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): A mixture of benzene and \( \mathrm{Mg} (\mathrm{OMe}) \mathrm{Br} \).

Step 1: Understanding the Reagents

Phenyl magnesium bromide (C6H5MgBr) is a Grignard reagent, while methanol (CH3OH) is an alcohol.

Step 2: Grignard Reagent Reaction with Alcohol

Grignard reagents react with alcohols to produce hydrocarbons. In this case, the aromatic phenyl group (C6H5-) will gain a proton (H+) from methanol, forming benzene (C6H6).

Step 3: Formation of By-product

After benzene is formed, the methoxy group (OMe) along with the magnesium from the Grignard goes on to form the by-product \(\mathrm{Mg} \mathrm{(OMe)} \mathrm{Br}\). This happens because the methanol deprotonates to form methoxide \(\mathrm{(OMe)}^{-}\), which then associates with the magnesium bromide ion \(\mathrm{Mg}^{2+}, \mathrm{Br}^{-}\).

Step 4: Select the Correct Answer

By considering the reaction steps, we find that phenyl magnesium bromide reacts with methanol to produce benzene and \(\mathrm{Mg} \mathrm{(OMe)} \mathrm{Br}\). So, the correct answer is option (b).

Key Concepts

Phenyl Magnesium Bromide

Phenyl magnesium bromide, known by its chemical formula \(\text{C}_6\text{H}_5\text{MgBr}\), is a type of Grignard reagent. Grignard reagents are organomagnesium compounds that are widely used in the synthesis of organics due to their strong nucleophilic properties. In simple words, they are potent reactants that can easily form bonds with other elements, making them very important in organic chemistry.
One of the unique aspects of phenyl magnesium bromide is its ability to interact with various types of molecules, including alcohols. This reagent consists of a phenyl group (\(\text{C}_6\text{H}_5^-\)) connected to a magnesium bromide (\(\text{MgBr}^+\)) through a polar covalent bond.
The phenyl group is particularly reactive, which allows it to exert influence over a wide range of organic reactions.

Alcohol Reactions

Alcohols, like methanol (\(\text{CH}_3\text{OH}\)), are simple organic compounds characterized by an \(-\text{OH}\) (hydroxyl) group. When Grignard reagents come into contact with alcohols, an interesting transformation occurs.
The reaction basically involves the Grignard reagent acting as a base, snatching the hydrogen (proton) from the \(-\text{OH}\) group of the alcohol. This results in the formation of hydrocarbons, along with another reaction by-product.
In simpler terms, in the presence of alcohol, Grignard reagents don’t typically act like nucleophiles adding to the \(-\text{OH}\) group; instead, they just convert into a hydrocarbon. This showcases the versatility of Grignard reactions and their ability to modify organic structures efficiently.

Formation of Hydrocarbons

Hydrocarbons are the simplest forms of organic compounds composed entirely of hydrogen and carbon atoms. In reactions involving Grignard reagents with alcohols, the phenyl group receives a proton (H+) from the alcohol. This action changes the phenyl group into benzene (\(\text{C}_6\text{H}_6\)).
To put it simply, the presence of methanol allows for the creation of benzene, a common hydrocarbon and the core unit of many large aromatic compounds. The transformation from phenyl magnesium bromide to benzene highlights the production of a simple, stable hydrocarbon as a result of the reaction.
Understanding this shift is crucial for students to predict the outcomes of similar organic chemistry reactions, especially those involving Grignard reagents.

Methanol Reaction

Methanol is one of the simplest alcohols, carrying the chemical formula \(\text{CH}_3\text{OH}\). When methanol reacts with phenyl magnesium bromide, a considerable shift occurs, transforming the methanol into a methoxide ion \((\text{OMe}^-\)).
This begins with the abstraction of the hydrogen from the \(-\text{OH}\) group by the Grignard reagent, resulting in the deprotonation of methanol.
The removal of this proton effectively transforms methanol into a stronger base (methoxide) and energizes it to interact further within the reaction process. Methanol acts as a crucial intermediate that allows the Grignard reagent to complete its transformation into a hydrocarbon.

By-product Formation

During the reaction between phenyl magnesium bromide and methanol, the transformation of the initial Grignard reagent into benzene means that there must be a counter balancing by-product. This is essential for maintaining chemical balance.
As methanol is deprotonated to methoxide, the methoxide ion \((\text{OMe}^-\)) attracts and binds with the magnesium bromide part of the Grignard reagent. This forms \(\text{Mg(OMe)Br}\) as a by-product.
Formation of such by-products is a critical consideration in organic reactions as it helps chemists understand elemental balance and the overall efficiency of a reaction. By-products can sometimes interfere with desired products, or they can offer valuable insight into refining reaction techniques.