The Formation of Halogenoalkanes

This lesson covers: 

1. How halogens undergo radical substitution reactions with alkanes
2. The reaction mechanism for the halogenation of alkanes
3. How chlorofluorocarbons are produced and their impact on the ozone layer

Photochemical halogenation of alkanes

Halogens react with alkanes in special light-induced reactions called photochemical reactions.

For the reaction to occur:

• Ultraviolet (UV) light must be present.
• This UV light provides the activation energy to start the reaction.

The overall reaction is a substitution, where a hydrogen atom in the alkane molecule is replaced by a halogen atom like chlorine or bromine.

Free radical substitution mechanism

Photochemical halogenation of alkanes follows a three-step free radical substitution mechanism:

1. Initiation - UV light produces reactive radicals
2. Propagation - Radicals react in a chain reaction
3. Termination - Radicals join to form stable molecules

Methane reacts vigorously with chlorine gas in the presence of UV light, as represented by the equation:

CH₄ + Cl₂ ➔ CH₃Cl + HCl

The three-stage mechanism for this reaction is detailed below.

Stage 1 - Initiation

• UV light breaks the Cl-Cl bond in chlorine via homolytic fission
• This gives two chlorine radicals (Cl•):

Cl₂ ➔ 2Cl•

• The unpaired electron makes Cl• highly reactive.

Stage 2 - Propagation

• The chlorine radical attacks a methane molecule in a substitution reaction:

Cl• + CH₄ ➔ CH₃• + HCl

• The methyl radical (CH₃•) attacks another chlorine molecule:

CH₃• + Cl₂ ➔ CH₃Cl + Cl•

• This propagation cycle continues until reagents are used up.

Stage 3 - Termination

• Two radicals join to form a stable covalent bond:

Cl• + CH₃• ➔ CH₃Cl

• Other combinations like 2CH₃• ➔ C₂H₆ or 2Cl• ➔ Cl₂ are also possible.

The end products of the termination step depend on which reagent is in excess:

• Excess chlorine - Further substitution on products like chloromethane occurs, producing a mixture of products such as CH₃Cl, CH₂Cl₂, CHCl₃, and CCl₄.
• Excess methane - Predominantly single substitution occurs to form chloromethane.

Chlorofluorocarbons and ozone depletion

Chlorofluorocarbons (CFCs) are halogenoalkanes containing chlorine, fluorine and carbon with no hydrogen atoms remaining.

Two examples of CFCs are trichlorofluoromethane (CCl₃F) and chlorotrifluoromethane (CClF₃).

Ozone (O₃) in the upper atmosphere serves as a vital "chemical sunscreen", absorbing harmful ultraviolet radiation from the sun.

CFCs pose a threat to this protective ozone layer, leading to increased risks of sunburn, skin cancer, and damage to plant life.

How CFCs damage the ozone layer

When CFC molecules reach the upper atmosphere, ultraviolet light breaks them down, producing reactive chlorine radicals (Cl•).

These chlorine radicals catalyse the destruction of ozone via the following propagation reactions:

Cl• + O₃ ➔ ClO• + O₂

ClO• + O₃ ➔ 2O₂ + Cl•

The overall reaction is:

2O₃ ➔ 3O₂

This "ozone depletion" caused by chlorine radicals gradually thins the Earth's protective ozone layer. Due to their detrimental impact, CFCs were globally banned in 1989.