Halogenalkanes: Free Radical Substitution and Ozone A Level Chemistry

Write an overall equation for the reaction between methane and chlorine to make chloromethane and ?

Name the mechanism

CH₄ + Cl₂ → CH₃Cl + HCl

Free Radical Substitution

What is a RADICAL?

A species that contains an UNPAIRED ELECTRON

Name the three stages of Free Radical Substitution and state what species react and forms at each stage

Initiation:
molecule → two radicals
Propagation (P1 & P2):
molecule+radical→molecule₊radical
Termination:
radical+radical→molecule

What condition is needed for initiation?

High Energy UV

During Free Radical Substitution, what molecule is produced for each Hydrogen atom that is substituted for a halogen?

Hydrogen halide (ie HCl)

Write equations for initiation, propagation and termination for the reaction between methane and chlorine to form chloromethane

Initiation: Cl₂→2Cl∙
Propagation 1: CH₄ + Cl∙→HCl + ∙CH₃
Propagation 2: Cl₂ + ∙CH₃ → CH₃Cl + Cl∙
Termination (possible steps include):
∙CH₃ + ∙CH₃ → CH₃CH₃
∙CH₃ + Cl∙ → CH₃Cl
Cl∙ + Cl∙ → Cl₂

Write equations for initiation, propagation and termination for the reaction between chloromethane and chlorine to form dichloromethane

Initiation: Cl₂→2Cl∙
Propagation 1: CH₃Cl +Cl∙→HCl+ ∙CH₂Cl
Propagation 2: Cl₂+∙CH₂Cl→CH₂Cl₂+Cl∙
Termination (possible steps include):
∙CH₂Cl + ∙CH₂Cl → CH₂ClCH₂Cl
∙CH₂Cl + Cl∙ → CH₂Cl₂
Cl∙ + Cl∙ → Cl₂

Which two steps do I add together (and cancel) to produce the overall equation?

Propagation 1 & 2
Propagation 1: CH₄ + Cl∙→HCl + ∙CH₃
Propagation 2: Cl₂ + ∙CH₃ → CH₃Cl + Cl∙
Overall : CH₄ + Cl₂→HCl + CH₃Cl

State the harmful effects of high energy UV light

Skin cancer
Damage to eyes
(and possible crop damage)

What is a CFC?

ChloroFluoroCarbon

Why are CFCs less reactive on earth than alkanes

Because the C-F bond is very strong

What happens to CFCs in the upper atmosphere when they are exposed to high energy UV light?
What is this step called?

The C-Cl bond is broken by the high energy uv light to form Cl∙ (chlorine radical/chlorine atom)
Initiation step

Why does the C-Cl bond break but the C-F bond does not break in the upper atmosphere

Because the C-F bond is much stronger than the Cl bond.
There is not enough energy in the UV light to break the C-F bond.

Write an initiation equation for CClF₃

CClF₃ → ∙CF₃ + ∙Cl

Write the two propagation equations for the reaction between chlorine radical and ozone

P1: Cl∙ + O₃→ClO∙ + O₂
P2: ClO∙ + O₃ → Cl∙ + 2O₂

What is the OVERALL equation for the breakdown of ozone by chlorine radicals

2O₃→3O₂

Why is the reaction between a chlorine radical ozone considered CATALYTIC?

Because the Cl∙ (Chlorine radical) is used in one step and regenerated in another.

What is the equation for the NATURAL formation and breakdown of ozone in the upper atmosphere

O∙ + O₂ ⇌O₃

What are the safer alternatives to CFCs and why are they safer?

HCFCs (HydroChloroFluoroCarbons)
HFCs (HydroFluoroCarbons)
The HCFCs and HFCs contain C-H bonds which break more easily. These molecules have mostly broken down BEFORE they reach the ozone layer, so they do not form radicals