Question

Why are all the carbon-carbon bonds in naphthalene not equal in length?

Answer 1

The carbon-carbon bond lengths in naphthalene are not equal because as an aromatic hydrocarbon, naphthalene'sstructure consists of a fused pair of benzene rings. Hence , it consists of alternating single and double bonds with different bond lengths.

Answer 2

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When we first think about it, we would think that resonance implies equal bond lengths. But, it has been shown by experiments such electron-density-deformation and X-ray structures (after many years of debate on this) that the bond angles and bond lengths during Benzene ring annulation change. This is called the Mills-Nixon effect.

When benzene rings anneal to form a bicyclic compound as in naphthalene, orbitals at the juction rehybridize under stearic strain, localizing the bond and electron density. Hence, bond 1 and 2 have higher bond order and less anti-bonding charectaristics. Bond 3 is not affected by strain and has a lower bond order and higher bond length. Bond 4, though unhindered by bond strain, has ring strain due to unequal bond lengths at 1,2 and 3. To counteract ring strain, it has more bonding characteristics and smaller bond length (this is better explained by wave-mechanical calculations by Linus Pauling).

Because of a electron localization and ring strain, bond 3 has a higher bond length compared to 1,2 and 4.