Chapter 12, Problem 12.1P

Interpretation Introduction

Interpretation:

The complete, detailed mechanism for the given reaction is to be drawn, and the major product of the reaction is to be predicted.

Concept introduction:

A carbene is a highly reactive species containing a carbon atom that has two bonds and a lone pair of electrons. Carbenes are electron-poor due to the lack of an octet and are ${\text{sp}}^{\text{2}}$ hybridized. The lone pair of electrons occupies a hybrid orbital, leaving the p orbital empty. Thus, these carbenes resemble carbocations, except that they don’t have a formal charge. At the same time, the lone pair also gives it a nucleophilic character. Diazomethane, ${\text{CH}}_{\text{2}}{\text{N}}_{\text{2}}$, is one precursor from which a carbene can be made.

Alkenes react with carbenes to form a cyclopropyl ring. Carbene simultaneously forms two bonds to the initially double-bonded carbon atoms. The lone pair on the carbine carbon forms a bond with one of the double-bonded carbons. Simultaneously, the $\text{π}$ bond pair from the initial double bond forms a bond with the carbine carbon. This leads to the formation of a cyclopropyl ring.

Answer to Problem 12.1P

The complete, detailed mechanism and the major product of the reaction are shown below:

Explanation of Solution

The given reaction is

In the first step of the reaction mechanism, the ${\text{CH}}_{\text{2}}{\text{N}}_{\text{2}}$ bond breaks heterolytically on heating, to form the reactive carbene intermediate.

Next, the reactive carbene adds to the double bond, simultaneously forming bonds with the two carbons of the initial double bond. The lone pair from the carbine forms a bond with one carbon, while the $\text{π}$ bond pair of the double bond forms a bond between the other carbon and the carbine. This results in the replacement of the $\text{π}$ bond of the double bond by a bridging $-{\text{CH}}_{\text{2}}-$ and forms a cyclopropyl ring.

Thus the complete mechanism can be drawn as follows:

Conclusion

Carbenes add to alkenes to form a cyclopropyl ring in place of the $\text{π}$ bond.