Problem 111 - A Complete Resource Book in Chemistry for JEE Main

Question

Chloroethane reacts with magnesium in dry ether to form \(X\). When \(X\) is hydrolyzed, a carbon compound \(Y\) and \(\mathrm{Z}\) are formed. Which of the following is \(\mathrm{Y}\) ? (a) \(\mathrm{C}_{2} \mathrm{H}_{4}\) (b) \(\mathrm{C}_{2} \mathrm{H}_{2}\) (c) \(\mathrm{C}_{2} \mathrm{H}_{6}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{6}\)

Step-by-Step Solution

Answer

Compound Y is ethane (fC_2H_6 ightf), option (c).

Step 1: Identify the Reaction Type

Chloroethane (fC_2H_5Cl ightf) reacts with magnesium in dry ether to form a Grignard reagent. A typical reaction here involves the formation of ethylmagnesium chloride (fC_2H_5MgCl ightf) as the intermediate. This is a part of Grignard reaction series.

Step 2: Hydrolyze the Grignard Reagent

Once ethylmagnesium chloride (fC_2H_5MgCl ightf) forms as the Grignard reagent, it can undergo hydrolysis by reaction with water. The general result of hydrolyzing a Grignard reagent is the formation of an alkane that corresponds to the organic group of the Grignard reagent and a magnesium hydroxide salt.

Step 3: Determine the Products of Hydrolysis

During hydrolysis of ethylmagnesium chloride (fC_2H_5MgCl ightf), the ethyl group (fC_2H_5 ightf) from the Grignard reagent is converted into ethane (fC_2H_6 ightf). The other product of the reaction is magnesium hydroxide (Mg(OH)f_2 ightf), but only the hydrocarbons are of interest here.

Step 4: Identify Compound Y

From the hydrolysis reaction, the main carbon compound formed is ethane (fC_2H_6 ightf). This matches the option (c). Thus, compound fiYf corresponds to ethane.

Key Concepts

Chloroethane

Chloroethane, represented by the chemical formula \( \text{C}_2\text{H}_5\text{Cl} \), is a simple alkyl halide. It's essentially an ethane molecule where one of the hydrogen atoms has been replaced by a chlorine atom. Chloroethane is known for its use in the production of tetraethyllead, although this application has significantly declined due to environmental concerns. In organic chemistry, chloroethane is pivotal in preparing Grignard reagents, as it reacts with magnesium in dry ether to form such intermediates.

Alkyl halides like chloroethane are characterized by their ability to act as electrophiles due to the strong polarization of the carbon-halogen bond. This makes them ideal participants in nucleophilic substitution reactions that form the basis for the synthesis of various organic compounds, including Grignard reagents.

Magnesium in dry ether

The reaction of chloroethane with magnesium in a dry ether solution is fundamental to forming Grignard reagents. Magnesium is a reactive metal known for its ability to insert itself into carbon-halogen bonds, creating organometallic compounds.

In this particular scenario, the dry ether acts as a solvent and is crucial because it stabilizes the resulting Grignard reagent. Ether has a low dielectric constant, which allows it to solvate the magnesium ion effectively without interfering with the reaction's progress. Furthermore, keeping the ether dry is necessary, as the presence of water would cause the premature hydrolysis of the Grignard reagent, rendering it ineffective in further reactions.

Ethylmagnesium chloride

Ethylmagnesium chloride, formulated as \( \text{C}_2\text{H}_5\text{MgCl} \), is the intermediate compound formed when chloroethane and magnesium react in dry ether. This compound is a classic example of a Grignard reagent, which is characterized by a carbon-magnesium bond. This bond is highly polar, with the carbon atom carrying a partial negative charge and exhibiting strong nucleophilic properties.

Grignard reagents like ethylmagnesium chloride are versatile in synthesis because they can act as nucleophiles. They attack electrophilic centers such as carbonyl compounds, facilitating the formation of new carbon-carbon bonds, which is crucial in organic synthesis. The use of ethylmagnesium chloride, specifically, highlights its role in forming hydrocarbons through reactions with various substrates.

Hydrolysis of Grignard reagent

The hydrolysis of a Grignard reagent like ethylmagnesium chloride involves its reaction with water, leading to the formation of an alkane. Here, ethylmagnesium chloride interacts with water to yield ethane \( \text{C}_2\text{H}_6 \), a simple alkane composed of two carbon atoms.

This hydrolysis reaction of a Grignard reagent generally results in the alcohol corresponding to the organic portion of the reagent being formed along with magnesium hydroxide. In this instance, however, the focus is on the hydrocarbon product, ethane, which corresponds to option (c) in the original exercise solution. This step is crucial as it transforms the Grignard reagent into a more stable organic compound, allowing for its practical application in chemical synthesis.