What are 3 characteristics of Benzene that go against the proposed Kekule model?

The Kekule model states Benzene consists of 3 pairs of alternating double C=C and single C-C bonds. A typical C=C bond would readily react with bromine to produce a dibromo-alkane and therefore would react with bromine water (bromine gas dissolved in water) to produce a colour change from brown to colourless. Benzene however doesn’t react with bromine water, producing no colour change and require much more forcing conditions, such as heat and UV light or a halogen carrier, to react.

Benzene’s bond lengths are also all equal and have a length of (139pm), thereby being in-between that of a typical C=C double bond’s length (134pm) and a C-C single bond’s length(154pm). The final characteristic of Benzene would be it’s lower than expected hydrogenation enthalpy (enthalpy of reaction with hydrogen gas). A typical C=C bond would have a hydrogenation enthalpy of 120 kJmol-1 and therefore 3x C=C bonds would produce a hydrogenation enthalpy of 360 kJmol-1, whereas benzene’s hydrogenation enthalpy is less at 208 kJmol-1.

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Answered by Chris G. Chemistry tutor

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