Describe why the hydroxyl group attached to an aromatic ring is more acidic than the hydroxyl group attached to an alkyl group. How does the presence of a nitro group in phenol affect its acidic character?
Solution
Ionisation is governed by the anion's stability.
Ar–OH → ArO: the negative charge delocalises into the ring (o/p positions) — stabilised, so aromatic –OH ionises more readily.
R–OH → RO: charge stays on O and the alkyl +I effect intensifies it — ionisation resisted.
A nitro group (−I, −M), particularly at o/p, pulls the phenoxide charge onto its own oxygens, stabilising the anion further — so nitrophenols are much stronger acids than phenol (2,4,6-trinitrophenol is almost a mineral acid).
Ar–OH → ArO: the negative charge delocalises into the ring (o/p positions) — stabilised, so aromatic –OH ionises more readily.
R–OH → RO: charge stays on O and the alkyl +I effect intensifies it — ionisation resisted.
A nitro group (−I, −M), particularly at o/p, pulls the phenoxide charge onto its own oxygens, stabilising the anion further — so nitrophenols are much stronger acids than phenol (2,4,6-trinitrophenol is almost a mineral acid).
CBSE Class 12 Board · 2004 · Foreign · Subjective