Q2(b). Acidity of dimethyl-4-nitrophenols: steric inhibition of resonance

Introduction. A para-nitro group raises the acidity of phenol (p-nitrophenol pKₐ 7.14) because, besides its -I effect, it delocalises the negative charge of the phenoxide onto its oxygens (-R). The -R effect needs the NO₂ group to be coplanar with the ring.

Scheme: Methyls flank NO₂ in the 3,5-isomer and flank OH in the 2,6-isomer.

1. In 3,5-dimethyl-4-nitrophenol the two methyl groups are ortho to NO₂. Their bulk forces the NO₂ group to twist out of the ring plane. The p orbitals of N and ring carbon are no longer parallel, so the phenoxide charge cannot be placed on the nitro oxygens: steric inhibition of resonance. Only the -I effect remains.
2. In 2,6-dimethyl-4-nitrophenol the NO₂ has H atoms beside it and stays planar; the phenoxide is stabilised by the quinonoid resonance form with the charge on NO₂.
3. The methyls of the 2,6-isomer sit beside O⁻; their weak +I effect and slight hindrance to solvation lower acidity only a little.
4. Hence the -R effect, the larger of the two, is lost only in the 3,5-isomer, which is about as weak an acid as a phenol with only a -I substituent.

Conclusion. 2,6-Dimethyl-4-nitrophenol is the stronger acid: its NO₂ group is coplanar and delocalises the phenoxide charge. In 3,5-dimethyl-4-nitrophenol the flanking methyls twist NO₂ out of plane (steric inhibition of resonance).

Application. Steric inhibition of resonance also explains why N,N-dimethyl-2,6-dimethylaniline is a stronger base than N,N-dimethylaniline and why o-substituted benzoic acids are stronger acids (ortho effect).