What is an acetal? Give examples. $OR$ Give the reaction of ethanal with $(a)$ ethanol and $(b)$ ethylene glycol in the presence of dry $HCl$ gas.

(N/A) An acetal is a gem-dialkoxy compound formed by the reaction of an aldehyde or ketone with two equivalents of a monohydric alcohol or one equivalent of a dihydric alcohol in the presence of an acid catalyst like dry $HCl$ gas.
$(i)$ Reaction of ethanal with ethanol:
$CH_3CHO + C_2H_5OH \xrightarrow{dry \ HCl} CH_3CH(OH)(OC_2H_5)$ (Hemiacetal)
$CH_3CH(OH)(OC_2H_5) + C_2H_5OH \xrightarrow{dry \ HCl} CH_3CH(OC_2H_5)_2 + H_2O$ ($1$,$1$-Diethoxyethane,an acetal)
$(ii)$ Reaction of ethanal with ethylene glycol:
$CH_3CHO + HOCH_2CH_2OH \xrightarrow{dry \ HCl} CH_3CH(O_2C_2H_4) + H_2O$ ($2$-Methyl$-1,3-$dioxolane,a cyclic acetal).