### 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 \( C_nH_{2n+2} \), 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 heat or light. - The general reaction can be represented as: \[ \text{R-H} + \text{X}_2 \rightarrow \text{R-X} + \text{HX} \] 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 heat or light. - **Propagation**: The halogen radical reacts with the alkane, replacing one hydrogen atom to form a new alkyl halide (R-X) and another radical (R•). This radical can then react with another halogen molecule, continuing the chain reaction. - **Termination**: The reaction can terminate when two radicals combine to form a stable product. 4. **Example**: - Consider the reaction of methane (CH4) with chlorine (Cl2): \[ \text{CH}_4 + \text{Cl}_2 \rightarrow \text{CH}_3\text{Cl} + \text{HCl} \] - Here, one hydrogen atom in methane is replaced by a chlorine atom, resulting in chloromethane (CH3Cl) and hydrogen chloride (HCl). #### Why Other Options Are Incorrect: - **B. Addition**: - Addition reactions involve the addition of atoms or groups to a molecule, typically seen in unsaturated compounds (alkenes and alkynes). Since alkanes are saturated, they do not undergo addition reactions. - **C. Condensation**: - Condensation reactions involve the joining of two molecules with the loss of a small molecule (often water). While alkanes can participate in condensation reactions under specific conditions (like forming larger hydrocarbons), this is not their most common reaction type. - **D. Polymerisation**: - Polymerisation is the process of combining small molecules (monomers) to form a larger molecule (polymer). Alkanes do not typically undergo polymerisation as they lack the necessary reactive sites (like double bonds) to initiate this process. ### Revision Summary: - Alkanes primarily undergo **substitution reactions**, particularly free radical substitution with halogens. - They are saturated hydrocarbons, making them less reactive than unsaturated hydrocarbons. - Addition, condensation, and polymerisation are not common reactions for alkanes due to their structural characteristics. - Understanding the mechanism of free radical substitution is crucial for predicting the products of alkane reactions.