Q34E - Organic Chemistry (Mcmurry)

Question

Bicyclohexadiene, also known as Dewar benzene, is extremely stable despite the fact that its rearrangement to benzene is energetically favored. Explain why the rearrangement is so slow.

Step-by-Step Solution

Answer

Benzene with a trans double bond would result from the thermal ring-opening. Compared to Dewar benzene, this would be even more challenging. Dewar benzene rearranges slowly as a result.

Step 1: Benzeneand Dewar Benzene, Relative Stabilities

Benzene is an extremely stable compound due to aromaticity (4n+2 π electron system with conjugation and planarity), while Dewar benzene is not aromatic as it doesn’t have a conjugated system. So, it was expected that the conversion of Dewar benzene to benzene should be quite fast. But the compound reverts to benzene with a chemical half-life of two days. This thermal conversion is relatively slow because it is symmetry forbidden based on orbital symmetry arguments.

Step 2: Why Conversion is Slow?

The Dewar benzene can get converted to Benzene by the opening of one of the 4 membered rings, which can take place as: 

Looking at this we can say that thermal opening would give benzene with a trans double bond. This would be even more strained than Dewar benzene. Thus, rearrangement of Dewar benzene is slow.