Problem 3 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The gas evolved on heating \(\mathrm{CH}_{3} \mathrm{MgBr}\) in methanol is : (a) Methane (b)Ethane (c) Propane (d) \(\mathrm{HBr}\)

Step-by-Step Solution

Answer

The gas evolved is (a) Methane.

Step 1: Understand the Reaction

When methylmagnesium bromide (\(\mathrm{CH}_{3} \mathrm{MgBr}\)) in methanol reacts, it undergoes a reaction where the alkyl group in Grignard reagent gets protonated, releasing a gas as a byproduct.

Step 2: Identify the Byproduct

The reaction between \(\mathrm{CH}_{3} \mathrm{MgBr}\) and methanol (\(\mathrm{CH}_{3}\mathrm{OH}\)) produces methane gas (\(\mathrm{CH}_{4}\)) when the methyl group in Grignard reagent is protonated by methanol.

Step 3: Write the Chemical Reaction

The reaction can be expressed as:\[\mathrm{CH}_{3}\mathrm{MgBr} + \mathrm{CH}_{3}\mathrm{OH} \rightarrow \mathrm{CH}_{4} + \mathrm{Mg(OH)Br}\]Here, methane (\(\mathrm{CH}_{4}\)) is the evolved gas.

Step 4: Confirm the Correct Gas

According to the reaction, the gas evolved is methane (\(\mathrm{CH}_{4}\)), which confirms that the answer is (a) Methane.

Key Concepts

Methylmagnesium Bromide

Methylmagnesium bromide, often represented as \(\mathrm{CH}_3\mathrm{MgBr}\), is one of the simplest examples of a Grignard reagent. Grignard reagents are organometallic compounds comprised of a magnesium atom bonded to a carbon atom. This bond is crucial because it injects a degree of "ionic character" into the compound, making it highly reactive and valuable in synthetic chemistry.
They are commonly used in reactions to form carbon-carbon bonds, a fundamental step in building larger organic molecules.
The reactivity of Grignard reagents, such as methylmagnesium bromide, arises from the polarized bond:

• The magnesium atom carries a partial positive charge.
• The carbon atom (in the methyl group) carries a partial negative charge.

This polarization allows the methyl group to act as a nucleophile, meaning it can donate electron pairs to form new bonds with electrophilic centers, such as partially positive carbons in carbonyl groups. Understanding this property is key when predicting their reactions with compounds like methanol.

Protonation Reaction

A protonation reaction occurs when a proton (\(\mathrm{H}^+\)) is added to an atom, ion, or molecule. When it comes to methylmagnesium bromide and methanol, this type of reaction is crucial.
In the context of the given exercise, **protonation** refers to the transfer of a hydrogen ion from methanol to the negatively charged carbon atom in methylmagnesium bromide. Here's why this happens:- The carbon in \(\mathrm{CH}_3\mathrm{MgBr}\) is a **nucleophile** because it is negatively charged due to its association with the magnesium atom.- Methanol provides a hydrogen atom which can readily transfer as a proton (\(\mathrm{H}^+\)), leading to the formation of methane gas (\(\mathrm{CH}_4\)).This reaction can be summarized by the following equation:
\[\mathrm{CH}_3\mathrm{MgBr} + \mathrm{CH}_3\mathrm{OH} \rightarrow \mathrm{CH}_4 + \mathrm{Mg(OH)Br} \]The methyl group acts as a base in this scenario, picking up a proton from the methanol, turning it into methane. Recognizing this molecular choreography helps us understand the transformation leading to gas evolution.

Gas Evolution

The term **gas evolution** refers to the formation and release of gas during a chemical reaction. In the context of the reaction between methylmagnesium bromide and methanol, the evolved gas is methane. This can be surprising and is the result of the protonation of the highly reactive Grignard reagent by methanol.
Understanding gas evolution requires us to think about the following:- **Formation of Methane**: During the reaction, the methyl group from \(\mathrm{CH}_3\mathrm{MgBr}\) gets protonated by methanol, forming methane gas which bubbles out as a visible sign of the reaction progress.- **Reaction Conditions**: Careful control of reaction conditions, such as temperature, can influence the rate and extent of gas evolution to ensure the reaction proceeds efficiently.The release of methane is not only a textbook confirmation of theoretical predictions but also a convenient cue that the reaction is complete. In labs, such visible changes are often used practically to signify that the reaction setup is progressing as anticipated. This understanding is valuable in both academic learning and applied chemical syntheses.