Phenol acidity — why phenol is more acidic than alcohol but less than carboxylic acid

The decreasing order of acidity is:

$\text{p}K_a$ values: acetic acid $\approx 4.76$, phenol $\approx 10$, ethanol $\approx 16$.

Now we need to explain why this order holds. The key principle: acidity depends on how well the conjugate base is stabilised after losing $\text{H}^+$.

When ethanol loses $\text{H}^+$, we get $\text{C}_2\text{H}_5\text{O}^{-}$ (ethoxide ion). The negative charge sits entirely on the oxygen atom. There’s no resonance to spread it out — the alkyl group actually destabilises the charge through its electron-donating inductive effect ($+I$ effect).

Result: the conjugate base is poorly stabilised, so ethanol is reluctant to donate $\text{H}^+$.

When phenol loses $\text{H}^+$, we get the phenoxide ion ($\text{C}_6\text{H}_5\text{O}^{-}$). Here, the negative charge on oxygen is delocalised into the benzene ring through resonance. We can draw multiple resonance structures where the negative charge sits on ortho and para carbons of the ring.

This delocalisation spreads the charge over a larger area, making phenoxide much more stable than ethoxide. Hence phenol is a stronger acid than ethanol.

But the charge is spread over carbon and oxygen atoms that are not equivalent — oxygen is more electronegative, so it holds the charge better. The stabilisation is good, but not as good as what carboxylate ions achieve.

When acetic acid loses $\text{H}^+$, we get the acetate ion ($\text{CH}_3\text{COO}^{-}$). The negative charge is delocalised equally over two equivalent oxygen atoms:

$\text{CH}_3\text{COO}^{-}: \quad \text{O}^{-}\text{---C=O} \longleftrightarrow \text{O=C---O}^{-}$

This is the most effective resonance stabilisation among the three because:

• Both resonance structures are equivalent (same energy)
• The charge is shared between two electronegative oxygen atoms
• The two $\text{C-O}$ bonds become identical (bond length 1.27 A, between single and double bond)

This makes acetate ion extremely stable, so acetic acid gives up $\text{H}^+$ most easily.