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Considering the three canonical (resonance) structures of naphthalene, which statement about its carbon-carbon bond lengths is correct?

1. All C-C bonds are of the same length
2. The C1-C2 bond is shorter than the C2-C3 bond
3. The C1-C2 bond is longer than the C2-C3 bond
4. None of these

Correct answer: The C1-C2 bond is shorter than the C2-C3 bond

Solution

Of the three Kekule structures of naphthalene, the C1-C2 bond is double in two of them, while the C2-C3 bond is double in only one. Greater double-bond character gives C1-C2 more pi character and therefore a shorter length (about 1.36 A vs 1.42 A for C2-C3).

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