CBSE Class 10 Science 2017 · Paper 31/2/1 · Question 19

Esters are sweet-smelling carbon compounds formed by the reaction of a carboxylic acid with an alcohol; they contain the ester group $-\text{COO}-$, for example ethyl ethanoate, ${\text{CH}}_3{\text{COOC}}_2{\text{H}}_5$. Preparation (esterification): a carboxylic acid is warmed with an alcohol in the presence of a little concentrated sulphuric acid as catalyst: ${\text{CH}}_3\text{COOH}+{\text{C}}_2{\text{H}}_5\text{OH}\stackrel{\text{conc. }{\text{H}}_2{\text{SO}}_4,\text{warm}}{\to }{\text{CH}}_3{\text{COOC}}_2{\text{H}}_5+{\text{H}}_2\text{O}$. When an ester reacts with sodium hydroxide, it is converted back into the alcohol and the sodium salt of the carboxylic acid: ${\text{CH}}_3{\text{COOC}}_2{\text{H}}_5+\text{NaOH}⟶{\text{CH}}_3\text{COONa}+{\text{C}}_2{\text{H}}_5\text{OH}$. Name: saponification (alkaline hydrolysis of an ester). Use: making soap, by heating fats or oils, which are esters of long-chain fatty acids, with sodium hydroxide; this gives the sodium salts of the fatty acids, which are soap.