxidation of aldehydes and ketones
Aldehydes are quite strong reducing agents, and therefore are easily oxidized by various oxidizing agents, for example: KMnO 4, K 2 Cr 2 O 7, OH, Cu(OH) 2. All reactions occur when heated:
3CH 3 CHO + 2KMnO 4 → CH 3 COOH + 2CH 3 COOK + 2MnO 2 + H 2 O
3CH 3 CHO + K 2 Cr 2 O 7 + 4H 2 SO 4 → 3CH 3 COOH + Cr 2 (SO 4) 3 + 7H 2 O
CH 3 CHO + 2KMnO 4 + 3KOH → CH 3 COOK + 2K 2 MnO 4 + 2H 2 O
5CH 3 CHO + 2KMnO 4 + 3H 2 SO 4 → 5CH 3 COOH + 2MnSO 4 + K 2 SO 4 + 3H 2 O
CH 3 CHO + Br 2 + 3NaOH → CH 3 COONa + 2NaBr + 2H 2 O
"silver mirror" reaction
With an ammonia solution of silver oxide, aldehydes are oxidized to carboxylic acids, which in an ammonia solution give ammonium salts (the “silver mirror” reaction):
CH 3 CH=O + 2OH → CH 3 COONH 4 + 2Ag + H 2 O + 3NH 3
CH 3 –CH=O + 2Cu(OH) 2 → CH 3 COOH + Cu 2 O + 2H 2 O
Formic aldehyde (formaldehyde) is usually oxidized to carbon dioxide:
5HCOH + 4KMnO4 (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 harsh conditions by strong oxidizing agents with rupture C-C connections and give a mixture of acids:
Carboxylic acids. Among the acids, formic and oxalic acids have strong reducing properties, which oxidize to carbon dioxide.
HCOOH + HgCl 2 =CO 2 + Hg + 2HCl
HCOOH+ Cl 2 = CO 2 +2HCl
HOOC-COOH+ Cl 2 =2CO 2 +2HCl
Formic acid, in addition to acidic properties, also exhibits some properties of aldehydes, in particular, reducing properties. At the same time, it is oxidized to carbon dioxide. For example:
2KMnO4 + 5HCOOH + 3H2SO4 → K2SO4 + 2MnSO4 + 5CO2 + 8H2O
When heated with strong dewatering agents (H2SO4 (conc.) or P4O10) it decomposes:
HCOOH →(t)CO + H2O
Catalytic oxidation of alkanes:
Catalytic oxidation of alkenes:
Oxidation of phenols:
C 6 H 5 -CHO + O 2 ® C 6 H 5 -CO-O-OH
The resulting perbenzoic acid oxidizes the second molecule of benzoaldehyde to benzoic acid:
C 6 H 5 -CHO + C 6 H 5 -CO-O-OH ® 2C 6 H 5 -COOH
Experiment No. 34. Oxidation of benzoaldehyde with potassium permanganate
Reagents:
Benzoaldehyde
Potassium permanganate solution
Ethanol
Work progress:
~3 drops of benzaldehyde are placed in a test tube, ~2 ml of potassium permanganate solution is added and heated in a water bath with shaking until the odor of aldehyde disappears. If the solution does not become discolored, then the color is destroyed with a few drops of alcohol. The solution is cooled. Benzoic acid crystals fall out:
C 6 H 5 -CHO + [O] ® C 6 H 5 -COOH
Experiment No. 35. Oxidation-reduction reaction of benzaldehyde (Cannizzaro reaction)
Reagents:
Benzoaldehyde
Alcohol solution of potassium hydroxide
Work progress:
Add ~5 ml of a 10% alcohol solution of potassium hydroxide to ~1 ml of benzoaldehyde in a test tube and shake vigorously. This generates heat and solidifies the liquid.
The redox reaction of benzoaldehyde in the presence of alkali proceeds according to the following scheme:
2C 6 H 5 -CHO + KOH ® C 6 H 5 -COOK + C 6 H 5 -CH 2 -OH
The potassium salt of benzoic acid (the oxidation product of benzoaldehyde) and benzyl alcohol (the reduction product of benzoaldehyde) are formed.
The resulting crystals are filtered and dissolved in a minimal amount of water. When adding ~1 ml of a 10% solution to the solution hydrochloric acid free benzoic acid precipitates:
C 6 H 5 -COOK + HCl ® C 6 H 5 -COOH¯ + KCl
Benzyl alcohol is in the solution remaining after separating the crystals of the potassium salt of benzoic acid (the solution has the smell of benzyl alcohol).
VII. CARBOXYLIC ACIDS AND THEIR DERIVATIVES
Experiment No. 36. Oxidation of formic acid
Reagents:
Formic acid
10% sulfuric acid solution
Potassium permanganate solution
Barite or lime water
Work progress:
~0.5-1 ml of formic acid, ~1 ml of a 10% sulfuric acid solution and ~4-5 ml of potassium permanganate solution are poured into a test tube with a gas outlet tube. The gas outlet tube is immersed in a test tube with a solution of lime or barite water. The reaction mixture is carefully heated by placing boiling stones in the test tube to ensure uniform boiling. The solution first turns brown, then becomes discolored, and carbon dioxide is released:
5H-COOH + 2KMnO4 + 3H2SO4 ® 5HO-CO-OH + K2SO4 + 2MnSO4 + 3H2O
HO-CO-OH ® CO 2 + H 2 O
Experiment No. 37. Reduction of an ammonia solution of silver hydroxide with formic acid
Reagents:
Ammonia solution of silver hydroxide (Tollens reagent)
Formic acid
This substance can be considered not only as an acid, but also as an aldehyde. The aldehyde group is outlined in brown.
Therefore, formic acid exhibits reducing properties typical of aldehydes:
1. Silver mirror reaction:
2Ag (NH3)2OH ® NH4HCO3 + 3NH3 + 2Ag + H2O.
2. Reaction with copper hydroxide when heated:
HCOONa + 2Cu (OH)2 + NaOH ® Na2CO3 + Cu2O¯ + 3H2O.
3. Oxidation with chlorine to carbon dioxide:
HCOOH + Cl2 ® CO2 + 2HCl.
Concentrated sulfuric acid takes water from formic acid. This produces carbon monoxide:
The acetic acid molecule contains a methyl group, the remainder of a saturated hydrocarbon - methane.
That's why acetic acid(and other saturated acids) will undergo radical substitution reactions characteristic of alkanes, for example:
CH3COOH + Cl2 + HCl
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