Problem 183 - A Complete Resource Book in Chemistry for JEE Main

Question

Of the following reagents the number of them which will react with \(\mathrm{CH}_{3} \mathrm{MgBr}\) to give \(\mathrm{CH}_{4}\) is \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}, \mathrm{CH}_{3} \mathrm{SH}, \mathrm{CH}_{3} \mathrm{NH}_{2}, \mathrm{RC} \equiv \mathrm{CR}, \mathrm{HCHO}\) \(\mathrm{RC} \equiv \mathrm{CH}\)

Step-by-Step Solution

Answer

4 reagents will react with \(\mathrm{CH}_3\mathrm{MgBr}\) to give \(\mathrm{CH}_4\): ethanol, methanethiol, methylamine, and a terminal alkyne.

Step 1: Identify Reactivity

We need to determine which of the reagents can react with \(\mathrm{CH}_3\mathrm{MgBr}\) to produce \(\mathrm{CH}_4\). \(\mathrm{CH}_3\mathrm{MgBr}\) is a Grignard reagent, which is a strong base and nucleophile. It reacts with compounds containing acidic hydrogen atoms (protic compounds), which can donate a hydrogen to form \(\mathrm{CH}_4\).

Step 2: Test Each Reagent

The following are the reagents:1. \(\mathrm{C}_2\mathrm{H}_5\mathrm{OH}\): Ethanol, an alcohol, which contains an O-H bond. This bond is acidic enough to react with a Grignard reagent.2. \(\mathrm{CH}_3\mathrm{SH}\): Methanethiol, contains an S-H bond, which is also acidic and will react with Grignard reagents.3. \(\mathrm{CH}_3\mathrm{NH}_2\): Methylamine, contains an N-H bond, which will react as well since it can donate a proton.4. \(\mathrm{RC}\equiv\mathrm{CR}\): An internal alkyne, which does not have an acidic hydrogen.5. \(\mathrm{HCHO}\): Formaldehyde, which does not have an acidic hydrogen capable of forming \(\mathrm{CH}_4\) with Grignard reagents.6. \(\mathrm{RC}\equiv\mathrm{CH}\): A terminal alkyne, which contains an acidic hydrogen that can react to give \(\mathrm{CH}_4\).

Step 3: Count Reactive Reagents

Count the number of reagents that have acidic hydrogens and will generate \(\mathrm{CH}_4\) through reaction with \(\mathrm{CH}_3\mathrm{MgBr}\). These are: 1. Ethanol (\(\mathrm{C}_2\mathrm{H}_5\mathrm{OH}\))2. Methanethiol (\(\mathrm{CH}_3\mathrm{SH}\))3. Methylamine (\(\mathrm{CH}_3\mathrm{NH}_2\))4. Terminal alkyne (\(\mathrm{RC}\equiv\mathrm{CH}\))Thus, there are 4 reagents that will react.

Key Concepts

Nucleophilic Addition

Nucleophilic addition is a common reaction involving Grignard reagents. Grignard reagents, such as \(\mathrm{CH}_3\mathrm{MgBr}\), are known for their strong nucleophilic properties. This means they are very effective at attacking electrophilic centers or positive parts of molecules.

During a nucleophilic addition, the Grignard reagent donates its electron-rich carbon atom to bond with an electron-deficient atom, usually a carbon in a carbonyl group. This reaction forms a new carbon-carbon bond, which is a fundamental process in organic synthesis. For instance, in the original exercise, potential reactions are examined to identify reagents that may act as electrophiles due to their specific structure and bonding characteristics.

Key points about nucleophilic addition:

• It involves a nucleophile (e.g., a Grignard reagent).
• The reaction forms a new bond, typically carbon-carbon.
• It requires an electrophilic site on the other molecule.

Understanding this concept is crucial when considering how Grignard reagents react with different compounds.

Protic Compounds

Protic compounds are those which can donate a hydrogen ion (proton), due to the presence of O-H, N-H, or S-H bonds. Grignard reagents react with these protic compounds because the protons can be abstracted, leading to the formation of a new molecule like methane, \(\mathrm{CH}_4\).

To visualize this, imagine each of the protic compounds like ethanol (\(\mathrm{C}_2\mathrm{H}_5\mathrm{OH}\)), methanethiol (\(\mathrm{CH}_3\mathrm{SH}\)), and methylamine (\(\mathrm{CH}_3\mathrm{NH}_2\)). When a Grignard reagent comes in contact with these molecules, it grabs the available hydrogen ion, leading to methane formation.

Notable attributes of protic compounds and their reactions include:

• Presence of hydrogen atoms that are easily donated.
• Reactions with Grignard reagents result in the production of alkanes (such as methane).
• They often function as electrophiles when interacting with Grignard reagents.

This understanding helps highlight why certain compounds in the original exercise are suitable for reacting with \(\mathrm{CH}_3\mathrm{MgBr}\).

Acidic Hydrogen

The term "acidic hydrogen" refers to the hydrogen atom that can be easily released as a proton (\(\mathrm{H}^+\)) in a chemical reaction. Compounds with acidic hydrogens are more reactive with Grignard reagents because they can easily donate this proton.

For example, terminal alkynes, which have the general formula \(\mathrm{RC}\equiv\mathrm{CH}\), possess acidic hydrogens. When they react with Grignard reagents, they can form alkanes, such as methane, as observed in the exercise. This kind of reaction showcases why knowing about acidic hydrogens is important.

Important factors about acidic hydrogens in reactions:

• Acidic hydrogens can be found in molecules with O-H, N-H, or S-H bonds.
• They are essential for the reactions between Grignard reagents and protic compounds.
• The ease of hydrogen donation is influenced by the electronegativity of the atoms bonded to hydrogen.

Grasping these ideas helps in predicting and understanding the behavior of different reagents when interacting with Grignard reagents.