(2)

Reactivity of aldehydes and ketones is more in nucleophilic addition reactions if:

(i) Partial positive charge on carbonyl carbon is more, ${\text{Nu}}^{-}$ is attracted more and hence more is reactivity.

(ii) Lesser is crowding around carbonyl carbon, easier is it for ${\text{Nu}}^{-}$ to approach, hence more is reactivity.

Presence of alkyl groups around carbonyl carbon decreases partial positive charge of the carbonyl carbon due to $+\mathrm{I}$ effect of alkyl groups. Also, bulky alkyl groups hinder attack of ${\text{Nu}}^{-}$ on carbonyl carbon. Hence presence of alkyl groups around carbonyl carbon decreases reactivity of aldehydes/ketones towards nucleophilic addition.