Stearic effect - relative reactivity of aldehydes and keto

The size of the alkyl group is much more than that of hydrogen. In aldehydes, there is one alkyl set although in ketones, there are two alkyl sets linked to the carbonyl group. The alkyl groups are bigger as compared to a hydrogen atom and these cause hindrance to the attacking group. This is termed as stearic hindrance. As the size and number of the alkyl sets increase, the hindrance to molest of nucleophile as well increases and the reactivity of a carbonyl decreases. The short of hindrance in nucleophilic attack is other reason for the better reactivity of formaldehyde. So, the reactivity follows the order: