Question

Predict which is most likely to be the stronger acid: \(\mathrm{CH}_{3} \mathrm{COOH}\) or \(\mathrm{CCl}_{3} \mathrm{COOH}\). Explain your reasoning.

Short answer

Trichloroacetic acid \(\mathrm{CCl}_{3} \mathrm{COOH}\) is likely to be the stronger acid because the three chlorine atoms enhance the stability of its conjugate base by withdrawing electron density. This makes it easier for \(\mathrm{CCl}_{3}\mathrm{COOH}\) to donate a proton and thus act as an acid.

Step-by-step solution

Compare the Structures

The two molecules for comparison are acetic acid \(\mathrm{CH}_{3}\mathrm{COOH}\) and trichloroacetic acid \(\mathrm{CCl}_{3}\mathrm{COOH}\). They are similar in structure, but in trichloroacetic acid, the three hydrogen atoms in the methyl group are replaced by chlorine atoms.

Assess the Effect of Chlorine Atoms

The chlorine atoms in trichloroacetic acid are highly electronegative and are also good at withdrawing electron density. This gives them the capability to stabilize the conjugate base after the acid donates a proton. Consequently, the negative charge of the conjugate base in trichloroacetic acid is better dispersed than in acetic acid.

Predict the Relative Acidity

The greater the ability of a molecule to stabilize its conjugate base (an anion formed by the loss of a proton), the stronger the acid. Therefore, \(\mathrm{CCl}_{3}\mathrm{COOH}\) will be a stronger acid than \(\mathrm{CH}_{3}\mathrm{COOH}\) due to its ability to spread out and stabilize the negative charge that occurs when the acid donates a proton.