Which statement about the reaction mechanism of alkanes with halogens under UV light is true?

• A. Free radical substitution
• B. Electrophilic addition
• C. Nucleophilic substitution
• D. Elimination

Answer: (A)

Free radical substitution is the mechanism here. Under UV light, halogen molecules split into halogen radicals. These radicals grab a hydrogen from the alkane, forming a hydrogen halide and leaving an alkyl radical. That alkyl radical then reacts with another halogen molecule to form the alkyl halide and regenerate the halogen radical, allowing the chain reaction to continue. This replaces a hydrogen atom with a halogen on the alkane.

Electrophilic addition isn’t correct because alkanes lack a double bond for an electrophile to attack. Nucleophilic substitution isn’t typical for unactivated alkanes with halogens under light, since there isn’t a good leaving group and no carbocation or SN mechanisms in play here. Elimination would form alkenes, which isn’t the observed primary pathway under these photochemical conditions.