A hydrocarbon C₅H₁₀ does not react with chlorine in dark but gives a single monochloro compound C₅H₉Cl in bright sunlight. Identify the hydrocarbon.
The hydrocarbon is cyclopentane.
The molecular formula C₅H₁₀ corresponds to either an alkene or a cycloalkane. Since the compound does not react with chlorine in the dark, it cannot be an alkene because alkenes undergo addition reactions with halogens even in the absence of light. This indicates the compound is a cycloalkane.
In bright sunlight, free radical substitution occurs. The fact that only a single monochloro compound is formed indicates that all hydrogen atoms in the molecule are equivalent. In cyclopentane, all 10 hydrogen atoms are identical (equivalent) as they are attached to similar secondary carbon positions on the ring. Therefore, substitution of any one hydrogen yields the same product: chlorocyclopentane (C₅H₉Cl).
Explanation
This question tests understanding of the difference between addition reactions of alkenes and free radical substitution reactions of alkanes/cycloalkanes. The key insight is that alkenes react with chlorine in the dark via electrophilic addition, while alkanes/cycloalkanes require UV light (bright sunlight) for free radical substitution. The formation of a single monochloro product indicates molecular symmetry where all hydrogens are equivalent - a characteristic of cyclopentane.
Solution Steps
Step 1: Analyze molecular formula C₅H₁₀ - could be alkene or cycloalkane
Step 2: Apply reactivity criteria - no reaction in dark rules out alkene
Step 3: Apply product criteria - single monochloro product indicates all equivalent hydrogens
Step 4: Conclude cyclopentane is the only structure with all equivalent hydrogens