Elimination Reactions of Halogenoalkanes (Oxford AQA International A Level Chemistry)
Revision Note
Written by: Philippa Platt
Reviewed by: Stewart Hird
Elimination Mechanism
In an elimination reaction, an organic molecule loses a small molecule
In the case of halogenoalkanes this small molecule is a hydrogen halide (e.g. HCl)
The halogenoalkanes are heated with ethanolic sodium hydroxide (no water present) causing the C-X bond to break heterolytically, forming an X- ion and leaving an alkene as an organic product
Under these conditions the OH- ion can act as a base removing an H+ ion from the halogenoalkane
Elimination of a halogenoalkane
Elimination with OH-
There are three curly arrows in this mechanism
Curly arrow from lone pair on :OH- ion to H atom bonded to the adjacent carbon atom
Curly arrow from the C–H bond adjacent to the appropriate C–C bond
Curly arrow from C-X bond to Br (X) atom
Which product will form?
Note that the reaction conditions should be stated correctly as different reaction conditions will result in different types of organic reactions
Mechanism favoured | Conditions | Product |
---|---|---|
Nucleophilic substitution | NaOH in water Room temperature Favoured by primary halogenoalkanes | Alcohol |
Elimination | NaOH in ethanol (no water present) High temperature Favoured by tertiary halogenoalkanes | Alkene |
Isomeric alkenes
Examiner Tips and Tricks
To help visualise the elimination mechanism you can think of a stair case as the shape is very similar. Double check you have drawn first curly arrow to the correct hydrogen atom.
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