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

Question: Explain the formation of 1-(bromomethyl)-cyclohexene as the major product when methylenecyclohexane is treated with NBS in dichloromethane. Answer: The formation of 1-(bromomethyl)-cyclohexene involves the selective bromination of the allylic position by NBS. In this reaction, NBS generates a low concentration of molecular bromine. A bromine atom abstracts an allylic hydrogen from methylenecyclohexane to form a carbon-centered radical. This allylic radical reacts with a molecule of bromine to form the brominated product and regenerate a bromine atom. The reaction is favored due to the stability of the allylic radical intermediate.

Step-by-step solution

Write down the given reaction

Methylenecyclohexane reacts with NBS in the presence of dichloromethane to form 1-(bromomethyl)-cyclohexene as the major product.

Examine the initial compound

Methylenecyclohexane consists of a cyclohexene ring, with an additional methylene (-CH2-) group connected to one of the carbons of the double bond in the ring.

Recognize the role of NBS in the reaction

NBS is used to selectively brominate allylic and benzylic positions in substrates, by generating a low concentration of molecular bromine (Br2). In this reaction, the allylic position is where the bromine atom will be added.

Understand the reaction mechanism

The mechanism for the bromination of methylenecyclohexane involves the following steps: 1. Formation of molecular bromine: NBS forms a low concentration of molecular bromine (Br2) in the presence of a radical initiator, like light (hv) or heat. \[ NBS \xrightarrow{hv} Br_2 \] 2. Initiation: A bromine atom abstracts an allylic hydrogen from methylenecyclohexane to form a carbon-centered radical. Since the methylene group is adjacent to the double bond, its hydrogen is allylic. \[ Br + CH_2CH(CH_2)_2CH=CH_2 \rightarrow CH_2CH(CH_2)_2CH=CH \cdot{} + HBr \] 3. Propagation: The allyl radical formed in the previous step reacts with a molecule of bromine to form the brominated product and regenerate a bromine atom. \[ CH_2CH(CH_2)_2CH=CH \cdot{} + Br_2 \rightarrow CH_2CH(CH_2)_2CH=CHBr + Br \] 4. Termination: The bromine radical reacts with another molecule of NBS, which stops the chain and forms the final product. \[ Br + NBS \rightarrow BrNBS \]

Recognize the formation of the major product

The reaction leads to the formation of 1-(bromomethyl)-cyclohexene because the bromine atom is added to the methylene group, which is connected to the carbon of the double bond in the cyclohexene ring. This reaction is favored due to the stability of the allylic radical intermediate.

Key concepts

Allylic Bromination

Allylic bromination is a selective chemical reaction that targets the allylic position, which is the carbon atom next to a carbon-carbon double bond in an alkene. It is a form of halogenation specific to bromine and is important due to the high reactivity of allylic carbons.

In the case of the reaction between methylenecyclohexane and NBS, the presence of an allylic methylene group makes it particularly susceptible to bromination. The process takes advantage of the resonance stability of the allylic radical, which is formed during the reaction. The resonance allows the positive charge to be distributed over several carbons, leading to increased stability and thus, a more favorable reaction pathway.

N-bromosuccinimide (NBS)

N-bromosuccinimide, commonly known as NBS, is a brominating agent used in organic chemistry for its ability to selectively add bromine atoms to allylic and benzylic positions.

NBS is particularly useful because it generates a low, steady concentration of the bromine radical upon activation, often by heat or light. This minimizes the risk of over-bromination or unwanted side reactions, making it an excellent choice for controlled halogenation. In the exercise, NBS, when activated, forms molecular bromine which in turn creates bromine radicals required for the allylic bromination to occur.

Reaction Mechanisms

Understanding reaction mechanisms is crucial for predicting and explaining the outcomes of chemical reactions. A reaction mechanism details the step-by-step sequence of elementary reactions that lead to the overall transformation of reactants into products.

In the provided exercise, the mechanism proceeds through initial generation of bromine radicals, abstraction of an allylic hydrogen to form a radical intermediate, then propagation of the reaction with another bromine molecule, and finally, termination when a bromine radical reacts with a fresh dose of NBS. Each step follows specific electron shifts and bond-breaking/forming processes that define the pathway from reactants to products.

Radical Halogenation

Radical halogenation is a chemical reaction where a halogen atom is introduced into an organic molecule via free radical intermediates. This type of halogenation involves breaking and forming bonds through the propagation of radical species, which are highly reactive due to an unpaired electron.

Factors that influence radical halogenation include the presence of radical initiators like heat or light, and the type of halogen involved, as each halogen has a different reactivity profile. Bromine, being a relatively large atom with a low bond dissociation energy, is often used because it forms relatively stable radicals that can add to carbon atoms with precision, offering a high degree of specificity in reactions like the one demonstrated in the exercise.