CBSE 12th Chemistry Question Paper 2022 (Set 4)

The order of reactivity towards nucleophilic addition of HCN is: Acetone < Acetophenone < Acetaldehyde < Benzaldehyde. Reactivity is governed by two factors: steric hindrance and electronic effects. Aldehydes are more reactive than ketones due to lesser steric hindrance and higher electrophilicity of the carbonyl carbon. Among aldehydes, acetaldehyde is more reactive than benzaldehyde due to the resonance stabilization of the benzene ring in the latter. Among ketones, acetone is more reactive than acetophenone for the same reason.

The acidic strength order is: (CH3)2CHCOOH < CH3CH(Br)CH2COOH < CH3CH2CH(Br)COOH. The acidity of carboxylic acids is increased by the -I (inductive) effect of electronegative substituents. As the electronegative Br atom gets closer to the -COOH group, its -I effect becomes stronger, thereby stabilizing the carboxylate anion more effectively. In (CH3)2CHCOOH, the +I effect of alkyl groups decreases acidity.

The limiting molar conductivity of weak electrolytes cannot be obtained graphically because the curve for weak electrolytes slopes asymptotically to the axis. It can be obtained using Kohlrausch's Law of Independent Migration of Ions, which states that at infinite dilution, the limiting molar conductivity of an electrolyte is the sum of the individual contributions of its constituent ions (Λm° = ν+λ°+ + ν-λ°-).

The increasing order of boiling points is: CH3CHO < CH3CH2OH < CH3COOH. CH3CHO has only dipole-dipole interactions, making it the lowest. CH3CH2OH has intermolecular hydrogen bonding. CH3COOH has the highest boiling point because it forms stable intermolecular hydrogen-bonded dimers in the vapor phase, which are stronger than the hydrogen bonding in alcohols.

(i) Benzoic acid does not undergo Friedel-Crafts reaction because the -COOH group is strongly deactivating and the Lewis acid catalyst (e.g., AlCl3) gets bonded to the oxygen atom of the -COOH group, further deactivating the ring. (ii) Oxidation of aldehydes is easier because they contain a hydrogen atom attached to the carbonyl group, which can be easily converted into an -OH group during oxidation to carboxylic acids; ketones lack this hydrogen.