Inductive effect and delocalization on relative acidity

Question

Why is the pKa of benzoic acid lower than that of phenol? That would mean benzoic acid is a stronger acid and has a more stable conjugate base. But, benzoic acid does not have a signficant amount of resonance and cannot resonate into the ring. Phenol can resonate into the ring. Does this mean in this case the inductive effect is stronger than the effect of the resonance??

Answer 1

In this specific example, the inductive effect might have a minor effect on the overall acidicity but it is not as significant as the types of resonance forms in each molecule. I imagine that it is that the combination of the types of resonance AND the inductive effect that make benzoic acid stronger, with resonance being the primary justification. The four factors when qualitatively assessing acid/base strength are (in order of importance) are atomic size (for the atom bearing the charge in the conjugate base), resonance, inductive effect, then orbital type (sp, sp2, sp3).

The carboxylic acid group is generally more acidic than an -OH group because of the resonance stabilization that occurs. The resonance within the carboxylate ion results in two equivalent resonance structures (which I believe is the key here) and the charge is delocalized onto the most electronegative atoms. While it helps that the negative charge on the conjugate base in phenol can be delocalized into the ring (and therefore on paper has more resonance forms than benzoic acid), it hurts the argument that the negative charge in many of the resonance forms ends up on carbon (a less electronegative element). Therefore, the resonance forms in phenol are not all equivalent in their contribution. Major contributing resonance forms typically have the charge on the most electronegative atom; that said, many of the resonance forms in phenol exist, but have minor contribution to the real molecule since the charges end up on carbon. Therefore the effects of resonance on stabilization are greater in benzoic acid than phenol.

I imagine that benzoic acid is still stronger due to the combination of BOTH resonance stabilization and inductive effect of the benzene ring. In terms of inductive effect, yes, the benzene ring is slightly electron withdrawing on its own when the conjugated pi system of the ring is left intact. Electrons are being pulled away from the carboxyl group, making the O-H bond in the carboxylic acid weaker than normal (therefore more acidic).

To address the inductive effect in phenol, yes the ring is also electron withdrawing before the -O-H deprotonation occurs (making it more acidic than a regular alcohol). Once deprotonation occurs and resonance stabilization through the ring happens, the inductive effect is significantly weakened due to the disruption of the conjugation pi system in the ring.

Answer 2

Carboxylic acids, as the name implies, are stronger acids than alcohols because the corresponding base is more stable. The resonance between the oxygens in this case are more important than any interaction with the ring. This has to do with the electronegativity of the oxygen in phenol relative to the carbons, and the fact that resonance in the phenoxide ion comes at the expense of aromaticity.