Question

Account for the following: (i) CH\(_3\)CHO is more reactive than CH\(_3\)COCH\(_3\) towards reaction with HCN.

Hint:

Hydrogen bonding raises boiling points. Carboxylic acids dimerize strongly via H-bonds unlike aldehydes and ketones.

Answer 1

(i) Aldehydes (like CH\(_3\)CHO) are more reactive than ketones (like CH\(_3\)COCH\(_3\)) due to two reasons: - **Steric hindrance**: Ketones have two bulky alkyl groups, whereas aldehydes have one H, making them less hindered. - **+I effect**: Alkyl groups in ketones donate electrons, reducing electrophilicity of carbonyl C. Thus, CH\(_3\)CHO is more reactive than acetone towards nucleophiles like HCN. \[ \boxed{\text{CH}_3\text{CHO}>\text{CH}_3\text{COCH}_3 \text{ in reactivity toward HCN}} \] (ii) Carboxylic acids are higher boiling liquids than aldehydes and ketones.
Solution:
(ii) Carboxylic acids form **extensive hydrogen bonds** (dimers) in liquid state due to the presence of –OH and =O groups. - This results in stronger intermolecular forces. - Aldehydes and ketones only have dipole–dipole interactions (no strong H-bonding). Hence, carboxylic acids have significantly higher boiling points. \[ \boxed{\text{Boiling point: Carboxylic acids}>\text{Aldehydes/Ketones}} \]