### Correct Option: A. Substitution #### Detailed Explanation: Alkanes are saturated hydrocarbons, meaning they consist only of carbon (C) and hydrogen (H) atoms, with single bonds between the carbon atoms. The general formula for alkanes is CₙH₂ₙ₊₂, where n is the number of carbon atoms. 1. **Nature of Alkanes**: - Alkanes are relatively unreactive due to the strong C-H and C-C bonds. They do not readily participate in addition reactions, which are more characteristic of unsaturated hydrocarbons (like alkenes and alkynes) that contain double or triple bonds. 2. **Substitution Reactions**: - The most common reaction that alkanes undergo is **substitution**, specifically **free radical substitution**. This occurs when an alkane reacts with a halogen (like chlorine or bromine) in the presence of UV light or heat. - The general reaction can be represented as: \[ \text{R-H} + \text{X}_2 \xrightarrow{\text{UV light}} \text{R-X} + \text{HX} \] where R is the alkyl group, X is a halogen, and HX is a hydrogen halide. 3. **Mechanism of Free Radical Substitution**: - **Initiation**: The halogen molecule (X₂) is broken down into two halogen radicals (X•) by UV light or heat. - **Propagation**: The halogen radical reacts with the alkane, removing a hydrogen atom and forming an alkyl radical (R•). The alkyl radical can then react with another halogen molecule to form the substituted product (R-X) and regenerate the halogen radical. - **Termination**: The reaction can terminate when two radicals combine to form a stable molecule. 4. **Example**: - Consider the reaction of methane (CH₄) with chlorine (Cl₂): \[ \text{CH}_4 + \text{Cl}_2 \xrightarrow{\text{UV light}} \text{CH}_3\text{Cl} + \text{HCl} \] - Here, methane is converted to chloromethane (CH₃Cl) through a substitution reaction. #### Why Other Options Are Incorrect: B. **Addition**: - Addition reactions involve the addition of atoms or groups to a molecule, typically seen in alkenes and alkynes due to their double or triple bonds. Alkanes, being saturated, do not undergo addition reactions because they lack the multiple bonds necessary for this type of reaction. C. **Condensation**: - Condensation reactions involve the joining of two molecules with the loss of a small molecule, often water. This type of reaction is more common in the formation of esters or amides from carboxylic acids and alcohols or amines, not in alkanes. D. **Polymerisation**: - Polymerisation is the process of linking monomer units to form a polymer. While alkenes can undergo polymerisation to form long-chain hydrocarbons, alkanes do not typically participate in this process due to their saturated nature. ### Revision Summary: - Alkanes primarily undergo **substitution reactions**, particularly free radical substitution with halogens. - The mechanism involves initiation, propagation, and termination stages. - Addition, condensation, and polymerisation are not typical reactions for alkanes due to their saturated structure. - Understanding the reactivity of alkanes is crucial for predicting their behavior in chemical reactions.