Alcohols, Phenols and Ethers: Edge Cases and Subtle Traps (3)

• $p$-nitrophenol has a strongly electron-withdrawing $-\text{NO}_2$ group at the para position. Withdraws electron density via both inductive ($-I$) and resonance ($-R$) effects.
• Phenol has a benzene ring. The phenoxide ion is stabilised by delocalisation into the ring.
• $p$-cresol has an electron-donating $-\text{CH}_3$ group at the para position. Pushes electron density into the ring ($+I$, $+H$ hyperconjugation).
• Ethanol is an aliphatic alcohol with no aromatic ring to stabilise the alkoxide.

Acidity tracks conjugate-base stability. The more stable the conjugate base, the more acidic the parent.

• $p$-nitrophenoxide: extra resonance with NO₂ delocalises the negative charge onto the nitro oxygens. Very stable.
• Phenoxide: charge delocalised over ring carbons. Moderately stable.
• $p$-cresoxide: methyl group destabilises (pushes electrons toward already-negative oxygen). Less stable than phenoxide.
• Ethoxide: no resonance, only $\sigma$-bonded carbon framework. Least stable.

$\text{decreasing acidity:}\quad p\text{-nitrophenol} > \text{phenol} > p\text{-cresol} > \text{ethanol}$

Approximate $\text{p}K_a$ values: $7.1, 9.9, 10.3, 16$.

The trap: students sometimes rank cresol more acidic than phenol, thinking “more carbons = more stable.” But in acid chemistry, electron-donating groups destabilise the conjugate base by adding negative charge to a position that’s already negative. So cresol is less acidic than phenol — the methyl group hurts, not helps.

The mirror-image trap is on basicity: amines with electron-donating groups are more basic, because they stabilise the positive conjugate acid. The same group has opposite effects on acid vs base strength. Always ask “which species needs to be stabilised?” — the conjugate.