Question

Toluene on reaction with n-bromosuccinimide gives (a) phenyl bromomethane (b) o-bromomethyl benzene (c) p-bromomethyl benzene (d) m-bromomethyl benzene

Short answer

The product is (a) phenyl bromomethane.

Step-by-step solution

Identify the Reaction

This problem involves the reaction of toluene with N-bromosuccinimide (NBS). NBS is commonly used in organic chemistry for selective bromination reactions, especially in the allylic or benzylic position of aromatic compounds.

Locate the Benzylic Position

Toluene has a methyl group attached to the benzene ring. The benzylic position, which is the carbon directly attached to the benzene ring in the methyl group, is the primary site of bromination with NBS.

Understand the Reaction Specificity

NBS preferentially brominates the benzylic carbon due to its stability and position. Therefore, the reaction will result in the substitution of a hydrogen atom in the methyl group of toluene with a bromine atom.

Determine the Product Structure

The substitution reaction converts the methyl group (H_3 ight)) to a bromomethyl group (BrH_2 ight)). This results in the formation of benzyl bromide (_6H_5CH_2Br ight)), which is referred to as phenyl bromomethane.

Key concepts

Benzylic Bromination

One key aspect of this exercise is understanding benzylic bromination. In organic chemistry, bromination involves the addition of a bromine atom to an organic molecule. When it comes to benzylic bromination, the focus is on the special position known as the benzylic position. This is the position directly attached to the aromatic ring of a benzene, usually occupied by a methyl group as in toluene.

The benzylic position is particularly reactive due to the stability provided by the resonance of the aromatic ring. It allows for efficient free radical reactions. This stability makes benzylic bromination a selective process. By targeting the benzylic position, we can replace a hydrogen atom with a bromine atom, forming a new product.

N-bromosuccinimide (NBS)

N-bromosuccinimide (NBS) is a reagent frequently used for bromination in organic chemistry. One of its prime features is its ability to selectively brominate at allylic or benzylic positions. This characteristic makes it less reactive in harsher conditions, thus preventing undesirable brominations in other positions of the molecule.

NBS is often used with initiators, like radical initiators, to proceed with the reaction. It acts through a radical mechanism, continuously generating brominating agents in the vicinity of the target site. Consequently, it provides precise control over the bromination process. Using NBS simplifies the procedure while minimizing side reactions, particularly in the presence of a double-bonded structure like benzene.

Reaction Mechanisms

Understanding the reaction mechanisms of benzylic bromination with NBS can greatly help in grasping the entire process. The reaction typically follows a free radical chain mechanism, which involves three key steps:

• Initiation: Radicals are generated from NBS and the initiator, often involving light or heating to commence the process.
• Propagation: These radicals react with the benzylic hydrogen of toluene, forming a new radical species and liberating hydrogen bromide (HBr).
• Termination: Radical species combine to form stable compounds, or the reaction continues to propagate with additional NBS until complete.

This chain mechanism is effective due to its self-sustaining ability, allowing the reaction to proceed smoothly without needing excess reagent amounts. Observing these mechanisms illuminates how selective the bromination process can be under controlled conditions.

Substitution Reactions

Substitution reactions are fundamental in organic chemistry, where one atom or group of atoms in a molecule is replaced with another. In the context of this exercise, the NBS-mediated reaction of toluene is a radical substitution reaction.

Typically, substitution reactions can be classified as nucleophilic or electrophilic, but when radicals are involved, it's a bit different. Here, the hydrogen of the methyl group in toluene is substituted by a bromine atom. This process takes advantage of the stabilization offered by the aromatic benzene ring.

The most notable outcome is the formation of benzyl bromide from toluene, known also as phenyl bromomethane. This reaction serves as a great example of how substitution reactions modify molecules to produce valuable derivatives in organic synthesis.