Question

Look at the five resonance structures for phenanthrene (Problem 15-26), and predict which of its carbon-carbon bonds is the shortest.

Short answer

Shorter bond at 6 and 7 carbon

Step-by-step solution

Characteristic of shorter bond

The carbon-carbon double bond that is present in most of the resonance structures of phenanthrene is the bond that shows more double bond character. This bond corresponds to a shorter bond compared to other double bonds.

Shorter bond in Phenanthrene

As mentioned in the diagram, the double bond between carbon 6 and 7 is present in four resonance structures out of five. Thus, this bond has a more double-bond character and is shorter than the other carbon bonds of phenanthrene. Therefore, the resonance structures showing the shorter bond is depicted as follows:

Shorter bond at 6 and 7 carbon