Question

Which carboxylic acid has the lower ${\mathbf{pK}}_{\mathbf{a}}$, pyruvic acid $\left({\mathrm{CH}}_3\mathrm{COCOOH}\right)$or acetoacetic acid $\left({\mathrm{CH}}_3{\mathrm{COCH}}_2\mathrm{COOH}\right)$ ? Explain your choice.

Short answer

Pyruvic acid is the carboxylic acid that has a lower ${\mathrm{pK}}_{\mathrm{a}}$value.

Step-by-step solution

Meaning of  pKa

The ${\mathrm{pK}}_{\mathrm{a}}$ value is determines the acidic strength of the acids. Higher the value of ${\mathbf{pK}}_{\mathbf{a}}$, lower will be the acidity of the compound.

The acidic strength increases due to the presence of the electron-withdrawing groups. The acidic strength also increases if the conjugate base is resonance stabilized.

Comparison of the acidic strength of pyruvic acid and acetoacetic acid

The structure of pyruvic acid is as follows:

Pyruvic acid

The structure of acetoacetic acid is as follows:

Acetoacetic acid

The acidic strength of acetoacetic acid is lower as compared to pyruvic acid. The pKa of acetoacetic acid is 3.58, whereas the pKa of pyruvic acid is 2.50. This implies that pyruvic acid is a stronger acid than acetoacetic acid.