Question

The correct order of decreasing acid strength of trichloroacetic acid $(A)$ trifluoroacetic acid $(B)$, benzoic acid $(C)$ and formic acid $(D)$ is

• A. $A>B>C>D$
• B. $A>C>B>D$
• C. $B>A>D>C$
• D. $B>A>C>D$

Answer 1

Answer: (C)

Presence of electron withdrawing group like $-F,-Cl$ ($-F$ is more electron withdrawing) increases the acidity of carboxylic acids by decreasing electron density on the oxygen atom of $O-H$ bond. Due to it the $O-H$ bond becomes weaker and proton is released easily.
Phenyl group $\left(-C_6{H}_5\right)$ is also an electron withdrawing group. But it decreases the acidity of - COOH group when attached to the chain. This is because of $+R$ effect which tends to destabilise the ion.
Thus, the order of acidity is
$\underset{(B)}{C{F}_{3}COOH}>\underset{(A)}{CC{l}_{3}COOH}>\underset{(D)}{\mathrm{HCOOH}}>\underset{(C)}{C_6{H}_{5}COOH}$