Bond Length in Resonance Structures

[Arya Bhalla 1H]

In class today we learned that the C-C bond in C6H6 is 1.39 angstroms long. However, a C-C double bond is 1.33 angstroms while a C-C single bond is 1.54 angstroms. Why is the length of the C-C bond closer towards the C-C double bond instead of simply being the average of the single and double bond?

[Mikayla 2G]

This is a good question, I was also wondering this. Potentially it is because the distance is not a linear equation, but I don't know exact reasoning

[Rachel Fox - 3F]

The length of the C-C bond in benzene is closer to the C-C double bond due to hybridization of the atoms. The C-C bond has sp2 hybridization, which means there is one sigma and 2 pi bonds. The overlap of these sp2 orbitals leads to the bond length being somewhere between C-C and C=C bond length. The atoms are most stable at that bond length as well, so the length of 1.39 angstroms would be the bond length. I also think that the delocalized electrons involved in pi bonding across the benzene ring could contribute to the bond length being closer to the double bond length, however, I am not certain. Hope this helps!

[Reagan Feldman 1D]

To add, we saw that the C-C single bond was a larger value because the single bond is longer and weaker. The double bond between C-C was a smaller value because it is a shorter and stronger bond. Double bonds pull the electrons from the two elements in closer, therefore decreasing the bond length.