Question

How to carry out halogenation process?

Answer 1

Halogenation reaction

• A type of chemical reaction that involves halogen atom substitution with some other substance.
• And the halogen that was substituted becomes part of that substance/ compound.
• In general, a halogenation reaction is a reaction that involves the addition of one or more halogens to a compound.

Free radical chain mechanism

• It is a chemical mechanism
• In this, unpaired electrons have molecules involved.
• Radical species can either be a initial compound or it can be product also.

Halogenation reaction mechanism

• $\mathrm{R}-\mathrm{H}$ represents alkane where R is any alkyl group.
• When the alkyl group gets attached to a hydrogen atom, the parent hydrocarbon of the alkyl group is formed.

$\mathrm{R}-\mathrm{H}+{\mathrm{X}}_2\to \mathrm{R}-\mathrm{X}+\mathrm{HX}$

• $\mathrm{R}-\mathrm{X}$ is the product formed where R is that alkyl group present in the reactant side and X is the halogen used.

For example Chlorination for methane

Step$1$: Initiation

• For initiation, halogen gets splits into two free radicals with the help of ultraviolet light.

$$\begin{gathered} \mathrm{Cl}-\mathrm{Cl}\stackrel{\mathrm{h\nu }}{\to }\mathrm{Cl}°+\mathrm{Cl}° \\ \mathrm{where}\mathrm{Cl}°\mathrm{represents}\mathrm{the}\mathrm{chlorine}\mathrm{radical} \end{gathered}$$

Step$2$: Propagation

• In this step, alkane reacts with halogen.
• Hydrogen gets separated from carbon resulting in methyl radical formation
• Methyl radical now reacts with the chlorine radical.
• Since chlorine is continuously formed, this can theoretically continue, just like a chain reaction till there are reactants.

$$\begin{gathered} {\mathrm{CH}}_4+\mathrm{Cl}°\stackrel{\mathrm{h\nu }}{\to }°{\mathrm{CH}}_3+\mathrm{H}-\mathrm{Cl} \\ °{\mathrm{CH}}_3+\mathrm{Cl}-\mathrm{Cl}\to {\mathrm{CH}}_3-\mathrm{Cl}+\mathrm{Cl}° \\ {\mathrm{CH}}_3\mathrm{Cl}+\mathrm{Cl}°\to °{\mathrm{CH}}_2\mathrm{Cl}+\mathrm{HCl} \\ °{\mathrm{CH}}_2+\mathrm{Cl}-\mathrm{Cl}\to {\mathrm{CH}}_2{\mathrm{CL}}_2+\mathrm{Cl}° \end{gathered}$$

Step$3$: Termination

• When termination occurs, two chlorine interacts or two methyl radicals combine to form ethane.

$$\begin{gathered} \mathrm{Cl}°+\mathrm{Cl}°\to \mathrm{Cl}-\mathrm{Cl} \\ {\mathrm{H}}_3\mathrm{C}°+°{\mathrm{CH}}_3\to {\mathrm{H}}_3\mathrm{C}-{\mathrm{CH}}_3 \end{gathered}$$

Hence, the halogenation process is carried out via a free radical chain mechanism.