Give reasons:

(a) Propanone is less reactive than ethanal towards nucleophilic addition reactions.

(b) O₂N–CH₂–COOH has lower pK_a value than CH₃COOH.

(c) (CH₃)₂CH–CHO undergoes aldol condensation whereas (CH₃)₃C–CHO does not.

(a) Propanone is less reactive than ethanal towards nucleophilic addition reactions due to steric and electronic reasons explained as follows-

Presence of 2 –CH₃ groups on propanone creates steric hinderance to the incoming nucleophile and also decreases the electrophilicity of carbonyl carbon because 2 –CH₃ groups are electron releasing in nature. On the other hand these two factors have less effect on ethanal due to presence of only one methyl group on it.

(b) O₂N–CH₂–COOH has lower pK_a value than CH₃COOH.

Lower the pK_a value, greater is the K_a value and more is the acidity.

In O₂N–CH₂–COOH has an EWG (-NO₂) attached which stabilizes the negative charge on conjugate base i.e. formed after removal of ‘H’ by –I effect. More is the stability of conjugate base, more acidic will be the corresponding acid and higher will be the K_a value and lower will be the pK_a value.

However, no such effect is present in CH₃COOH. Therefore, O₂N–CH₂–COOH has lower pK_a value than CH₃COOH

(c) (CH₃)₂CH–CHO undergoes aldol condensation whereas (CH₃)₃C–CHO does not because the main condition for aldol condensation to occur is the presence of hydrogen.

In (CH₃)₂CH–CHO there is 1 hydrogen present whereas in (CH₃)₃C–CHO there is no hydrogen, thus it does not undergo aldol condensation.