Question

Give reasons for the following:HCHO is more reactive than CH$_3$CHO towards addition of HCN.

Hint:

No alkyl group → no hindrance → more reactive carbonyl carbon!

Answer 1

Nucleophilic addition reactions are characteristic of aldehydes and ketones. Formaldehyde (HCHO) features two hydrogen atoms bonded to the carbonyl carbon and lacks alkyl substituents. Acetaldehyde (CH$_3$CHO), conversely, possesses one alkyl group exhibiting a +I effect, which donates electron density to the carbonyl carbon, thereby diminishing its partial positive charge. A more pronounced positive charge on the carbonyl carbon enhances its electrophilicity, leading to increased reactivity towards nucleophiles. Furthermore, larger alkyl groups introduce greater steric hindrance, impeding the approach of attacking nucleophiles. Consequently, HCHO demonstrates superior reactivity compared to CH$_3$CHO in the context of HCN addition.