Question

Arrange the following acids in increasing order of their acidic strengths: HCOOH, FCH₂COOH, NO₂CH₂COOH, ClCH₂COOH

• A. HCOOH $<$FCH$_2$COOH$<$ NO$_2$CH$_2$COOH $<$ClCH$_2$COOH
• B. HCOOH $<$ NO$_2$CH$_2$COOH $<$ ClCH$_2$COOH $<$ FCH$_2$COOH
• C. NO$_2$CH$_2$COOH $<$ HCOOH $<$ ClCH$_2$COOH $<$ FCH$_2$COOH
• D. HCOOH $<$ ClCH$_2$COOH $<$ FCH$_2$COOH $<$ NO$_2$CH$_2$COOH

Answer 1

The Correct Option is D

The acidic strength of carboxylic acids is determined by the nature of substituents on the alpha carbon. Electron-withdrawing groups enhance acidity by stabilizing the carboxylate anion formed after proton dissociation (H⁺).

• HCOOH (Formic Acid): Lacks substituents, making it the least acidic.
• ClCH₂COOH (Chloroacetic Acid): Chlorine's electron-withdrawing effect increases acidity relative to formic acid.
• FCH₂COOH (Fluoroacetic Acid): Fluorine, being more electronegative than chlorine, exerts a stronger electron-withdrawing effect, resulting in higher acidity than chloroacetic acid.
• NO₂CH₂COOH (Nitroacetic Acid): The nitro group's potent electron-withdrawing capabilities, via resonance and inductive effects, make it the strongest acid among those listed.

The ascending order of acidic strength is dictated by the substituents' electron-withdrawing capacity:

HCOOH < ClCH₂COOH < FCH₂COOH < NO₂CH₂COOH