Elimination reactions

An elimination reaction removes a small molecule, such as water or a hydrogen halide, from a larger organic molecule and leaves a C=C double bond, so the product is an alkene. At A-level there are two: a halogenoalkane heated with potassium hydroxide dissolved in ethanol loses HX, and an alcohol heated with a concentrated acid catalyst loses H₂O. In both, the atoms removed come from two neighbouring carbon atoms.

What is an elimination reaction?

In an elimination reaction, one molecule splits into two: an organic product with a double bond and a small molecule. An H atom leaves one carbon and a group (a halogen, or OH) leaves the carbon next to it. The two electrons from the broken C-H bond are used to make the new C=C bond.

Elimination is the reverse of addition: adding HBr to propene gives 2-bromopropane, and eliminating HBr from 2-bromopropane gives propene back.

Elimination from halogenoalkanes

AQA A-level Chemistry uses 2-bromopropane with potassium hydroxide as its example. With hot ethanolic KOH, the main product is propene:

The elimination mechanism

Here the hydroxide ion acts as a base: it removes an H⁺ ion. The mechanism happens in one step, shown with three curly arrows:

1. A lone pair on the oxygen of OH⁻ forms a bond to a hydrogen on a carbon next to the C-Br carbon (curly arrow from the lone pair to that H).
2. The electrons in that C-H bond move to form the second bond of the C=C (curly arrow from the C-H bond to between the two carbons).
3. The C-Br bond breaks, and both electrons go to the bromine, which leaves as Br⁻ (curly arrow from the C-Br bond to Br).

The products are the alkene, water (from OH⁻ + H⁺) and a bromide ion. Draw every curly arrow starting from a lone pair or a bond, never from an atom.

Substitution or elimination?

The same reagent can also act as a nucleophile and attack the δ+ carbon, giving an alcohol by nucleophilic substitution. AQA asks you to explain the role of hydroxide as both nucleophile and base. The two reactions happen at the same time, and the conditions decide which wins:

Elimination from alcohols (dehydration)

AQA says alkenes can be formed from alcohols by acid-catalysed elimination reactions. Because water is removed, it is also called dehydration. For ethanol:

The mechanism for dehydrating an alcohol

1. A lone pair on the oxygen of the OH group bonds to an H⁺ ion from the acid, making a protonated alcohol, R-OH₂⁺.
2. The C-O bond breaks and a water molecule leaves, taking both electrons. This leaves a carbocation.
3. An H⁺ ion is lost from a carbon next to the positive carbon, and the electrons from that C-H bond form the C=C.

The H⁺ used in step 1 is given back in step 3, so the acid is a catalyst. A common practical is making cyclohexene from cyclohexanol, then purifying it by distillation and washing.

Worked example: more than one alkene

What alkenes form when 2-bromobutane, CH₃CHBrCH₂CH₃, is heated with ethanolic KOH?

• The H can be removed from either carbon next to the C-Br carbon: carbon 1 or carbon 3.
• Removing an H from carbon 1 gives but-1-ene, CH₂=CHCH₂CH₃.
• Removing an H from carbon 3 gives but-2-ene, CH₃CH=CHCH₃.
• But-2-ene has E and Z isomers, so there are three alkenes: but-1-ene, E-but-2-ene and Z-but-2-ene.

The same three alkenes form when butan-2-ol is dehydrated. For how E and Z are named, see stereoisomers. By contrast, 2-bromopropane gives only propene, because removing an H from either end carbon makes the same molecule.

Check your understanding.

Common mistakes

• Removing H from the wrong carbon. It must come from a carbon next to the one carrying the halogen or OH.
• Starting a curly arrow from the OH⁻ charge or from H. Start it from a lone pair on the oxygen.
• Mixing up the conditions. Ethanolic and hot for elimination; aqueous and warm for substitution.
• Calling OH⁻ a nucleophile in elimination. In elimination it acts as a base.
• Missing an isomer. Check both neighbouring carbons, then check whether the alkene has E and Z forms.
• Forgetting the acid is a catalyst in dehydration. H⁺ is used and then given back.

Practise with a short timed paper

Halogenoalkanes, alcohols and alkenes are in the first year of A-level Chemistry. The daily A-level Chemistry: Year 1 paper is a short timed paper of 8 questions, with a new one every day. To see the shapes of the alkenes, build them in the Molecule Builder.

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