Question

Which compound in each pair has the lower ${\mathbf{pK}}_{\mathbf{a}}$? Which compound in each pair has the stronger conjugate base?

a.

b.

Short answer

a.

Compound 2 has lower ${\mathrm{pK}}_{\mathrm{a}}$ and compound 1 has a stronger conjugate base.

b.

Compound 1 has lower ${\mathrm{pK}}_{\mathrm{a}}$ and compound 2 has a stronger conjugate base.

Step-by-step solution

pKa and acid strength

Strong acids have lower ${\mathbf{pK}}_{\mathbf{a}}$,while weak acids have higher ${\mathbf{pK}}_{\mathbf{a}}$ values.

The presence of electron-withdrawing groups on a compound increases its acidity.

Conjugate base

The removal of a proton from a compound generates its conjugate base.

Stronger the acid, weaker is its conjugate base.

Comparing the acidity of the given compounds 

a.

Compounds in a

Compound 2 is more acidic as it contains the electron-withdrawing chlorine atom, and hence it will have lower ${\mathrm{pK}}_{\mathrm{a}}$.

Since compound 1 is a weak acid, its conjugate base will be stronger.

b.

Compounds in b

Compound 1 is more acidic as it contains the electron-withdrawing cyanide (CN) group.

Since compound 2 is a weak acid, its conjugate base will be stronger.