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 \()+\mathrm{Cl}_{2}\) uv ligh (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

Option (a) provides the best yield of \( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl} \).

Step 1: Understand the Reaction

The reaction between ethane \( \mathrm{C}_{2} \mathrm{H}_{6} \) and chlorine \( \mathrm{Cl}_{2} \) under ultraviolet (UV) light is a free radical substitution reaction. This process replaces a hydrogen atom in ethane with a chlorine atom, forming ethyl chloride \( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl} \).

Step 2: Option A Analysis

In option (a), ethane is in excess with chlorine and the reaction occurs under UV light. Having ethane in excess ensures that more chlorine radicals will collide with ethane molecules, thus increasing the formation of \( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl} \).

Step 3: Option B Analysis

Option (b) involves mixing ethane and chlorine in the dark at room temperature. Free radical reactions require energy in the form of light (often UV) to initiate, so this condition does not favor the formation of \( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl} \) effectively.

Step 4: Option C Analysis

For option (c), chlorine is in excess and the reaction is under UV light. Excess chlorine can lead to further substitution reactions where more than one chlorine atom replaces hydrogen in ethane, reducing the yield of \( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl} \).

Step 5: Option D Analysis

In option (d), ethane and chlorine are both present in equal amounts, and the reaction occurs under UV light. This condition could lead to multiple substitutions, but not as pronounced as when chlorine is in excess.

Step 6: Evaluate Conditions for Highest Yield

For achieving the highest yield of \( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl} \), conditions should minimize the chances of further substitution yet allow the reaction to proceed efficiently. An excess of ethane under UV light provides such a condition.

Key Concepts

Ultraviolet Light Initiation

In free radical substitution reactions, ultraviolet (UV) light plays a crucial role. UV light provides the necessary energy to break the chlorine molecule (\(\mathrm{Cl}_{2}\)) into two chlorine radicals. This process is called photodissociation. Here's how it begins:

• When UV light hits a chlorine molecule, the energy absorbed is sufficient to break the \(\mathrm{Cl–Cl}\) bond. This creates two highly reactive chlorine radicals, each denoted as \(\cdot\mathrm{Cl}\).
• These chlorine radicals are key players in the next phase of the reaction. They are eager to engage with other molecules due to their unpaired electron.

Understanding this step is vital, as the generation of these radicals sets the entire chain of reactions into motion, leading eventually to the formation of ethyl chloride.

Ethyl Chloride Formation

Ethyl chloride (\(\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{Cl}\)) forms through the interaction of ethane and chlorine radicals. This process unfolds as:

• After UV light creates chlorine radicals, these radicals attack the hydrogen atom in ethane (\(\mathrm{C}_{2}\mathrm{H}_{6}\)). This step generates a newly formed hydrogen chloride (\(\mathrm{HCl}\)) and an ethyl radical (\(\mathrm{C}_{2}\mathrm{H}_{5}\cdot\)).
• Subsequently, the ethyl radical rapidly reacts with another chlorine molecule (\(\mathrm{Cl}_{2}\)) forming ethyl chloride (\(\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{Cl}\)) and regenerates a chlorine radical, perpetuating the chain reaction.

The chain reaction proceeds with these steps repeating, which efficiently leads to the accumulation of ethyl chloride if the conditions favor ethane in excess. Thus, having ethane in excess is crucial as it limits further substitution reactions, ensuring a higher yield of the desired product, ethyl chloride.

Chemical Reaction Analysis

To maximize the yield of ethyl chloride, it is essential to analyze the reaction conditions closely. The effectiveness of different conditions can vary, impacting the outcome:

• **Option A Analysis:** Here, ethane is in excess with UV light promoting the reaction. An excess of ethane increases contact with chlorine radicals, ensuring more focused substitution to yield a higher concentration of ethyl chloride.
• **Option B Analysis:** This option, lacking UV light, prevents the formation of chlorine radicals, thereby halting the substitution process entirely. Such conditions are unfavorable for the production of ethyl chloride.
• **Option C Analysis:** Excess chlorine under UV light may lead to over-substitution. Multiple hydrogen atoms in ethane may be replaced, reducing ethyl chloride concentration, as the reaction might proceed to form dichloroethane or other over-substituted products.
• **Option D Analysis:** Equal amounts of chlorine and ethane under UV light can still lead to some over-substitution, although less so than in an excess of chlorine.

In summary, the goal is to optimize conditions such that ethyl chloride is primarily formed without excessive byproducts. Analyzing these reaction dynamics allows chemists to design conditions for the highest yield of the desired product, ethyl chloride.