Question #bcaa2

Aug 5, 2016

Phenol is hydroxybenzene, which has a proton on the hydroxyl ($\text{OH}$) group. Its $\text{pKa}$ is about $9.9$, which makes it somewhat acidic.

When you deprotonate phenol, its anionic form has multiple resonance structures:

Because of the electron delocalization, the anion (conjugate base) is stabilized (relative to an aliphatic alcohol), and so the phenol molecule itself is more acidic than an aliphatic alcohol.

(The conjugate acid of a weaker base than an aliphatic alcohol is a stronger acid than an aliphatic alcohol)

For example, the $\text{pKa}$ of ethanol is about $15.9$, which means phenol is a stronger acid than ethanol.

Or, relative to benzene, the present $\text{OH}$ is an electron-donating group, which donates electron density into the aromatic ring, away from the proton.

That makes the hydroxyl proton more partially positive than in benzene, whose hydrogens are all belonging to the aromatic ring. That means it's more acidic, and so phenol is more prone to deprotonation than benzene.

As a note, the $\text{pKa}$ of benzene is about $43$, so that's a significant difference in acidity.