Addition of HCN to the carboxyl compounds is a nucleophilic addition reaction.
The reactivity towards HCN addition decreases as the + I effect of the alkyl groups increases and/or the steric hindrance to the nucleophilic attack by CN– at the carboxyl carbon increases. Thus the reactivity decreases in the order
In other words, reactivity increases in the reverse order, i. e.
Ditert‐butyl Ketone < tert‐Butyl methyl Ketone < Acetone < Acetaldehyde