Q4-57E - Organic Chemistry (Mcmurry)

Question

The German chemist J. Bredt proposed in 1935 that bicycloalkenes such as 1-norbornene, which have a double bond to the bridgehead carbon, are too strained to exist. Explain.

Step-by-Step Solution

Answer

When a model of 1-norbornene is made, ${\mathrm{sp}}^2$ hybridization is not possible since the double bonded carbon atoms are not in the same plane. 

Step 1: sp 2 hybridization in the compound

In the structure of 1-norbornene, there is a $\mathrm{\pi }$ bond which shows that the carbon atoms have ${\mathrm{sp}}^2$ hybridization. But when a model of the same compound is built, the starred carbon atoms must be in the same plane for the $\mathrm{\pi }$ bond to occur. But these carbon atoms are out of plane. 

Step 2: Bridgehead double bonds

The bridgehead double bonds cannot be made in the model of 1-norbornene since the bicyclic ring is not in the same plane. 

Thus 1-norbornene having bridgehead double bond does not exist.