Question

why the c-c bond distance in benzene is unusual

Answer 1

The C−CC−C bonds in benzene are 140 pm140 pm long which puts them in between single bonds and double bonds (given at 147−154 pm147−154 pm and 134−135 pm134−135 pm respectively, sourced from here and here) as expected. However, since the two resonance structures for benzene contribute equally, leading to six identical bonds, one might expect that the bond lengths would be exactly intermediate between a single and a double bond, but in fact the bond lengths seem to be closer to a double bond than a single bond.