Addition reactions of alkenesEdexcel International A Level Chemistry: Revision notes

Section 1

Why alkenes undergo addition

The C=C bond contains a pi bond whose electron density lies above and below the plane of the molecule. This makes alkenes much more reactive than alkanes. In an addition reaction the pi bond breaks and two atoms or groups add, one to each carbon of the C=C, giving a single saturated product. There is no other product, so the atom economy is 100%.

The reactions to know are with hydrogen, halogens, hydrogen halides, steam and acidified potassium manganate(VII).

Key termsaddition reactionpi bondatom economy

Section 2

Addition of hydrogen

Alkenes react with hydrogen gas in the presence of a nickel catalyst (heated, about 150 °C) to form an alkane. This is hydrogenation.

$CH_2{=}CH_2 + H_2 \rightarrow CH_3CH_3$

The same reaction hardens vegetable oils to make margarine. A molecule with two C=C bonds takes two molecules of hydrogen.

Key termshydrogenationnickel catalyst

Section 3

Addition of halogens and hydrogen halides

Halogens add at room temperature to give di-substituted halogenoalkanes, with one halogen atom on each carbon of the former double bond:

$CH_2{=}CH_2 + Br_2 \rightarrow CH_2BrCH_2Br$ (1,2-dibromoethane)

Hydrogen halides add to give mono-substituted halogenoalkanes:

$CH_2{=}CH_2 + HBr \rightarrow CH_3CH_2Br$ (bromoethane)

With an unsymmetrical alkene such as propene, HBr can add in two ways, so two isomers form; 2-bromopropane is the major product.

Key termsdi-substituted halogenoalkanemono-substituted halogenoalkane

Common mistake

In 1,2-dibromoethane the two Br atoms are on different carbon atoms. Do not name or draw 1,1-dibromoethane.

Section 4

Addition of steam: hydration

Alkenes react with steam in the presence of an acid catalyst to form alcohols. Industrially ethene and steam react over phosphoric acid on silica at about 300 °C and 60 to 70 atm:

$CH_2{=}CH_2 + H_2O \rightarrow CH_3CH_2OH$

The reaction is reversible, but unreacted ethene is recycled. The only product is ethanol, so the atom economy is 100%.

Key termshydrationphosphoric acid

Section 5

Oxidation to a diol

Alkenes are oxidised by cold, dilute, acidified potassium manganate(VII) to a diol, an alcohol with two OH groups on adjacent carbons. The purple solution turns colourless.

$CH_2{=}CH_2 + [O] + H_2O \rightarrow CH_2(OH)CH_2(OH)$ (ethane-1,2-diol)

$[O]$ represents oxygen supplied by the oxidising agent, the manganate(VII).

Key termsdioloxidising agent

Section 6

Test for a C=C double bond

Add bromine water (or bromine in an organic solvent) to the sample and shake. If the sample contains C=C, bromine adds across the double bond and the orange colour disappears, leaving a colourless solution. An alkane gives no change.

The result is 'colourless', not 'clear': a clear solution can still be coloured.

Key termsbromine waterunsaturated

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Addition reactions of alkenes

Margarine manufacturers harden liquid vegetable oils, whose molecules contain C=C bonds, by reacting them with hydrogen gas in the presence of a nickel catalyst. Relative atomic masses are given where needed, and the molar volume of a gas at room temperature and pressure is 24.0 dm³ mol⁻¹.
A student shakes a sample of the vegetable oil with bromine water. State the colour change and explain what it shows.2 marks
Ethanol for industrial use is made from ethene, which comes from the cracking of oil. Ethene also undergoes several other addition reactions that a chemist is investigating.
Write the equation for the reaction of ethene with steam to make ethanol, and name a suitable catalyst.2 marks
A technician has two unlabelled colourless liquids, one hexane and one hex-1-ene. Separately, she studies the oxidation of propene by cold, dilute, acidified potassium manganate(VII).
Describe a test that would identify which liquid is hex-1-ene, stating the observation for each liquid.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).