Unit 2 Section 4 Elimination Reactions

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Elimination of a Halogen From a Haloalkane

If you warm a haloalkane with hydroxide ions dissolved in ethanol instead of water, an elimination reaction happens, and you end up with an alkene.

Example:

Reacting bromoethane with potassium hydroxide dissolved in warm ethanol under reflux produces an elimination reaction and forms ethene, water and potassium bromide.

CH3CH2Br + KOH >>> C2H4 + H20 + KBr

In the reaction, H and Br are eliminated from neighbouring carbon atoms in CH3CH2Br to leave CH2=CH2.

1. OH- acts as a base and takes a proton, H+, from the carbon on the left. This makes water.

2. The left carbon now has a spare pair of electrons, so it forms a double bond with the other carbon.

3. To form the double bond, the right carbon has to let go of the Br, which drops off as a Br- ion.

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Nucleophilic Substitution vs Elimination - part 1

You can control what type of reaction happens by changing the conditions.

By reacting a haloalkane with water under reflux, the molecule will predominantly undergo a nucleophilic substitution reaction to form an alcohol.

You'll still get a bit of elimination to form an alkane but not a lot

Example:

Reacting bromoethane with water (NaOH or KOH) under reflux will produce ethanol.

This is because under aqueous conditions, the OH- acts as a nucleophile - it donates an electron pair to delta positive carbon atom.

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Nucleophilic Substitution vs Elimination - part 2

By reacting a haloalkane with ethanol under reflux, the molecule will predominantly undergo an elimination reaction to form an alkene.

Example:

Reacting bromoethane with ethanol (NaOH or KOH) under reflux will produce ethene.

This is because under anhydrous conditions, the OH- acts as a base - it removes a hydrogen atom from the haloalkane.

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Nucleophilic Substitution vs Elimination - part 3

Both of these reactions have their uses.

The elimination reaction is a good way of getting a double bond into a molecule.

Loads of other organic synthesis reactions use alkenes, so the elimination is a good starting point for making lots of different organic chemicals.

The substitution reaction allows you to produce any alcohol molecule that you need.

And alcohols can be the starting point for sythesis reactions that produce aldehydes, ketones, esters and carboxylic acids. 

So haloalkanes are very useful as a starting material for making other organic compounds.

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