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 \()+\mathrm{Cl}_{2} \stackrel{\text { uv light }}{\longrightarrow}\) (b) \(\mathrm{C}_{2} \mathrm{H}_{6}+\mathrm{Cl}_{2} \underline{\text { dark }, \text { room temperature }}\) (c) \(\mathrm{C}_{2} \mathrm{H}_{6}+\mathrm{Cl}_{2}\) (excess) \(\stackrel{\text { uv light }}{\longrightarrow}\) (d) \(\mathrm{C}_{2} \mathrm{H}_{6}+\mathrm{Cl}_{2} \stackrel{\text { uv light }}{\longrightarrow}\)

Short answer

Option (a) provides the best yield of C2H5Cl with excess C2H6 and UV light.

Step-by-step solution

Identify Reaction Type

The reaction involves the chlorination of ethane to form chloroethane, which is a free radical substitution reaction requiring specific conditions.

Evaluate Effect of UV Light

UV light is necessary to break the Cl-Cl bond in chlorine, forming free radicals that initiate the chain reaction. Options (a), (c), and (d) use UV light.

Determine Impact of Reactant Excess

Having an excess of chlorine ( option (c)) will lead to further chlorination of chloroethane, reducing the yield of the desired product, C2H5Cl. An excess of ethane, as in option (a), favors higher yield of C2H5Cl by limiting further substitution.

Consider Dark and Room Temperature Conditions

Option (b) mentions dark and room temperature conditions, which are not conducive to initiating the free radical chain reaction necessary for chlorination.

Select Best Conditions

Considering all the factors, option (a) with excess C2H6 and UV light provides the best conditions for maximizing the yield of C2H5Cl by limiting further chlorination.

Key concepts

Free Radical Substitution

In the fascinating world of organic chemistry, free radical substitution plays a pivotal role. This type of reaction involves the substitution of an atom, typically a hydrogen atom, with a free radical. Free radicals are atoms or groups of atoms that contain an unpaired electron, making them highly reactive. This reactivity is the key to the free radical substitution process.
This reaction follows a chain mechanism that can be explained in three stages:

• Initiation: This is where the free radicals are generated, usually by breaking a bond in a molecule using energy, such as UV light. For example, when chlorine (\(\mathrm{Cl}_2\)) is exposed to UV light, the bond between the two chlorine atoms breaks, forming two chlorine radicals (\(\mathrm{Cl}\cdot\)).
• Propagation: This stage involves the free radicals reacting with stable molecules to form new radicals. In the chlorination of ethane, a chlorine radical reacts with an ethane molecule (\(\mathrm{C}_2\mathrm{H}_6\)), abstracting a hydrogen atom to form chloroethane (\(\mathrm{C}_2\mathrm{H}_5\mathrm{Cl}\)) and a new ethyl radical.
• Termination: The chain reaction ends when two radicals collide and combine to form a stable product, effectively removing the reactive species from the reaction mixture.

Understanding these stages helps explain why free radical substitution is a critical mechanism in organic synthesis, particularly in the preparation of halogenated compounds.

Chlorination of Ethane

Chlorination of ethane is a classic example of a free radical substitution reaction. It involves the substitution of a hydrogen atom in ethane (\(\mathrm{C}_2\mathrm{H}_6\)) with a chlorine atom, resulting in the product chloroethane (\(\mathrm{C}_2\mathrm{H}_5\mathrm{Cl}\)). This reaction is significant due to its practical applications, such as in the industrial production of chloroethane, which is used in various chemical syntheses.
To better understand this process, consider:

• Initiation by UV Light: The presence of UV light is crucial as it provides the energy needed to dissociate molecular chlorine into two chlorine radicals, which are key to the initiation process.
• Selective Substitution: The reaction proceeds by selectively substituting one hydrogen atom in ethane with a chlorine atom. This initial substitution gives the desired product, chloroethane, but care must be taken to enhance selectivity and minimize unwanted multiple substitutions.
• Product Yield: Maximizing the yield of chloroethane involves controlling reaction conditions to minimize further chlorination, which could produce dichloroethane and other by-products.

Chlorination of ethane, though simple in terms of reactants, requires intricate control of conditions to ensure optimal selectivity and yield.

Reaction Conditions

The conditions under which a reaction takes place are critical in determining the efficiency and selectivity of the reaction. In free radical substitution reactions like the chlorination of ethane, several conditions must be controlled carefully for the best results.
Key reaction conditions include:

• Use of UV Light: UV light is essential as it initiates the formation of radicals by breaking the Cl-Cl bond in chlorine molecules. Without UV light, the reaction does not proceed efficiently.
• Reactant Ratios: The proportion of reactants plays a pivotal role. An excess of ethane ensures that once chloroethane is formed, it does not undergo further chlorination. This strategy limits the formation of unwanted by-products.
• Temperature and Light Conditions: Reactions that occur in the dark or at room temperature are not favorable because they lack the energy necessary to initiate radical formation. Therefore, adequate light and thermal conditions are crucial.

Understanding and manipulating these reaction conditions are vital for achieving the highest yield of the desired product in chemical synthesis, especially in industrial settings where efficiency and cost-effectiveness are paramount.