JEE Main · 2019 · 09 Apr 2019 · Shift 2 · Aldehydes Ketones and Carboxylic Acids

Acetal formation is a nucleophilic addition at the carbonyl, and an aldehyde reacts faster than a ketone because its carbonyl carbon is less hindered and more electrophilic, whereas a ketone has two electron-donating alkyl groups shielding that carbon. So propanal, an aldehyde, outruns acetone, a ketone. On top of that the reaction is an equilibrium, so taking methanol in excess pushes it toward the acetal by Le Chatelier's principle. The fastest case therefore pairs propanal with methanol in excess.