Question

Some pairs of acids are given below. Select the pair in which second acid is stronger than first.

• A. $C{H}_{3}COOHandC{H}_{2}FCOOH$
• B. $CH{F}_{2}COOHandC{H}_{2}ClCOOH$
• C. $C{H}_{2}ClCOOHandC{H}_{2}BrCOOH$
• D. $C{H}_{3}C{H}_{2}CHFCOOHandC{H}_{3}CHFC{H}_{2}COOH$

Answer 1

The correct option is A $C{H}_{3}COOHandC{H}_{2}FCOOH$

$\text{Stability of conjugate base}\propto \text{Acidic strenght}$
-I effect will stabilise the conjugate base
In option (a), -I effect of F stabilise the conjugate base. Hence the second acid is more acidic.

In option (b), Flourine has effective -I effect than Cl and also there are 2 F so conjugate base of first acid is more stable and has high acidic strength.


In option (c), Cl has effective -I effect than Br so conjugate base of first acid is more stable and has high acidic strength.


In option (d), Flourine is attached to 1 carbon of first acid and 2 carbon of second acid. -I effect decrease with increase in distance. Since in first acid F is located to near site, the conjugate base is more stable and acidic.


Hence, option (a) is correct answer.