On treatment with sodium hydroxide, the ester can be converted back to __________.

On treatment with sodium hydroxide, which is an alkali, the ester is converted back to alcohol and sodium salt of carboxylic acid. This reaction is known as saponification because it is used in the preparation of soap.

${\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_5 \stackrel{\mathrm{NaOH}}{\to } {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH} + {\mathrm{CH}}_3\mathrm{COONa}$