In the forward acid-catalysed process, the first step is protonation which produces an oxonium ion that is a much better electrophile than the aldehyde In the forward base-catalysed process, methoxide is a stronger nucleophile as compared with methanol in the acid-catalysed reaction The first step in the reverse acid-catalysed reaction is addition of a proton to the oxygen of the hemiacetal that bears the alkyl group The first step in the reverse base-catalysed reaction is attack of hydroxide at the carbon of the hemi-acetal