Chemistry Reactions Involving `-COOH` Group and Uses of Carboxylic Acids

Topics Covered :

● Reactions Involving `-COOH` Group
● Reduction
● Decarboxylation
● Substitution Reactions in Hydrocarbon Part
● Halogenation
● Ring Substitution
● Uses of Carboxylic Acids

Reactions Involving `–COOH` Group :

Following are the reactions given by `color{red}(-COOH)` group :

Reduction :

`=>` Carboxylic acids are reduced to primary alcohols by lithium aluminium hydride or better with diborane.

● Diborane does not easily reduce functional groups such as ester, nitro, halo, etc.

● Sodium borohydride does not reduce the carboxyl group.

`color{red}(R-COOH underset((ii) H_3O^(+)) overset((i) LiAlH_4//text(ether or) B_2H_4)→ R- CH_2OH)`

Decarboxylation :

`=>` Carboxylic acids lose carbon dioxide to form hydrocarbons when their sodium salts are heated with sodalime (`color{red}(NaOH)` and `color{red}(CaO)` in the ratio of `3 : 1`). The reaction is known as decarboxylation.

`color{red}(R- COONa undersettext(Heat) overset(NaOH & CaO)→ R- H +Na_2CO_3)`

`=>` Alkali metal salts of carboxylic acids also undergo decarboxylation on electrolysis of their aqueous solutions and form hydrocarbons having twice the number of carbon atoms present in the alkyl group of the acid. The reaction is known as `color{green}(text(Kolbe electrolysis))`.

`color{red}(undersettext(Sodium acetate)(2CH_3COO^(-) text()^+Na) + 2H_2O oversettext(Electrolysis)-> CH_3 CH_3 + 2CO_2 + H_2 + 2NaOH)`

The reaction is supposed to follow the path shown in fig.

Substitution Reactions in the Hydrocarbon Part :

Following substitution reactions are given by carboxylic acids :

Halogenation :

`=>` Carboxylic acids having an `color{red}(α)`-hydrogen are halogenated at the `color{red}(α)`-position on treatment with chlorine or bromine in the presence of small amount of red phosphorus to give `color{red}(α)`-halocarboxylic acids. The reaction is known as `color{green}(text(Hell-Volhard-Zelinsky reaction))`.

`color{red}(R-CH_2 -COOH underset((ii) H_2O) overset((i) X_2// text(Red phosphorus)) → undersettext(X = Cl , Br α - Halocarboxylic acid)(R- underset(underset(X)(|))C-COOH))`

Ring substitution :

`=>` Aromatic carboxylic acids undergo electrophilic substitution reactions in which the carboxyl group acts as a deactivating and meta-directing group.

`=>` They do not undergo Friedel-Crafts reaction (because the carboxyl group is deactivating and the catalyst aluminium chloride (Lewis acid) gets bonded to the carboxyl group).

Uses of Carboxylic Acids :

`=>` Methanoic acid is used in rubber, textile, dyeing, leather and electroplating industries.

`=>` Ethanoic acid is used as solvent and as vinegar in food industry.

`=>` Hexanedioic acid is used in the manufacture of nylon-6, 6.

`=>` Esters of benzoic acid are used in perfumery.

`=>` Sodium benzoate is used as a food preservative.

`=>` Higher fatty acids are used for the manufacture of soaps and detergents.