Question

Place the following in the correct order of acidic nature $CH\equiv C-COOH;C{H}_2=CH-COOH;C{H}_{3}C{H}_{2}COOH$
(I) (II) (III)

• A. I > II > III
• B. III > II > I
• C. II > I > III
• D. I > III > II

Answer 1

The correct option is A I > II > III

As more stable is conjugate base (carboxylate ion) more is acidic strength.
$\left(I\right)HC\equiv \underset{\underset{sp}{\uparrow }}{C}-CO{O}^{-}\left(II\right)C{H}_2=\underset{\underset{s{p}^2}{\uparrow }}{CH}-CO{O}^{-}\left(III\right)C{H}_3-\underset{\underset{s{p}^3}{\uparrow }}{C{H}_2}-CO{O}^{-}$
% s-character $\propto$ Electronegativity of carbon $\propto$ stability of anion.
$\text{Thus Stability of conjugate base is}III<II<I$
$\text{and hence, correct order of acidity is}III<II<I$