Answer to Question #168244 in Organic Chemistry for Sam

Question

• Explain the accepted structure of benzene in terms of hybridisation, sigma and pi bonding, and delocalisation of electrons. Include examples of evidence (such as infrared, x-ray and thermochemical analysis) that support the structure of benzene as being very stable and regularly shaped. Include within your explanation why this evidence also disproves the originally suggested Kekulé structure for benzene, including reference to the bond lengths of C-C, C=C and those within benzene

Answer 1

In benzene, all of the carbon-carbon bond lengths are equal. Therefore, the Kekule structure shown below is an incorrect representation of benzene.

It is incorrect because it suggests that there are two different types of carbon-carbon bonds in benzene, a carbon-carbon double bond and a carbon-carbon single bond. Typically, the bond length in a carbon-carbon double bond is around 135 pm, a carbon-carbon single bond is around 147 pm long, while the carbon-carbon bond length in benzene is roughly in between at 140 pm.

Benzene is often drawn as follows to indicate that all of the carbon-carbon bonds have the same length and the pi electron system is continuous and uniformly distributed.