The correct option is Substitution.
Explanation
Methane ($CH_4$) is an alkane, which is a saturated hydrocarbon containing only single covalent bonds. Saturated hydrocarbons are generally unreactive towards many reagents but undergo specific reactions, such as halogenation, in the presence of ultraviolet (UV) light or at high temperatures.Analysis of the Reaction Type
- Substitution (Correct): When methane reacts with chlorine in the presence of sunlight, one or more hydrogen atoms of the methane molecule are replaced by chlorine atoms. For example, the formation of chloromethane ($CH_3Cl$) and hydrogen chloride ($HCl$). Since hydrogen atoms are replaced by chlorine atoms, this is classified as a substitution reaction. Specifically, it follows a free radical substitution mechanism.
- Addition (Incorrect): Addition reactions are characteristic of unsaturated hydrocarbons (alkenes and alkynes) possessing double or triple bonds. In these reactions, atoms are added to the molecule by breaking pi bonds. Methane, being saturated, cannot undergo addition reactions.
- Oxidation/Reduction (Incorrect context): While the carbon atom's oxidation state changes during chlorination, in the context of organic reaction mechanisms distinguishing between structural changes, this process is defined as substitution. Oxidation typically refers to combustion or the addition of oxygen/removal of hydrogen in a broader functional group context.
Key Takeaway
Alkanes (saturated hydrocarbons) undergo free radical substitution reactions in the presence of sunlight, where hydrogen atoms are replaced by halogens.