Problem 31 - A Complete Resource Book in Chemistry for JEE Main

Question

The reaction conditions leading to provide the best yield of \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\) are (a) \(\mathrm{C}_{2} \mathrm{H}_{6}(\) excess \()+\mathrm{Cl}_{2} \stackrel{\text { uv ligh }}{\longrightarrow}\) (b) \(\mathrm{C}_{2} \mathrm{H}_{6}+\mathrm{Cl}_{2} \stackrel{\text { dark, room temperature }}{\longrightarrow}\) (c) \(\mathrm{C}_{2} \mathrm{H}_{6}+\mathrm{Cl}_{2}\) (excess) \(\stackrel{\text { uv ligh }}{\longrightarrow}\) (d) \(\mathrm{C}_{2} \mathrm{H}_{6}+\mathrm{Cl}_{2} \stackrel{\text { uv ligh }}{\longrightarrow}\)

Step-by-Step Solution

Answer

(a) is the best condition for maximizing chloroethane yield with ethane in excess under UV light.

Step 1: Understanding the Reaction

The reaction being considered is the chlorination of ethane (246) to form chloroethane (254). This is a substitution reaction where a chlorine atom replaces a hydrogen atom in ethane. The reaction requires initiation by UV light because it involves free radical substitution.

Step 2: Analyzing Options

The goal is to find the conditions that maximize the yield of 254. The best yield is achieved when side reactions are minimized. Let's examine the given conditions: - In option (a), ethane is in excess with UV light. This means that chlorine atoms are more likely to react with ethane rather than with already formed 254, reducing the chances of forming poly-chlorinated products. - Option (b) is conducted in the dark at room temperature, where the reaction will not proceed significantly because free radicals are not being generated. - Option (c) has chlorine in excess with UV light, which increases the chances of further chlorination beyond 254, leading to the formation of dichloroethanes and other poly-chlorinated products. - Option (d) with equimolar amounts of 246 and 28, along with UV light, means that additional chlorinated products could also form once the initial 254 is produced.

Step 3: Selecting the Best Condition

Reviewing these conditions, option (a)—246 in excess with UV light—provides the simplest environment to favor the formation of 254. Having ethane in excess assumes that available chlorine will preferentially attach to ethane molecules, lowering the likelihood of forming multi-chloro products.

Key Concepts

Free Radical Substitution

In the chlorination of ethane, free radical substitution is a key mechanism that helps us understand how chloine replaces a hydrogen atom in ethane to form chloroethane. This type of chemical reaction involves the homolytic cleavage of the chlorine molecule (chlorine (\(\mathrm{Cl}_2\)), generating two free radicals, each containing an unpaired electron. These free radicals are highly reactive and can initiate further chemical reactions.Free radical substitution is triggered by UV light, which is needed to break the bond between the chlorine atoms effectively. Once the chlorine radicals are generated, they proceed to attack the ethane molecule. They substitute a hydrogen atom, forming a new bond and creating hydrogen chloride (\(\mathrm{HCl}\)). This substitution is not random, but a series of steps interlinked to form a chain reaction:

• Initiation: This stage involves the absorption of UV light, splitting chlorine molecules into radicals.
• Propagation: The chlorine radical attacks ethane to form chloroethane and a new hydrogen radical. The hydrogen radical then reacts with another chlorine molecule, regenerating a chlorine radical and forming HCl.
• Termination: This occurs when two radicals join and create a stable molecule, ending the chain reaction.

This chain reaction ensures that the chlorination of ethane continues efficiently until no more radicals are available to propagate the reaction.

Reaction Conditions

The conditions under which the reaction takes place are crucial to its success and the yield of chloroethane. The chlorination of ethane proceeds best with specific factors that support free radical formation and limit side reactions. UV light plays an essential role in breaking down chlorine into radicals, making it indispensable for this process. Let's discuss different scenarios:

• Excess Ethane with UV Light: Having ethane in excess ensures that chlorine radicals primarily react with ethane molecules. This reduces the potential for further chlorination and thus minimizes the creation of unwanted poly-chlorinated byproducts. This condition ultimately leads to a higher yield of chloroethane.
• Dark or Room Temperature Conditions: Without UV light, the reaction does not initiate effectively as radicals are not generated. Conducting the reaction in darkness or at room temperature halts the creation of reactive free radicals, virtually halting the chlorination process.
• Excess Chlorine with UV Light: Keeping chlorine in excess increases the chances that chloroethane, once formed, will react again to produce dichloroethanes and other multi-substituted products, thereby reducing the specific yield of chloroethane.

Thus, the reaction conditions need careful optimization to ensure maximum production of the desired product.

Chloroethane Formation

The formation of chloroethane (\(\mathrm{C}_2\mathrm{H}_5\mathrm{Cl}\)) involves specific pathways and controlled conditions to be efficient. In this chemical reaction, one of the ethane molecule's hydrogen atoms is replaced by a chlorine atom, leading to the creation of chloroethane. This formation is part of the broader chlorination process.To achieve this substitution, we employ the free radical mechanism facilitated by UV light. Here’s the general path:

• Ethane (\(\mathrm{C}_2\mathrm{H}_6\)) encounters a chlorine radical formed from the homolytic cleavage of a chlorine molecule induced by UV light.
• The chlorine radical attacks ethane, resulting in the abstraction of one hydrogen atom and the simultaneous formation of chloroethane along with a new radical, \(\mathrm{HCl}\).

Chloroethane features a single methyl group substituted by a chlorine atom, providing its distinct properties compared to ethane.While forming chloroethane, another challenge is preventing further reactions, such as the formation of poly-chlorinated products. By controlling the environment, for example, using excess ethane under UV light, we prioritize chloroethane formation and limit subsequent reactions that might lead to undesirable by-products. This strategic choice ensures a higher yield of the target product.