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Steric inhibition of resonance occurs in which of the following? Consider: A: a ring bearing -NMe2 flanked by two ortho methyl groups B: a ring bearing -NH2 flanked by two ortho methyl groups C: a ring bearing -SO3H flanked by two ortho methyl groups D: a ring bearing -NO2 flanked by two ortho methyl groups E: a ring bearing -Br with one para methyl group

1. In A and B only
2. In A, B, C and E
3. C only
4. In A only

Correct answer: In A only

Solution

Steric inhibition of resonance requires a substituent large enough that flanking ortho groups twist it out of the ring's plane, breaking conjugation. The bulky -NMe2 group between two ortho methyls is forced out of plane (A). The small -NH2 (B) is not twisted appreciably, and the other cases either lack the geometry or the bulk to lose conjugation.

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