Question

Which of the following reagents is employed to convert toluene to benzyl bromide? (A) \(\mathrm{Br}_{2}, \mathrm{AlBr}_{3}\) (B) \(\mathrm{PBr}_{3}\) (C) \(\mathrm{Br}_{2}, \mathrm{FeBr}_{3}\) (D) NBS

Short answer

The correct reagent to convert toluene to benzyl bromide is (D) NBS, as it can selectively brominate the benzylic position of toluene.

Step-by-step solution

Understand the structures of the starting and target compounds and known reagent properties

Toluene has the chemical formula C6H5CH3, where benzyl bromide has the formula C6H5CH2Br. The main difference between these two compounds is that we need to substitute the hydrogen on the methyl group of toluene with a bromine atom. Now let's analyze each given reagent: (A) \(\mathrm{Br}_{2}, \mathrm{AlBr}_{3}\): This combination is used for electrophilic aromatic substitution (EAS) reactions, which typically results in bromination of the aromatic ring. (B) \(\mathrm{PBr}_{3}\): This reagent is used to convert alcohols (R-OH) to alkyl bromides (R-Br). Since toluene does not have an alcohol group, it cannot convert toluene to benzyl bromide. (C) \(\mathrm{Br}_{2}, \mathrm{FeBr}_{3}\): This combination is also used for electrophilic aromatic substitution (EAS) reactions, just like the option A. It will brominate the aromatic ring, not the methyl group. (D) NBS: N-bromosuccinimide (NBS) is a reagent known for its ability to brominate allylic and benzylic positions selectively.

Identify the correct reagent

Out of the given options, only NBS (option D) can selectively brominate the methyl group of toluene (benzylic position). Therefore, NBS is the correct reagent used for converting toluene to benzyl bromide. So the correct answer is (D) NBS.

Key concepts

Electrophilic Aromatic Substitution (EAS)

Electrophilic aromatic substitution is a crucial reaction in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. It's widely employed in synthesizing diverse aromatic compounds. The process begins with the attack of an electrophile on the aromatic ring, which temporarily disrupts the pi system.

This generates an intermediate called a sigma complex or arenium ion, where the aromaticity is lost. The system quickly restores aromaticity by losing a proton, returning to a stable structure with the new electrophile attached. Typical electrophiles include halogens with a Lewis acid catalyst like FeBr3 or AlCl3, nitronium ions for nitration, and sulfonic acid groups for sulfonation.

However, for toluene's transformation to benzyl bromide, EAS isn't the right pathway since it would target the ring, and we need to modify the methyl group, which is a benzylic position.

Benzylic Bromination

Benzylic bromination is a specific type of halogenation that occurs at the benzylic position—the carbon atom directly adjacent to an aromatic ring. This reaction is particularly important when modifying the properties of aromatic compounds by transforming the benzylic H atoms.

It's distinct from aromatic bromination; the reactivity is due to the resonance stabilization of the intermediate radicals when a hydrogen atom is abstracted from the benzylic carbon. N-bromosuccinimide (NBS) is a classic reagent for this purpose, providing a low concentration of bromine that preferentially reacts with the benzylic position.

In the context of the exercise, NBS is the perfect reagent for converting toluene to benzyl bromide, as it will not brominate the ring but specifically the methyl group, thanks to the benzylic activation.

Functional Group Conversion

Functional group conversion is an integral part of organic synthesis, allowing chemists to alter the structure and reactivity of organic molecules. It involves changing one functional group into another to achieve the desired chemical properties or reactivity.

This strategy is a key step to piece together complex molecules from simpler ones. For instance, alcohol groups can be transformed into halides using reagents like PBr3 or SOCl2, carboxylic acids into esters using alcohols under acidic conditions, and amines into diazonium salts using nitrous acid.

Each transformation requires a specific reagent that will target only the intended functional group while leaving the rest of the molecule intact. This precision is a cornerstone of organic synthesis, reflected in the textbook problem where toluene's methyl group is specifically targeted without affecting the aromatic ring.