Explain why aldehydes are more reactive than ketones towards nucleophilic addition reactions ?

Explanation

Aldehyde are more Reactive than ketone :
1. Inductive effect : The relative reactivities of aldehyde and ketone in nucleophilies addition reactions may be attributed to the amount of positive charge on the carbon. A greater positive charge means a higher reactivity. If the positive charge is dispersed throughout the molecule the carbonyl compound become more stable and its reactivity decrease. Now, alkyl group is an electron releasing group ( + I Inductive effect). Therefore, electron releasing power of two alkyl group in ketone is more than that of one is aldehyde. As result, the electron deficiency carbon atom in the carbonyl group is satisfied more in ketone than in aldehyde. Therefore, the reduced positive charge on carbon in case of ketone discourages the attack of nucleophiles. Hence, electronically ketones are less reactive than aldehyde.

Sterle effect : The size of the alkyl grou is more than that of hydrogen. In aldehyde there is ope alkyl group but in ketones, there are two alkyl groups attached to the carbonyl group. The alkyl group are larger than a hydrogen atom and these cause hindrance to the attaking group. This is called sterik hindrance. As the number and size of the alkyl groups in causes, the hindrance .to the attack of nucleophilies also increase and Reactivity decrease. The lack of hindrance in nucleophilies attack is another reason for the greater of formaldehyde. Thus, the reactivity follow the other.