Question

The order of decreasing acid strength of carboxylic acids is:

• A. \({FCH}_2{COOH}>{ClCH}_2{COOH}>{NO}_2{CH}_2{COOH}>{CNCH}_2{COOH} \)
• B. \({CNCH}_2{COOH}>{FCH}_2{COOH}>{NO}_2{CH}_2{COOH}>{ClCH}_2{COOH} \)
• C. \({NO}_2{CH}_2{COOH}>{FCH}_2{COOH}>{ClCH}_2{COOH}>{CNCH}_2{COOH} \)
• D. \({FCH}_2{COOH}>{NO}_2{CH}_2{COOH}>{ClCH}_2{COOH}>{CNCH}_2{COOH} \)
• E. \({NO}_2{CH}_2{COOH}>{CNCH}_2{COOH}>{FCH}_2{COOH}>{ClCH}_2{COOH} \)

Hint:

The acidity of carboxylic acids increases with strong electron-withdrawing groups (-I or -R effects), such as NO\(_2\), CN, F, Cl, in decreasing order of influence.

Answer 1

Answer: (E)

Step 1: Understanding Acid Strength in Carboxylic Acids The acid strength of carboxylic acids is influenced by the electron-withdrawing or electron-donating effects of substituents. Electron-withdrawing groups (EWG) increase acidity by stabilizing the conjugate base through inductive (-I) or resonance (-R) effects. 
Step 2: Evaluating the Given Functional Groups 
\(-{NO}_2\) (Nitro group): Strongest electron-withdrawing group via -I and -R effects → strongest acid. 
\(-{CN}\) (Cyano group): Strong -I effect, but slightly weaker than nitro. 
\(-{F}\) (Fluorine): Strong -I effect, but lacks resonance stabilization. 
\(-{Cl}\) (Chlorine): Weaker -I effect than fluorine due to its lower electronegativity. 
Step 3: Arranging in Order of Acid Strength Stronger acids have stronger electron-withdrawing substituents. The correct order of acidity is: \[ {NO}_2{CH}_2{COOH}>{CNCH}_2{COOH}>{FCH}_2{COOH}>{ClCH}_2{COOH} \] This matches option (E).