Question

Which of the following compounds would have the smallest value for $pKa$?

• A. ${\mathrm{CHF}}_2{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{COOH}$
• B. ${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CF}}_2\mathrm{COOH}$
• C. ${\mathrm{CH}}_2{\mathrm{FCHFCH}}_2\mathrm{COOH}$
• D. ${\mathrm{CH}}_3{\mathrm{CF}}_2{\mathrm{CH}}_2\mathrm{COOH}$

Answer 1

The correct option is B

${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CF}}_2\mathrm{COOH}$

The explanation for the correct option

(B) ${\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CF}}_{\mathbf{2}}\mathbf{COOH}$

• As $pKa$increases, acidic strength will decrease. Thus the compound with the smallest $pKa$value will be more acidic.
• $-\mathrm{F}$ is a strong electron-withdrawing group. Thus, it will have a strong $-\mathrm{I}$ effect.
• This strong electron-withdrawing nature of two $-\mathrm{F}$ pulls the electron density from $-\mathrm{COOH}$ group so that it can easily release its proton ${\mathrm{H}}^{+}$.
• Therefore ${\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CF}}_{\mathbf{2}}\mathbf{COOH}$ is more acidic and has the smallest $pKa$ value.
• Hence option B is correct

The explanation for the incorrect options

(A) ${\mathrm{CHF}}_2{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{COOH}$

• $-\mathrm{F}$ group is present. But here, $-\mathrm{F}$is present at third carbon from the carboxyl group.
• As the distance from the carboxyl group increases, its electron-withdrawing effect will decrease. Thus the acidic strength also gets reduced.
• Thus $pKa$will not be the smallest.
• Hence option A is incorrect

(C) ${\mathrm{CH}}_2{\mathrm{FCHFCH}}_2\mathrm{COOH}$

• Here also $-\mathrm{F}$ group is present at more distance from the carboxyl group compared to option B.
• Thus acidic strength will get reduced.
• Hence option C is incorrect

(D) ${\mathrm{CH}}_3{\mathrm{CF}}_2{\mathrm{CH}}_2\mathrm{COOH}$

• $-\mathrm{F}$ group is at more distance from the carboxyl group compared to option B.
• Therefore here also acidic strength will get reduced due to the same reason.
• So more will be the $pKa$ value.
• Hence option D is incorrect.

Therefore the correct answer is option (B) ${\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CF}}_{\mathbf{2}}\mathbf{COOH}$