Question

The most acidic carboxylic acid among the following is:

• A. $C{H}_{3}C{H}_{2}COOH$
• B. $C{H}_2=CHCOOH$
• C. $CH\equiv C-COOH$
• D. $C{H}_{3}C{H}_{2}C{H}_{2}COOH$

Answer 1

The correct option is C $CH\equiv C-COOH$

We know that, more the stability of the conjugate base, more is the acidic strength. The conjugate base of the given carboxylic acids are $C{H}_{3}C{H}_{2}CO{O}^{-}$, $C{H}_2=CHCO{O}^{-}$,
$CH\equiv C-CO{O}^{-}$ and $C{H}_{3}C{H}_{2}C{H}_{2}CO{O}^{-}$
respectively. Among these,the first and the fourth option's conjugate bases will be destabilized by the positive inductive effect of the alkyl group. Since, $sp$ hybridised carbons are more electronegative than $s{p}^2$ hybridised carbons, thus $CH\equiv C-CO{O}^{-}$ will be more stable compared to $C{H}_2=CHCO{O}^{-}$. Hence, $CH\equiv C-COOH$ will be most acidic.