Problem 33 - A Complete Resource Book in Chemistry for JEE Main

Question

Chlorination of toluene in presence of light and heat followed by treatment with aqueous \(\mathrm{NaOH}\) gives (a) o-cresol (b) p-cresol (c) 2,4 -dihydroxytoluene (d) benzoic acid

Step-by-Step Solution

Answer

The main product is benzyl alcohol, which does not match the given options.

Step 1: Understand the Reaction Conditions

The chlorination of toluene in the presence of light and heat is a free radical reaction that predominantly affects the side chain (alkyl group) of toluene, leading to the formation of benzyl chloride. This occurs because the benzylic position is the most reactive due to resonance stabilization of the formed radical.

Step 2: Reaction with Chlorine

The toluene undergoes a free radical halogenation reaction, resulting in the substitution of a hydrogen atom in the methyl group of toluene with a chlorine atom, forming benzyl chloride (C6H5CH2Cl).

Step 3: Treatment with Aqueous Sodium Hydroxide (NaOH)

When benzyl chloride reacts with aqueous NaOH, a nucleophilic substitution reaction occurs. The chloride ion (
B[cin benzyl chloride is replaced by a hydroxide ion (OH-), forming benzyl alcohol (C6H5CH2OH).

Step 4: Analyze Product Formation

Benzyl alcohol does not match any of the given choices. In phenyl-ring-related reactions, NaOH can also undergo further reactions that are not directly related to the side chain. However, in this context, benzyl alcohol itself does not convert into any of the options given (a), (b), or (c); it cannot form cresols or dihydroxytoluene directly.

Key Concepts

Free Radical Halogenation

Free radical halogenation is a crucial concept to understand when studying organic reactions, especially those involving alkanes and aromatic compounds like toluene.
In the presence of light or heat, a halogen, such as chlorine, can initiate a chain reaction with hydrocarbons. This process involves three main steps: initiation, propagation, and termination.

• Initiation: This is where the diatomic chlorine molecule (asc Cl_2) is split into two reactive chlorine radicals. This is achieved through light or heat, which provides the energy needed to cleave the chlorine-chlorine bond.
• Propagation: In this step, the reactive chlorine radical abstracts a hydrogen atom from the methyl group in toluene, forming a new radical at the benzylic position. This radical then reacts with another chlorine molecule, resulting in benzyl chloride and regenerating the chlorine radical.
• Termination: The reaction can be terminated when two radicals collide and form a stable molecule, thus stopping the chain reaction.

In the case of toluene, this reaction predominantly targets the benzylic hydrogen due to the stability provided by resonance. This is how benzyl chloride is formed.

Nucleophilic Substitution Reaction

Following the formation of benzyl chloride, the next reaction involves a nucleophilic substitution. This type of reaction is vital in organic chemistry and involves the replacement of an atom or group (usually a halogen) by a nucleophile.
In the process involving benzyl chloride, the nucleophile is a hydroxide ion (aasc OH-).

Here is what happens:

• The nucleophile, OH-, attacks the slightly positive carbon atom in benzyl chloride, which is bonded to the chlorine atom. This carbon is electrophilic due to the electron-withdrawing nature of the chlorine atom.
• The result is the displacement of the chloride ion (aasc Cl-), which leaves, and the carbon atom now bonds with the hydroxide ion instead, forming benzyl alcohol.

Such reactions are highly relevant because they demonstrate how functional groups can be transformed, allowing for a wide variety of chemical products.

Benzyl Chloride

Benzyl chloride is an essential intermediate in various chemical synthesis processes. Obtained from the chlorination of toluene, it is a clear example of a compound where the functional group (in this case, chlorine) is attached to a benzylic position.
This position is highly reactive due to its ability to stabilize radicals and carbocations through resonance with the aromatic ring.

Key Characteristics of Benzyl Chloride:

• This compound is characterized by its ability to easily undergo nucleophilic substitution reactions.
• When in the presence of strong bases or nucleophiles, benzyl chloride readily reacts because the benzylic carbon can comfortably support the formation of intermediates.

Benzyl chloride is not only a precursor to benzyl alcohol but also to various other compounds through substitution or transformation reactions. Understanding its behavior is fundamental for students of organic chemistry.

Benzyl Alcohol

Benzyl alcohol, formed from the substitution of a hydroxide ion for chlorine in benzyl chloride, is an important organic compound. It's a primary alcohol, featuring a hydroxyl group attached to a benzylic carbon, giving it unique properties.

Characteristics of Benzyl Alcohol:

• It is a moderate solvent, frequently used in the preparation of other organic compounds.
• Due to its alcohol functional group, it can engage in further reactions, such as oxidation to form other products like benzaldehyde or benzoic acid.

In the context of the original problem, benzyl alcohol itself is not one of the direct answers offered but represents an important step in understanding substitution reactions and product formation.