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International Edexcel AS chemistry
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Topic 4: introductory organic chemistryThe fundamentals2 Topics
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Functional groups and homologous series2 Topics
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Nomenclature2 Topics
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Structural isomerism2 Topics
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Types of reaction2 Topics
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Substitution reactions2 Topics
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Hazards, risks and risk assessments
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Alkanes from crude oil1 Topic
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Alkanes as fuels1 Topic
Lesson 6, Topic 1
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Substitution reactions notes
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- Halogens react with alkanes in photochemical reactions.
- Photochemical reactions are started by light. This reaction requires ultraviolet light to start

A hydrogen atom is substituted (replaced) by chlorine or bromine. This is a radical substitution reaction



- Sunlight provides the energy to break the Cl-Cl bond, this is photodissociation
- The bond splits equally and each atom keeps one electron, this is homolytic fission
- The atom becomes a highly reactive radical, Cl•, because of its unpaired electron

- This equation can be represented using curly arrows to show the movement of electrons when forming the free radicals


- Cl• attacks the methane molecule

- The new ethyl radical • CH3, can attack another Cl2 molecule

- The new Cl•, can attack another CH4 molecule, and so on, until all the Cl2 of CH4 molecules are used up

- If two free radicals join together, they make a stable molecule
- There are many possible termination reactions


Desired product

Trace impurity
- Some of the products made in this way will be trace impurities in the final sample

- Further substitution is possible for example the producing dichloromethane, trichloromethane and tetrachloromethane rather than the desired product chloromethane

Dichloromethane

Trichloroethane

Tetrachloroethane
- A mixture of products form, both by further substitution and through termination steps
- The likelihood of further reactions being produced can be reduced by adding an excess of methane
- The mixture of products can be separated by fractional distillation since their boiling temperatures will be different
- This has an economic cost in industrial processes