HALOGENATION OF ALKANES

Introduction to Alkanes

Alkanes are a group of hydrocarbons, which means they are compounds that only contain carbon and hydrogen atoms. They are also known as saturated hydrocarbons because they contain only single bonds between the carbon atoms, making them highly stable and unreactive.

The general formula for alkanes is CnH2n+2, where n is the number of carbon atoms in the molecule. For example, methane, which is the simplest alkane, has the molecular formula CH4, and ethane, which has two carbon atoms, has the formula C2H6.

Here are some chemical equations that illustrate the properties and reactions of alkanes:

1) Combustion: Alkanes are highly flammable and react vigorously with oxygen to produce carbon dioxide and water vapor. This reaction is exothermic, meaning it releases heat.

For example, the combustion of methane can be represented by the following equation:

CH4 + 2O2 -> CO2 + 2H2O + energy

 

2) Halogenation: Alkanes can undergo a substitution reaction with halogens (such as chlorine or bromine) to form halogenated derivatives. This reaction requires UV light or heat as a catalyst.

For example, the reaction between methane and chlorine can be represented by the following equation:

CH4 + Cl2 -> CH3Cl + HCl

 

3) Isomerization: Alkanes can be converted into isomeric forms, which are molecules with the same molecular formula but different structures. This reaction can be achieved through thermal or catalytic cracking.

For example, the isomerization of butane can be represented by the following equation:

C4H10 -> C4H9CH3 (2-methylpropane) + H2

 

4) Hydrogenation: Alkanes can be hydrogenated, meaning they are reacted with hydrogen gas in the presence of a catalyst to form saturated hydrocarbons with higher carbon content.

For example, the hydrogenation of ethene (not an alkane but a typical reaction) can be represented by the following equation:

C2H4 + H2 -> C2H6

 

Free radical mechanism

Halogenation of alkanes is a reaction in which a halogen molecule (e.g., chlorine, bromine) is added to an alkane. This reaction is an example of a free radical substitution reaction, which involves the substitution of one atom or group of atoms for another through a series of free radical intermediates.

The overall reaction for the halogenation of methane (CH4) with chlorine gas (Cl2) is as follows:

CH4 + Cl2 → CH3Cl + HCl

This reaction involves three distinct steps: initiation, propagation, and termination.

1) Initiation: The reaction is initiated by the homolytic cleavage (breaking) of the chlorine molecule into two chlorine free radicals:

Cl2 → 2Cl•

This step is usually triggered by the application of heat or light, which provides the energy needed to break the relatively strong chlorine-chlorine bond.

 

2) Propagation: Once the chlorine free radicals are formed, they can react with the methane molecule to form a methyl free radical and a hydrogen chloride molecule:

Cl• + CH4 → •CH3 + HCl

The methyl free radical can then react with another chlorine molecule to form a molecule of chloromethane (CH3Cl) and another chlorine free radical:

•CH3 + Cl2 → CH3Cl + Cl•

The new chlorine free radical can then react with another methane molecule to start the process again:

Cl• + CH4 → •CH3 + HCl •CH3 + Cl2 → CH3Cl + Cl•