Question

Which of the following alcohols is expected to have a lowest $p{K}_a$ value?

• A. Ethanol
• B. 1-Propanol
• C. 2,2,2-trifluoroethanol
• D. 2-chloroethanol

Answer 1

The correct option is C 2,2,2-trifluoroethanol

$\text{Acidic strength}\propto K_a\propto \frac{1}{p{K}_a}\propto -I\text{of groups attached.}$
Option a : $C{H}_{3}C{H}_2{O}^{-}$ destabilized by $+I$ effect of alkyl group attached.
Option b : $C{H}_{3}C{H}_{2}C{H}_2{O}^{-}$ destabilized by $+I$ effect of alkyl group attached.
Option c :


$-I$ effect of fluorine stabilizes the conjugate base.
Option D :


$-I$ effect of chlorine stabilizes the conjugate base.
Since $-I$ effect of fluorine is greater than that of chlrone, so $p{K}_a$ value of 2,2,2-trifluorethanol is the least.