Question

The major product formed when methylenecyclohexane is treated with NBS in dichloromethane is 1-(bromomethyl)-cyclohexene. Account for the formation of this product.

Short answer

Answer: The major product formed is 1-(bromomethyl)-cyclohexene. This product is formed because NBS acts as an electrophilic bromine source, generating Br· radicals that attack the nucleophilic allylic carbon of methylenecyclohexane. The reaction proceeds through a free-radical mechanism involving allylic radical and allylic carbocation intermediates, ultimately resulting in the formation of the brominated product with syn stereochemistry.

Step-by-step solution

Understand the reactants and reaction conditions

First, let's familiarize ourselves with methylenecyclohexane, NBS, and dichloromethane. Methylenecyclohexane is a cyclohexene molecule with a double bond between two carbon atoms and a CH2 group attached to one of the carbons. NBS is an electrophilic bromine source that can generate Br· radicals, and it is commonly used for allylic or benzylic bromination reactions. Dichloromethane (DCM) is a common organic solvent that does not react with the other reactants and provides a suitable environment for the reaction to occur.

Identify the electrophilic species

In this reaction, NBS acts as the electrophilic species, as it can produce Br· radicals which can attack the π system of methylenecyclohexane.

Identify the nucleophilic species

Methylenecyclohexane has an alkene functionality, making the double bond the nucleophilic species. Specifically, the allylic carbon next to the CH2 group is the most reactive site due to the stabilization of the formed intermediates.

Understand the mechanism of the reaction

The reaction proceeds through a free-radical mechanism. First, NBS generates Br· radicals via homolytic cleavage of the Br-Br bond. Then, the Br· radical attacks the allylic carbon of methylenecyclohexane to form an allylic radical intermediate. Next, another molecule of NBS reacts with the allylic radical, ultimately forming an allylic carbocation intermediates. Finally, this carbocation is attacked by the Br- ion, resulting in the formation of 1-(bromomethyl)-cyclohexene.

Stereochemistry of the product

The bromine in the product can have two possible stereochemistries: syn addition (bromine is added to the same face of the double bond) or anti addition (bromine is added to the opposite face of the double bond). The experimental observation is that 1-(bromomethyl)-cyclohexene is formed as the major product, suggesting a preferred stereochemistry. In this case, the observed product adopts a syn stereochemistry, where bromine is added to the less hindered face of the double bond, in line with the formation of more stable intermediates along the reaction pathway.