Explain the following observations:
i) The boiling point of ethanol is higher than that of methoxymethane.
ii) Phenol is more acidic than ethanol.
iii) o- and p-nitrophenols are more acidic than phenol.
i) The boiling point of ethanol is higher than that of methoxymethane.
ii) Phenol is more acidic than ethanol.
iii) o- and p-nitrophenols are more acidic than phenol.
Solution
(i) Ethanol molecules associate through O–H···O hydrogen bonds; methoxymethane, with no O–H, has only weak dipole forces — so ethanol (351 K) boils far above its isomer (249 K).
(ii) Phenoxide spreads its charge over the ring; ethoxide concentrates it on O (worsened by +I of ethyl) — phenol ionises, ethanol barely does.
(iii) A nitro group at o/p withdraws electrons (−I, −M) and carries the phenoxide's negative charge onto its own oxygens through resonance — the anion is extra-stable, so o-/p-nitrophenols ≫ phenol in acidity.
(ii) Phenoxide spreads its charge over the ring; ethoxide concentrates it on O (worsened by +I of ethyl) — phenol ionises, ethanol barely does.
(iii) A nitro group at o/p withdraws electrons (−I, −M) and carries the phenoxide's negative charge onto its own oxygens through resonance — the anion is extra-stable, so o-/p-nitrophenols ≫ phenol in acidity.
CBSE Class 12 Board · 2009 · All India · Subjective