The dipole moment of nitrobenzene (a) is 3.95 D, while that of 2,6-dimethylnitrobenzene (b) is 3.55 D.
(a) (b)
What best explains the lower dipole moment of (b)?

Question

The dipole moment of nitrobenzene (a) is 3.95 D, while that of 2,6-dimethylnitrobenzene (b) is 3.55 D. (a) (b) What best explains the lower dipole moment of (b)?

TestHub · Accepted Answer

Part of nitrobenzene's dipole comes from resonance structures that place positive charge on the ring and extra negative charge on the oxygens. In (b) the ortho methyls twist the N O 2 ​ group out of the ring plane, this resonance contribution shrinks (steric inhibition of resonance), and the dipole moment falls from 3.95 D to 3.55 D.

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