Problem 47 - A Complete Resource Book in Chemistry for JEE Main

Question

Toluene when refluxed with \(\mathrm{Br}_{2}\) in the presence of light mainly gives (a) p-bromotoluene (b) benzyl bromide (c) o-bromotoluene (d) mixture of o- and p-bromotoluene

Step-by-Step Solution

Answer

(b) benzyl bromide

Step 1: Understanding the Reaction

Toluene is an aromatic hydrocarbon that reacts with bromine, usually in the presence of catalysts or light. The presence of light suggests that this is a free radical substitution reaction rather than electrophilic aromatic substitution.

Step 2: Identifying the Reaction Type

When toluene is exposed to bromine in the presence of light, the reaction favors a side-chain halogenation due to the formation of free radicals. This means the bromine atom will attach to the carbon adjacent to the benzene ring rather than the aromatic ring itself.

Step 3: Predicting the Major Product

In free radical bromination, the most stable radical is formed, which in case of toluene is the benzyl radical. A bromine atom attaches to the benzylic position, forming benzyl bromide. This is because the benzylic radical is stabilized by resonance with the aromatic ring.

Key Concepts

Free Radical Substitution

Free radical substitution is a type of organic chemistry reaction where a free radical, usually a halogen, replaces a hydrogen atom in an organic molecule. In this process, bonds are broken and formed through one-electron movements. A common example is the halogenation of hydrocarbons like toluene. The reaction typically requires heat or light to initiate since energies are needed to generate reactive free radicals.
This reaction proceeds through three main steps: initiation, propagation, and termination.

• Initiation: Light or heat causes the homolytic cleavage of a halogen molecule ( ext{X}_2) into two halogen radicals.
• Propagation: A halogen radical abstracts a hydrogen atom from the hydrocarbon, forming a new radical and HX. The organic radical then reacts with another halogen molecule to form the final halogenated product and regenerate the radical.
• Termination: Two radicals combine to form a stable molecule, ending the chain reaction.

Aromatic Hydrocarbons

Aromatic hydrocarbons are compounds that contain one or more benzene rings, which are known for their stability derived from delocalized \( ext{π}\) electrons. Each carbon in the ring is sp² hybridized, allowing for an even distribution of electrons through resonance. Toluene is a common example, consisting of a benzene ring with a methyl group attached.
This unique structure gives aromatic hydrocarbons distinct chemical properties, such as the ability to undergo electrophilic substitution reactions more readily than non-aromatic hydrocarbons. However, under certain conditions like the presence of light, reactions may occur at the substituent side chains rather than the ring itself.

• The aromatic ring imparts stability to radicals formed at the benzylic position, making such positions more reactive in free radical processes.
• Benzene's resonance stabilization ensures that direct substitution on the ring happens through different mechanisms compared to those on the side chain.

Halogenation

Halogenation is the reaction where halogen atoms are introduced into an organic compound. In aromatic hydrocarbons like toluene, halogenation can occur under two distinct strategies: one affects the aromatic ring, and the other the side chain.

• Side Chain Halogenation: In the presence of light, as seen with toluene and \( ext{Br}_2\), the bromine preferentially attacks the benzylic hydrogen. This is due to free radical formation, making benzyl bromide the primary product.
• Aromatic Ring Halogenation: This typically requires a catalyst and results in substituting a hydrogen on the ring with a halogen.

It is crucial to differentiate between these types to predict the correct products in aromatic systems.

Benzyl Radical Stability

The stability of benzyl radicals plays a crucial role in free radical halogenation reactions. In such reactions, the benzylic position of a methyl group attached to a benzene ring becomes a favorable site for radical formation.
The key reason for this increased stability is resonance. The benzylic radical can resonate with the aromatic ring, allowing the unpaired electron's charge to be spread across several atoms rather than being localized. This delocalization minimizes energy, stabilizing the radical.

• The resonance stabilization of benzyl radicals makes reactions at the benzylic position more energetically favorable compared to primary or secondary alkyl radicals.
• This is why in reactions like the one involving toluene and bromine, benzyl bromide is the major product formed.

Understanding the principles of radical stability is key to mastering substitution reactions, especially in the context of organic synthesis.