Question

For the following acids
$\underset{\left(I\right)}{(C{H}_3{)}_{3}CC{H}_{2}C{O}_{2}H}\underset{\left(II\right)}{(C{H}_3{)}_3{N}^{\oplus }C{H}_{2}C{O}_{2}H}\underset{\left(III\right)}{C{H}_{3}COOH}$
$p{K}_a$ value will be in order:

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

Answer 1

The correct option is C I > III > II

EWG increases acidic strength
EDG decreases acidic strength
$I.C{H}_3-\stackrel{\stackrel{C{H}_3}{|}}{\underset{\underset{C{H}_3}{|}}{C}}-C{H}_2-CO{O}^{-}II.C{H}_3-\stackrel{\stackrel{C{H}_3}{|}}{\underset{\underset{C{H}_3}{|}}{N^{\oplus }}}-C{H}_2-CO{O}^{-}III.C{H}_3-\stackrel{\stackrel{O}{||}}{C}-O^{-}$

$K_a\propto$ stability of carboxylate ion.

Stability of ion is II > III > I since $\left(C{H}_3{)}_3{N}^{\oplus }C{H}_2\right)$ group shows -I effect and stablize carboxylate ion in $\left(II\right)$ and hence more stable than $\left(I\right)$ and $\left(III\right)$.
$\left(I\right)$ is less stable than $\left(III\right)$ since $\left(C{H}_3{)}_{3}CC{H}_2\right)$ shows +I and hence destablises carboxylate ion $\left(I\right)$.
$\Rightarrow K_a\text{order is}II>III>I\Rightarrow p{K}_a\text{is}II<III<I$