Which condition is essential for the reaction between ethane and bromine to produce bromoethane?

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Examoogle · Accepted Answer

The correct answer is Option A: Presence of sunlight. The reaction between ethane and bromine is a free-radical substitution reaction that requires ultraviolet (UV) light to initiate. The light energy is necessary to undergo homolytic fission of the bromine molecule into highly reactive bromine radicals.

Step-by-Step Solution & Concepts:
The reaction between ethane and bromine is a free-radical substitution reaction that requires ultraviolet (UV) light to initiate. The light energy is necessary to undergo homolytic fission of the bromine molecule into highly reactive bromine radicals.

Working Principle: The reaction proceeds via a free-radical mechanism: initiation, propagation, and termination. In the initiation step, $Br_{2}$ absorbs UV light to form two bromine radicals ($Br\bullet$). These radicals then abstract a hydrogen atom from ethane ($C_{2}H_{6}$) to form an ethyl radical ($C_{2}H_{5}\bullet$), which subsequently reacts with $Br_{2}$ to yield bromoethane ($C_{2}H_{5}Br$).

Key Formulas: $CH_{3}CH_{3} + Br_{2} \xrightarrow{h
u} CH_{3}CH_{2}Br + HBr$ — The overall substitution reaction of ethane with bromine.; $Br_{2} \xrightarrow{h
u} 2Br\bullet$ — The initiation step showing homolytic fission by light energy ($h
u$).

Key Points: This is a photochemical reaction involving free radicals.; The reaction does not occur in the dark at room temperature.; Substitution reactions are characteristic of alkanes.; The byproduct of the reaction is hydrogen bromide ($HBr$).

Advantages: Allows the functionalization of unreactive alkanes.

Disadvantages: Lacks regioselectivity as reaction can continue to produce poly-substituted products like 1,1-dibromoethane.

Applications: Synthesis of alkyl halides.; Organic chemical synthesis pathways.

Additional Info: Option B (Iron filings) is used as a catalyst in electrophilic aromatic substitution, such as bromination of benzene, not for alkanes.. Option C (High pressure) is used in industrial processes like polymerization or ammonia synthesis (Haber process).. Option D (Absence of solvent) is not a requirement; many radical reactions are performed in non-polar solvents like $CCl_{4}$.

Answer: A is correct — The presence of sunlight (UV radiation) provides the energy required for the homolytic fission of bromine into reactive radicals.

Exam Tip: Always distinguish between alkanes (substitution reactions requiring UV light) and alkenes (addition reactions that occur readily with bromine water even in the dark).

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Option B: Presence of a catalyst like iron filings

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Option C: High pressure only

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Option D: Absence of any solvent

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