Question

Why is it preferable to produce chloroethane by the reaction of \(\mathrm{HCl}(g)\) with ethene than by the reaction of \(\mathrm{Cl}_{2}(g)\) with ethane? (See Exercise 62.)

Short answer

The production of chloroethane is preferable by the reaction of \(\mathrm{HCl}(g)\) with ethene over the reaction of \(\mathrm{Cl}_{2}(g)\) with ethane because the former exhibits a faster reaction rate, a simpler and more direct reaction mechanism (electrophilic addition), and fewer side reactions. The latter, on the other hand, proceeds via a more complex chain mechanism that involves initiation, propagation, and termination steps, resulting in side reactions and byproducts, which reduce the overall yield and potentially raise environmental and safety concerns.

Step-by-step solution

Write the reaction equations

Write the balanced chemical equations for the two reactions: 1. Reaction of \(\mathrm{HCl}(g)\) with ethene: \(\mathrm{C}_{2}\mathrm{H}_{4}(g) + \mathrm{HCl}(g) \rightarrow \mathrm{C}_{2}\mathrm{H}_{5}\mathrm{Cl}(g)\) 2. Reaction of \(\mathrm{Cl}_{2}(g)\) with ethane: \(\text{A. Initiation step: \( \mathrm{Cl}^{\cdot}_{2}(g) \longrightarrow 2\mathrm{Cl}^{\cdot}(g) \)\\ B. Propagation step: \) \( \mathrm{C}_{2}\mathrm{H}_{6}(g) + \mathrm{Cl}^{\cdot}(g) \rightarrow \mathrm{C}_{2}\mathrm{H}_{5}\mathrm{Cl}(g) + \mathrm{H}^{\cdot}(g)\)

Compare reaction rates

The reaction of \(\mathrm{HCl}(g)\) with ethene is a faster reaction than the reaction of \(\mathrm{Cl}_{2}(g)\) with ethane because the second reaction requires the homolytic cleavage of the \(\mathrm{Cl}-\mathrm{Cl}\) bond, which is an energetically unfavorable process. Moreover, the reaction with ethane is a radical chain mechanism, which involves the formation and propagation of reactive radical intermediates, making the reaction slower and more complex.

Compare reaction mechanisms

The reaction of \(\mathrm{HCl}(g)\) with ethene is an electrophilic addition reaction, where the \(\mathrm{H}^{\delta+}\) atom from \(\mathrm{HCl}\) acts as an electrophile and forms a bond with the nucleophilic ethene double bond. This addition reaction is a relatively simple and direct mechanism. On the other hand, the reaction of \(\mathrm{Cl}_{2}(g)\) with ethane is a radical halogenation reaction, which proceeds via a more complex chain mechanism involving initiation, propagation, and termination steps. This complexity in the reaction mechanism can lead to side reactions and byproducts, reducing the overall yield of chloroethane.

Consider side reactions

While the reaction of \(\mathrm{HCl}(g)\) with ethene is highly selective for chloroethane, the reaction of \(\mathrm{Cl}_{2}(g)\) with ethane has several side reactions. These side reactions include the formation of more highly chlorinated products (e.g., 1,1-dichloroethane, 1,2-dichloroethane, etc.), as well as the reaction of \(\mathrm{Cl}_{2}(g)\) with both hydrogen and chlorine radicals. These side reactions reduce the overall yield of chloroethane and can cause environmental and safety concerns due to the generation of hazardous byproducts. In conclusion, it is preferable to produce chloroethane by the reaction of \(\mathrm{HCl}(g)\) with ethene rather than the reaction of \(\mathrm{Cl}_{2}(g)\) with ethane because of the faster reaction rate, simpler reaction mechanism, and fewer side reactions that reduce the yield and produce potentially harmful byproducts.

Key concepts

Ethene and HCl Reaction

Producing chloroethane by reacting ethene with hydrogen chloride (HCl) is highly efficient. This process is an electrophilic addition, where ethene's carbon-carbon double bond ( \( \mathrm{C=C} \) ) acts as a nucleophile. The \( \mathrm{H}^{\delta+} \) ion from HCl easily bonds with the double bond, forming chloroethane in one simple step.
This reaction is straightforward and direct, reducing the chances for side reactions. Its simplicity contributes to faster production rates compared to other methods.

Radical Halogenation

Radical halogenation involves the reaction of chlorine ( \( \mathrm{Cl}_{2} \) ) with ethane to produce chloroethane. This method includes three main stages: initiation, propagation, and termination. Initially, \( \mathrm{Cl}_{2} \) molecules break into radicals under energy input, usually from light or heat.
During propagation, chlorine radicals react with ethane, substituting hydrogen atoms with chlorine. This process is less direct and efficient because it generates multiple reactive intermediates, increasing complexity.

Reaction Mechanisms

Understanding reaction mechanisms helps decide the best ways to produce chemicals like chloroethane. The electrophilic addition of HCl to ethene is straightforward—often preferred due to its simplicity.
In contrast, radical halogenation is complex and involves several steps. These steps can lead to chain reactions, where an atom or radical is continuously regenerated, introducing multiple chances for the reaction to deviate from the desired outcome. This complexity often makes this approach less efficient.

Chemical Yield

The efficiency of a chemical reaction is measured by its yield—the percentage of desired product obtained. The reaction of HCl with ethene reliably produces a high yield of chloroethane due to its direct mechanism and minimal side reactions.
In radical halogenation, the complexity and presence of side reactions often lead to lower yields. Competing pathways and byproducts reduce the efficiency, meaning more resources are needed for the same amount of product.

Side Reactions in Chemical Synthesis

Side reactions are undesired pathways that consume reactants without producing the main product. These can occur more frequently in complex reactions like radical halogenation.
Producing chloroethane using chlorine and ethane often results in byproducts like dichloroethane, as multiple chlorine atoms may attach to the carbon chain. These side reactions not only lower the yield but also complicate purification and may produce environmental hazards.

• More steps mean more opportunities for unwanted side products.
• Increased risk of toxic byproducts requires careful handling and waste treatment.