Question

What is the correct decreasing order for acid strength?

• A. $NO_{2}CH_{2}COOH>FCH_{2}COOH>CNCH_{2}COOH>ClCH_{2}COOH$
• B. $NO_{2}CH_{2}COOH>NCCH_{2}COOH>FCH_{2}COOH>ClCH_{2}COOH$
• C. $ \, CNCH_{2}COOH>O_{2}NCH_{2}COOH>FCH_{2}COOH>ClCH_{2}COOH$
• D. $FCH_{2}COOH>NCCH_{2}COOH>NO_{2}CH_{2}COOH>ClCH_{2}COOH$

Answer 1

Answer: (B)

Acidic character $ \propto $ stability of conjugate base (anion)
Stability of anion $ \propto - \text{I}$
Hence greater the $-I$ effect greater will be the acidity
Order of $-I$ effect $-NO_{2}>CN>-F>-Cl$ Hence, the order of acidity $\left(k_{a}\right)$ will be
$\text{O}_{2} \text{NCH}_{2} \text{COOH} > \text{NCCH}_{2} \text{COOH} > \text{FCH}_{2} \text{COOH} > \text{Cl CH}_{2} \text{COOH}$