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 \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\) with excess ethane and UV light.

Step 1: Understanding the Reaction

The exercise involves the production of ethyl chloride (\(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\)) from ethane (\(\mathrm{C}_{2} \mathrm{H}_{6}\)) and chlorine (\(\mathrm{Cl}_{2}\)) under certain conditions. The reaction is an example of halogenation, often initiated with ultraviolet (UV) light, and can be affected by the proportion of reactants.

Step 2: Evaluating Reaction Conditions

Each option presents different conditions for the halogenation process: - (a) uses excess ethane and UV light. - (b) takes place in the dark at room temperature. - (c) uses excess chlorine and UV light. - (d) uses equal amounts and UV light. Only UV light will provide the energy necessary to initiate the reaction.

Step 3: Identifying Best Yield Strategy

To maximize the yield of \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\)the monohalogenation productit's best to use conditions where ethane is in excess. This minimizes the likelihood of further substitution reactions, which would otherwise convert \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\) to more substituted or over-chlorinated products.

Step 4: Choosing the Correct Option

Among the given options, (a) provides excess \(\mathrm{C}_{2} \mathrm{H}_{6}\) with UV light, facilitating a high yield of the desired product \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\) with less chance of forming side products. Option (c), with excess chlorine, tends to produce more over-chlorinated derivatives, and (d) without excess doesnt preferentially push towards \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\) alone.

Key Concepts

Halogenation

Halogenation is an important chemical reaction in organic chemistry, involving the addition of a halogen atom (such as chlorine or bromine) to an organic molecule. In the context of the exercise, ethane undergoes halogenation to form ethyl chloride. This process can be thought of as replacing a hydrogen atom on ethane with a halogen atom, in this case, chlorine.
In the halogenation of ethane, chlorine ( \( \mathrm{Cl}_2 \)) is the halogen used, and UV light is often necessary to initiate the reaction. When considering the different options for halogenation, using excess ethane ensures that the reaction yields primarily monohalogenated products such as ethyl chloride, by minimizing further reactions, where additional chlorine atoms could potentially substitute more hydrogen atoms. This selectivity is crucial for industrial applications where high yield and minimal side products are desired.

Ethyl Chloride Production

Ethyl chloride, also known as chloroethane, is produced through the halogenation of ethane. The reaction is simple: ethane reacts with chlorine in the presence of UV light to produce ethyl chloride and hydrogen chloride (HCl) as a byproduct.
The equation for the reaction is:\[\mathrm{C}_2\mathrm{H}_6 + \mathrm{Cl}_2 \xrightarrow{\text{UV light}} \mathrm{C}_2\mathrm{H}_5\mathrm{Cl} + \mathrm{HCl}\]This reaction is quite sensitive to reactant conditions. When ethane is present in excess, the desired monohalogenated product, ethyl chloride, is predominantly formed. This setup ensures that chlorine molecules preferentially react with fresh ethane molecules, rather than further chlorinating the formed ethyl chloride. For practical purposes, achieving a high yield of ethyl chloride requires thoughtful control over reactant ratios and reaction conditions.

UV Light Initiated Reactions

UV light plays a crucial role in initiating halogenation reactions. This type of light provides the energy required to break the relatively strong bond in chlorine molecules ( \( \mathrm{Cl}_2 \)), resulting in the formation of highly reactive chlorine radicals. These radicals then proceed to attack the C-H bonds in ethane, leading to the formation of ethyl chloride.

• UV light causes homolytic cleavage of \( \mathrm{Cl}_2 \), splitting it into two chlorine radicals.
• These radicals are extremely reactive and set off a chain reaction.
• The process ensures that the halogenation occurs efficiently and selectively.

Without the presence of UV light, as in option (b) of the exercise, the reaction would not proceed efficiently at room temperature. This highlights the importance of UV light not just as a catalyst, but as an essential energy source for initiating the entire reaction process.