Aldehydes And Ketones

(I) Acetone reacts with semicarbazide (H N–NH–CONH ) to form a semicarbazone derivative via nucleophilic addition followed by elimination of water: (II) Two molecules of benzaldehyde are treated with conc. NaOH.
Solution:
(II) This is the **Cannizzaro reaction** (disproportionation of aldehydes with no -hydrogen): (III) Butan-2-one is treated with Zn–Hg and conc. HCl.
Solution:
(III) This is the **Clemmensen reduction**, converting ketones to alkanes: (a)(ii) Arrange the following in increasing order of acidic strength: (I) CH CH CH COOH, BrCH CH CH COOH, CH CHBrCH COOH, CH CH CHBrCOOH
Solution:
(I) Acidic strength increases with electron-withdrawing groups closer to –COOH: (II) Benzoic acid, 4-Methoxybenzoic acid, 4-Nitrobenzoic acid, 3,4-Dinitrobenzoic acid
Solution:
(II) Electron withdrawing groups increase acidity, donating groups decrease it.

(i) Aldehydes (like CH CHO) are more reactive than ketones (like CH COCH ) 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 CHO is more reactive than acetone towards nucleophiles like 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.

Aldol condensation occurs in aldehydes and ketones having -hydrogen atoms. Let's analyze: - (A) CH CHO (Acetaldehyde): Has -hydrogens → undergoes aldol
- (B) CH COCH (Acetone): Has -hydrogens → undergoes aldol
- (C) C H CHO (Benzaldehyde): No -hydrogens → does not undergo aldol
- (D) C H CHO (Propionaldehyde): Has -hydrogens → undergoes aldol
So, only benzaldehyde cannot undergo aldol condensation.