Chemistry Chemical Reactions Involving Cleavage of `C–OH` Bond of Carboxylic Acids

Topics Covered :

● Reactions Involving Cleavage of `C-OH` Bond
● Formation of Anhydride
● Esterification
● Reactions with `PCl_5`, `PCl_3` and `SOCl_2`
● Reaction with Ammonia

Reactions Involving Cleavage of `C–OH` Bond :

Reactions involving cleavage of `color{red}(C-OH)` are as follows :

Formation of anhydride :

Carboxylic acids on heating with mineral acids such as `color{red}(H_2SO_4)` or with `color{red}(P_2O_5)` give corresponding anhydride.

Esterification :

Carboxylic acids are esterified with alcohols or phenols in the presence of a mineral acid such as concentrated `H_2SO_4` or `HCl` gas as a catalyst.

`color{red}(RCOOH + R'OH overset(H^(+))⇌ RCOOR' + H_2O)`

`color{green}(text(Mechanism of Esterification of Carboxylic Acids ))` : The esterification of carboxylic acids with alcohols is a kind of nucleophilic acyl substitution.

● Protonation of the carbonyl oxygen activates the carbonyl group towards nucleophilic addition of the alcohol.

● Proton transfer in the tetrahedral intermediate converts the hydroxyl group into –`color{red}(text()^+OH_2)` group, which, being a better leaving group, is eliminated as neutral water molecule.

● The protonated ester so formed finally loses a proton to give the ester.

Reactions with `PCl_5, PCl_3` and `SOCl_2` :

The hydroxyl group of carboxylic acids, behaves like that of alcohols and is easily replaced by chlorine atom on treating with `color{red}(PCl_5, PCl_3)` or `color{red}(SOCl_2)`. Thionyl chloride `color{red}((SOCl_2))` is preferred because the other two products are gaseous and escape the reaction mixture making the purification of the products easier.

`color{red}(RCOOH +PCl_5 → RCOCl+PCl_3+HCl)`

`color{red}(3RCOOH +PCl_3 → 3RCOCl +H_3PO_3)`

`color{red}(RCOOH +SOCl_2 → RCOCl_2+SO_2+HCl)`

Reaction with ammonia :

Carboxylic acids react with ammonia to give ammonium salt which on further heating at high temperature give amides.

For example :

`color{red}(CH_3COOH +NH_3 ⇌ undersettext(Ammonium acetate)(CH_3CO O^(-) overset(+)NH_4) underset(-H_2O) overset(Delta)→ undersettext(Acetamide) (CH_3CONH_2))`