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Which of the following statements about dipole moments is INCORRECT?

1. mu(chlorobenzene) < mu(1,3-dichlorobenzene)
2. mu(1,4-dichlorobenzene) = mu(1,3,5-trichlorobenzene)
3. mu(CCl4) = mu(CH4)
4. mu(para-dihydroxybenzene) > mu(para-dicyanobenzene)

Correct answer: mu(para-dihydroxybenzene) > mu(para-dicyanobenzene)

Solution

A: mu(ClBz) ~ 1.7 D; for 1,3-diClBz the two C-Cl dipoles add at 60 deg giving resultant ~ 2*1.7*cos30 ~ 2.9 D. TRUE (ClBz < 1,3-diClBz). B: Both para-1,4-diClBz and 1,3,5-triClBz have zero net dipole by symmetry. TRUE (both = 0). C: CCl4 and CH4 are both perfectly tetrahedral; net mu = 0 for each. TRUE. D: Both para-dihydroxybenzene and para-dicyanobenzene are centrosymmetric; net mu = 0 for both. The statement '>' is INCORRECT — they are equal.

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