Flashcards in AS AQA CHEM 14.3 ELIMINATION REACTIONS IN HALOALKANES Deck (23):
How to haloalkanes typically react?
Haloalkanes typically react by nucleophilic substitution but under different conditions may react by elimination.
Elimination reaction summary
A hydrogen halide is eliminated from the molecule leaving a double bond in its place so that an alkene is formed.
Affect of an OH⁻ ion acting as a base.
OH- ion can act as a base, removing an H+ ion from the haloalkane.
Conditions of reaction when OH⁻ acts as a base.
Sodium/potassium hydroxide is dissolved in ethanol an mixed with the haloalkane.
No water is present.
Product of elimination reaction:
Product is ethene. Ethen burns and also decolourises bromine solution, showing that it has a carbon-carbon double bond.
Mechanism of elimination reaction.
1. OH- ion uses its lone pair to form a bond with one of the hydrogen atoms on the carbon next to the C-Br bond. These hydrogen atoms are very slightly δ+.
2. Electron pair from the C-H bond now becomes part of a carbon-carbon double bond.
3. Bromide takes the pair of electrons in the C-Br bond and becomes and ion.
Uses of elimination reactions
To make molecules with carbon-carbon double bonds.
Result of hydrogen ions reaction with haloalkanes as a nucleophile or as a base.
There is competition between elimination and substitution reactions. A mixture of an alcohol and an alkene is formed.
Reaction that predominates when hydrogen ions act as nucleophiles and bases depend on:
1. Reaction conditions - aqueous / ethanolic solution
2. Type of haloalkane (primary, secondary or tertiary).
Hydroxide ions at room temperature dissolved in water (aqueous) favour:
Hydroxide ions at high temperature, dissolved in ethanol favour:
Effect of the type of haloalkane: Primary haloalkanes
Tend to react by substitution.
Effect of the type of haloalkane: Tertiary haloalkanes
Tend to react by elimination.
Effect of the type of haloalkane: Secondary haloalkanes
Will react by elimination and substitution.
Classification - Primary
At the end of a chain.
Classification - Secondary
In the body of the chain.
Classification - Tertiary
At a branch in the chain.
Haloalkanes containing both chlorine and fluorine atoms but no hydrogen.
Reactions of CFCs
Unreactive under normal conditions
Uses of chlorofluorocarbons: Short chain CFCs
Cases and were used as aerosol propellants, refrigerants and blowing agents for foams like expanded polystyrene.
Uses of chlorofluorocarbons: Long chain CFCs
Used as dry-cleaning and de-greasing solvents.
Problems with CFCs
They eventually end up in the atmosphere where they decompose to give chlorine atoms.
Chloride atoms decompose ozone causing a hole in the Earth's ozone layer which will take years to recover.