The reaction of halogens with alkanes in the presence of sunlight is an example of

Analyze the question: We are asked what type of reaction occurs when a halogen (e.g. Cl₂ or Br₂) reacts with an alkane in sunlight (UV light).

Explain the concept: Alkanes are saturated hydrocarbons, so they contain only single C-C and C-H bonds and have no double bonds to open up. This means they cannot undergo addition reactions. Instead, they react by substitution, where a hydrogen atom is replaced by a halogen atom. Because sunlight starts the reaction by splitting the halogen molecule into free radicals, it is also called free radical substitution or photochemical halogenation.

Step-by-step solution:

Take methane reacting with chlorine:

CH₄ + Cl₂ → CH₃Cl + HCl (in sunlight)

1. Sunlight breaks the Cl-Cl bond: Cl₂ → 2Cl·
2. A chlorine radical removes a hydrogen from methane: CH₄ + Cl· → CH₃· + HCl
3. The methyl radical attacks another Cl₂: CH₃· + Cl₂ → CH₃Cl + Cl·

One hydrogen has been replaced by one chlorine, which is substitution. The reaction can continue, giving CH₂Cl₂, CHCl₃ (trichloromethane, or chloroform) and CCl₄ (tetrachloromethane).

Checking the other options:

• A. Oxidation: this usually involves adding oxygen or removing hydrogen with an oxidizing agent, which is not the main classification here.
• B. Addition: this happens with unsaturated compounds (alkenes, alkynes), not alkanes.
• C. Hydrogenation: this adds H₂ to unsaturated compounds.
• D. Substitution: correct.

Correct answer: D. Substitution reaction

Exam tip: Remember "Alkanes = Substitution, Alkenes = Addition." Alkanes are saturated and substitute, while alkenes are unsaturated and add. If you see "sunlight/UV + alkane + halogen," think substitution.