The correct order of decreasing acidity of the following aliphatic acids is

Solution & Explanation

\textbf{Answer:} (A) $\mathrm{HCOOH}>\mathrm{CH}{}_3\mathrm{COOH}>(\mathrm{CH}{}_3){}_2\mathrm{CHCOOH}>(\mathrm{CH}{}_3){}_3\mathrm{CCOOH}$. \textbf{Solution:} Electron-donating ($+I$) alkyl groups destabilise the carboxylate anion and reduce acidity. As more or larger alkyl groups are attached near the $-\mathrm{COOH}$, the $+I$ effect grows and acid strength falls. With no alkyl group, $\mathrm{HCOOH}$ is the strongest; acidity then decreases through $\mathrm{CH}{}_3\mathrm{COOH}$ and $(\mathrm{CH}{}_3){}_2\mathrm{CHCOOH}$ to the weakest $(\mathrm{CH}{}_3){}_3\mathrm{CCOOH}$.