Question
Why carbon-carbon distance in benzene is intermediate between carbon-carbon single and double bond?
Solution
Each carbon atom of benzene is sp2 hybridised. The unhybridised p-orbital of each carbon atom overlaps to a small but equal extent with the unhybridised p-orbitals of both the adjacent carbon atoms. This leads to the formation of molecular orbital embracing all the six carbon atoms. Due to the delocalisation of 
–electron clouds above and below the plane of the benzene ring, all the carbon-carbon bonds acquire slight double bond character. Thus carbon-carbon bond length in benzene lies in between C–C and C=C bond length.
Standard C–C bond length = 154 pm
Standard C=C bond length = 134 pm
But C–C bond length in benzene = 139 pm.
