Question

In the following reaction,
$\text{Carbonyl compound}+MeOH\stackrel{HCl}{\rightleftharpoons }\text{Acetal}$,
rate of the reaction is the highest for:

• A. Acetone as substrate and methanol in stoichiometric amount
• B. Acetone as substrate and methanol in excess
• C. Propanal as substrate and methanol in excess
• D. Propanal as substrate and methanol in stoichiometric amount

Answer 1

The correct option is C Propanal as substrate and methanol in excess

Ketone as substrate is less reactive than aldehyde towards nucleophilic addition. After the formation of hemiacetal, the hydroxy group is protonated and loss of water molecule takes place. The carbocationic centre thus produced is attacked by second molecule of methanol. So, excess amount of methanol will favour formation of acetal more.