Question

Question: The carbon–carbon bond lengths in naphthalene are not equal. Use a resonance argument to explain why bond (a) is shorter than bond (b).

Short answer

Answer

Bond (a) has more double bond character, making it shorter than bond (b).

Step-by-step solution

Bond length

Several bond parameters can be utilized to describe covalent bonds, and one among them is the bond length. Single bonds are considered to have shorter bond lengths than double bonds.

Resonance 

Electron delocalization can be explained with the help of certain Lewis structures, which are designated as resonance structures. A resonance hybrid is the resultant of several resonance structures.

Comparison of bond (a) and bond (b) in naphthalene

Naphthalene comprises three resonance structures, and they can be given as:

Resonance structures of naphthalene

In two of the resonance structures of naphthalene, bond (a) is a double bond, and bond (b) is a single bond. The more is the double bond character, the shorter is the bond length. Hence, bond (a) is shorter than bond (b).