Problem 225 - A Complete Resource Book in Chemistry for JEE Main

Question

Phenyl magnesium bromide reacts with methanol to give (a) a mixture of anisole and \(\mathrm{Mg}(\mathrm{OH}) \mathrm{Br}\) (b) a mixture of benzene and \(\mathrm{Mg}(\mathrm{OMe}) \mathrm{Br}\) (c) a mixture of toluene and \(\mathrm{Mg}(\mathrm{OH}) \mathrm{Br}\) (d) a mixture of phenol and \(\mathrm{Mg}(\mathrm{Me}) \mathrm{Br}\)

Step-by-Step Solution

Answer

The correct answer is (b).

Step 1: Understanding the Reactants

The exercise involves phenyl magnesium bromide, which is a Grignard reagent, and methanol as the reactants. Grignard reagents are highly reactive with protic solvents, like alcohols, due to the presence of acidic hydrogen atoms.

Step 2: Grignard Reagent Reactivity

Grignard reagents react with compounds containing acidic hydrogen (like those in alcohols) to release an alkane. Here, the phenyl magnesium bromide ( ext{PhMgBr}) will react with the hydrogen of methanol ( ext{MeOH}) to form benzene ( ext{C}_6 ext{H}_6) by forming the intermediate  ext{Ph-H}.

Step 3: Formation of the By-product

Alongside benzene, the magnesium part of the Grignard reagent combines with the remaining methoxy group and bromide to form  ext{Mg(OMe)Br}. This happens because the methoxy takes the place of the hydrogen ion in the Grignard reagent.

Step 4: Choosing the Correct Answer

The description of the reaction in the exercise matches (b), where phenyl magnesium bromide reacts with methanol to produce a mixture of benzene and  ext{Mg(OMe)Br}.

Key Concepts

Understanding Phenyl Magnesium Bromide

Phenyl magnesium bromide is a type of chemical compound called a Grignard reagent. These reagents consist of a magnesium atom bonded to an organic group, the phenyl group in this case, and a bromine atom. They are highly valuable in organic chemistry due to their ability to form new carbon-carbon bonds. Another interesting property is their reactivity with various functional groups, especially those with acidic hydrogen atoms.

When phenyl magnesium bromide encounters reagents containing acidic hydrogens, such as alcohols, it initiates a vigorous reaction. The reason for this reactivity is the polar nature of the carbon-magnesium bond, making it an excellent nucleophile capable of reacting with electrophilic sites. This makes Grignard reagents very useful in forming complex organic molecules.

In the case of the exercise, phenyl magnesium bromide reacts quickly with methanol because methanol contains an acidic hydrogen atom. This interaction leads to the production of specific products that include benzene and another by-product.

Methanol's Role in the Reaction

Methanol is an alcohol and features prominently in many organic reactions due to the presence of an -OH group containing an acidic hydrogen. When methanol reacts with a reactive species like phenyl magnesium bromide, an important transformation occurs.

- **Acidic Hydrogen Reaction**: Methanol's hydroxyl group provides an acidic hydrogen that reacts with the phenyl group from the Grignard reagent. This forms benzene, a simple aromatic compound, as the phenyl group takes on a hydrogen atom.

- **By-product Formation**: As the hydrogen ion from methanol is transferred, the methoxy group replaces it in the magnesium bromide, forming magnesium methoxide bromide (\( \text{Mg(OMe)Br} \)). This by-product is a result of the methanol component, which rearranges to pair with bromine and magnesium from the Grignard reagent. This careful balancing of components leads to the overall completion of the reaction.

Alkane Formation in Grignard Reactions

Alkane formation is a hallmark of many Grignard reactions when they involve protic solvents, like alcohols. In the context of the exercise, the alkane produced is benzene, formed as a direct result of the reaction between phenyl magnesium bromide and methanol.

- **Mechanism Insight**: The mechanism of Grignard reactions with alcohol involves the transfer of a hydrogen atom. The phenyl group, part of phenyl magnesium bromide, captures an acidic hydrogen from methanol. This results in the creation of benzene as the phenyl ring gains its hydrogen atom back, effectively turning it into an alkane.

- **Reaction Outcome**: The rapid transformation highlights the Grignard reagent's role in withdrawing acidic protons from alcohols to produce hydrocarbons. In this case, that hydrocarbon is benzene, which is a product of significant importance due to its potential applications in synthesizing various organic compounds relevant to many fields ranging from pharmaceuticals to industrial chemistry.

This straightforward conversion of a Grignard reagent through interaction with an alcohol emphasizes the efficiency of this chemical reaction in synthesizing alkanes alongside other notable reaction products like magnesium complexes.