In benzene and 1,3-cyclohexadiene, which one has greater C=C bond length?

Question

In benzene and 1,3-cyclohexadiene, resonance will occur, so both will have a partial pi character. Technically, all bonds in benzene have partial pi character, whereas in 1,3-cyclohexadiene, some have partial pi character and the rest are pure sigma. So considering this, 1,3-cyclohexadiene must have a greater bond length. But the answer in the book says benzene.

Answer 1

It would be interesting to know the sources cited in your question (both the book, as well as the primary literature) as reference because one complementary approach to the (assumed) spectroscopic determination is the analysis of the crystalline state. While there are multiple records about pure benzene, it seems there is no public model of the crystalline structure of pure 1,3-cyclohexadiene ; on the other hand, there are multiple records where the two motifs are present simultaneously.

From those with little geometric distortion by organometallic bonding, a framework of 1,4-benzenedicarboxylates with incorporated (guest molecules) of 1,3-cyclohexadiene nevertheless appears instructive:

(CCDC entry IXODUV, based on Wang et al. in 2011InorgChem2028)

For the moderately distorted 1,3-cyclohexadiene, the intermolecular distances are $\pu{1.35 A}$, $\pu{1.45 A}$, and $\pu{1.33 A}$ along the sequence of double, single, double bond $\ce{C=C-C=C}$. In the p-terephthalate, the model describes all aromatic $\ce{C\bond{~-}C}$ bonds with $\pu{1.38 A}$. Despite the constraints of the structure model, it is an example for the difference of aromatic $\ce{C\bond{~-}C}$ bonds from conjugated double bonds.

In the context of your question, the paper is instructive because the framework of the host, the p-terephthalate, chemically stays the same and with only minor changes of the lattice constants if it is loaded with guest molecules of benzene, 1,3-cyclohexadiene, 1,4-cyclohexadiene, cyclohexene, or cyclohexane.