NEET 2025 · ChemistryPrevious Year Question
The correct order of decreasing acidity of the following aliphatic acids is:
- A.$\ce{HCOOH}$ > $\ce{CH3COOH}$ > $\ce{(CH3)2CHCOOH}$ > $\ce{(CH3)3CCOOH}$✓
- B.$\ce{HCOOH}$ > $\ce{(CH3)3CCOOH}$ > $\ce{(CH3)2CHCOOH}$ > $\ce{CH3COOH}$
- C.$\ce{(CH3)3CCOOH}$ > $\ce{(CH3)2CHCOOH}$ > $\ce{CH3COOH}$ > $\ce{HCOOH}$
- D.$\ce{CH3COOH}$ > $\ce{(CH3)2CHCOOH}$ > $\ce{(CH3)3CCOOH}$ > $\ce{HCOOH}$
Correct Answer
(A) $\ce{HCOOH}$ > $\ce{CH3COOH}$ > $\ce{(CH3)2CHCOOH}$ > $\ce{(CH3)3CCOOH}$
Solution & Explanation
\textbf{Answer:} (a) \textbf{Solution:} Acidity is governed by the electron-donating $+I$ inductive effect of alkyl groups. More or larger alkyl groups push electron density toward $\ce{-COOH}$, destabilising the carboxylate anion and lowering acidity. Formic acid $\ce{HCOOH}$ has no alkyl group and is strongest; acidity then falls as the $+I$ effect increases: $\ce{HCOOH}$ > $\ce{CH3COOH}$ (one $\ce{CH3}$) > $\ce{(CH3)2CHCOOH}$ > $\ce{(CH3)3CCOOH}$ (three $\ce{CH3}$, weakest).
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