Aldehydes, Ketones and Carboxylic Acids Part 3

Concept:The acidic strength of carboxylic acids depends on the substituent's inductive effect and resonance stabilization of the conjugate base.Explanation:First, identify the major product for each reaction.Reaction A: $\mathrm{PhNH}_2$ undergoes diazotization followed by replacement with $\mathrm{CuCN}$ and hydrolysis to give benzoic acid ($\mathrm{PhCOOH}$).Reaction B: $\mathrm{CH}_3\mathrm{CH}_2\mathrm{CHO}$ is oxidised by Tollen's reagent to propanoic acid ($\mathrm{CH}_3\mathrm{CH}_2\mathrm{COOH}$).Reaction C: $\mathrm{CH}_4$ is oxidised in two steps to formic acid ($\mathrm{HCOOH}$).Reaction D: $\mathrm{PhCH}_2\mathrm{MgBr}$ reacts with $\mathrm{CO}_2$ followed by hydrolysis to phenylacetic acid ($\mathrm{PhCH}_2\mathrm{COOH}$).Now compare acidic strength: Formic acid ($\mathrm{HCOOH}$) is the strongest due to −I effect of H and small size.Benzoic acid ($\mathrm{PhCOOH}$) is next because the phenyl ring stabilizes the conjugate base via resonance.Phenylacetic acid ($\mathrm{PhCH}_2\mathrm{COOH}$) is weaker than benzoic acid because the $\mathrm{CH}_2$ group interrupts resonance, making the inductive effect weaker.Propanoic acid ($\mathrm{CH}_3\mathrm{CH}_2\mathrm{COOH}$) is the weakest because the alkyl group has +I effect, decreasing acidity.Thus the order is: $\mathrm{HCOOH} > \mathrm{PhCOOH} > \mathrm{PhCH}_2\mathrm{COOH} > \mathrm{CH}_3\mathrm{CH}_2\mathrm{COOH}$, i.e., C $>$ A $>$ D $>$ B.Answer:Option A: C $\rightarrow$ A $\rightarrow$ D $\rightarrow$ B.