Rank the following compounds in order of decreasing acidity: HO HO HO Cl Decreasing acidity
The Correct Answer and Explanation is:
To rank the compounds in order of decreasing acidity, we analyze how substituents on the aromatic ring influence the acidity of the carboxylic acid group. The compounds are:
- Benzoic acid (left compound)
- p-Methylbenzoic acid (middle compound)
- p-Chlorobenzoic acid (right compound)
Ranking (left to right in decreasing acidity):
p-Chlorobenzoic acid > Benzoic acid > p-Methylbenzoic acid
Explanation:
Acidity in carboxylic acids is largely affected by the ability of substituents to stabilize the negative charge on the carboxylate ion formed after deprotonation. This stabilization occurs through inductive and resonance effects.
- p-Chlorobenzoic acid:
The chlorine atom is an electron-withdrawing group due to its electronegativity. Through an inductive effect, it pulls electron density away from the aromatic ring and the carboxyl group. This stabilizes the negative charge on the carboxylate anion, making the acid more acidic. Though chlorine has lone pairs that can participate in resonance, in the para position this resonance effect is less significant than its inductive effect, which dominates and increases acidity. - Benzoic acid:
Benzoic acid has no additional substituents on the aromatic ring. Therefore, its acidity is the baseline for comparison. There are no electron-donating or withdrawing groups to influence the electron density of the carboxyl group. - p-Methylbenzoic acid:
The methyl group is an electron-donating group through hyperconjugation and inductive effects. It increases electron density on the ring and thus on the carboxylate group, destabilizing the negative charge. This makes the acid less acidic than benzoic acid.
By considering these effects, the correct ranking in order of decreasing acidity is:
p-Chlorobenzoic acid > Benzoic acid > p-Methylbenzoic acid.
