why ethanal is more reactive towards nucleophilic addition reactions than propanone??

Ketones are less reactive for nucleophillic addition reaction than aldehydes because:

• Inductive effect: The relative reactivities of aldehydes and ketones in nucleophilic 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 becomes more stable and its reactivity decreases.

Now, alkyl group is an electron releasing group (+I inductive effect). Therefore, electron releasing power of two alkyl groups in propanone is more than that of one alkyl group in ethanal. As a result, the electron deficiency of carbon atom in the carbonyl group is satisfied more in propanone than in ethanal. Therefore, the reduced positive charge on carbon in case of propanone discourages the attack of nucleophiles. Hence propanone are less reactive than ethanal.

• Stearic effect: The size of the alkyl group is more than that of hydrogen. In ethanal, there is one alkyl group but in propanone, there are two alkyl groups attached to the carbonyl group. The alkyl groups are larger than a hydrogen atom and these cause hindrance to the attacking group. This is called stearic hindrance. As the number and size of the alkyl groups increase, the hindrance to the attack of nucleophile also increases and the reactivity of a carbonyl decreases.