They being saturated and cannot undergo addition reaction.

Substitution reaction:

Substitution reactions are the characteristic reactions of alkanes. In a substitution reaction, an atom or group of atoms is replaced by another atom or group of atoms. This reaction occurs when a halogen (such as chlorine or bromine) reacts with an alkane.

Mechanism of substitution reaction:

The substitution reaction of alkanes involves three steps: initiation, propagation, and termination.

1. Initiation: In the initiation step, a halogen molecule (X2) is broken down into two halogen atoms (X•) by the input of energy, usually in the form of heat or light. For example, chlorine (Cl2) can be broken down into two chlorine atoms (Cl•) by exposure to ultraviolet (UV) light.

2. Propagation: In the propagation step, the halogen atom (X•) reacts with an alkane molecule (R-H) to form a hydrogen halide (HX) and an alkyl halide (R-X). The halogen atom replaces one of the hydrogen atoms in the alkane. This step can occur multiple times, leading to the substitution of more hydrogen atoms with halogen atoms.

3. Termination: In the termination step, the reactive species from the propagation step (alkyl radical and halogen atom) combine to form an unreactive product, such as a stable molecule or a dimer.

Example of substitution reaction:

An example of a substitution reaction is the reaction between methane (CH4) and chlorine (Cl2) to form chloromethane (CH3Cl) and hydrogen chloride (HCl). The reaction can be represented by the following equation:

CH4 + Cl2 → CH3Cl + HCl

In this reaction, one of the hydrogen atoms in methane is replaced by a chlorine atom, resulting in the formation of chloromethane and hydrogen chloride.

Significance of substitution reactions:

Substitution reactions are important because they allow for the synthesis of a wide range of organic compounds. By replacing one atom or group of atoms with another, various functional groups can be introduced into a molecule, resulting in different chemical and physical properties. Substitution reactions also play a crucial role in the preparation of many commercially important compounds, such as pharmaceuticals, plastics, and solvents.

In conclusion, the characteristic reaction of alkanes is substitution. In a substitution reaction, a halogen atom replaces one or more hydrogen atoms in an alkane molecule, leading to the formation of alkyl halides.

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