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In which of the following species are the C-C single bond and C=C double bond lengths NOT equal (i.e., bond lengths are unequal)?

1. Benzene
2. 1,3-butadiene
3. 1,3-cyclohexadiene
4. Graphite

Correct answer: 1,3-butadiene

Solution

Benzene and graphite have fully delocalized pi systems making all C-C bonds equal in length. 1,3-butadiene is an open-chain conjugated diene with distinct single (about 1.47 A) and double (about 1.34 A) bonds, so its bond lengths are unequal.

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