NEET 2025 · ChemistryPrevious Year Question

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

Answer: (A) $HCOOH$ > $CH X_{3} COOH$ > $(CH X_{3}) X_{2} CHCOOH$ > $(CH X_{3}) X_{3} CCOOH$ . \textbf{Answer:} (a) \textbf{Solution:} Acidity is governed by the electron-donating $+ I$ inductive effect of alkyl groups.

A. $HCOOH$ > $CH X_{3} COOH$ > $(CH X_{3}) X_{2} CHCOOH$ > $(CH X_{3}) X_{3} CCOOH$ ✓
B. $HCOOH$ > $(CH X_{3}) X_{3} CCOOH$ > $(CH X_{3}) X_{2} CHCOOH$ > $CH X_{3} COOH$
C. $(CH X_{3}) X_{3} CCOOH$ > $(CH X_{3}) X_{2} CHCOOH$ > $CH X_{3} COOH$ > $HCOOH$
D. $CH X_{3} COOH$ > $(CH X_{3}) X_{2} CHCOOH$ > $(CH X_{3}) X_{3} CCOOH$ > $HCOOH$

Correct Answer

(A) $HCOOH$ > $CH X_{3} COOH$ > $(CH X_{3}) X_{2} CHCOOH$ > $(CH X_{3}) X_{3} CCOOH$

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 $- COOH$ , destabilising the carboxylate anion and lowering acidity. Formic acid $HCOOH$ has no alkyl group and is strongest; acidity then falls as the $+ I$ effect increases: $HCOOH$ > $CH X_{3} COOH$ (one $CH X_{3}$ ) > $(CH X_{3}) X_{2} CHCOOH$ > $(CH X_{3}) X_{3} CCOOH$ (three $CH X_{3}$ , weakest).

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