Product of ethene + Br₂

1,2-Dibromoethane, CH₂BrCH₂Br.

How is ethanol made from ethene?

Ethene is reacted with steam using a phosphoric acid catalyst (about 300 °C, 60 to 70 atm).

Equation for hydration of ethene

C₂H₄ + H₂O → C₂H₅OH

Product of ethene with cold acidified KMnO₄

Ethane-1,2-diol; the purple solution turns colourless.

Equation for oxidation of ethene to a diol

CH₂=CH₂ + [O] + H₂O → CH₂(OH)CH₂(OH)

Test for a C=C double bond

Shake with bromine water: orange to colourless.

Why is the atom economy of addition 100%?

All the reactant atoms end up in the single product.

What is hydrogenation used for in industry?

Hardening vegetable oils to make margarine.

Why are alkenes more reactive than alkanes?

The C=C has a pi bond with high electron density above and below the plane, which is easily attacked.

Why say 'colourless' rather than 'clear'?

A solution can be clear but still coloured; the bromine colour disappears, so it is colourless.

What does [O] represent in an oxidation equation?

Oxygen supplied by the oxidising agent.

Addition reactions of alkenesEdexcel International A Level Chemistry: Flashcards

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Conditions for hydrogenation of an alkene

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Conditions for hydrogenation of an alkene: Hydrogen gas with a nickel catalyst, heated (about 150 °C); forms an alkane.
Product of ethene + Br₂: 1,2-Dibromoethane, CH₂BrCH₂Br.
Product of ethene + HBr: Bromoethane, CH₃CH₂Br.
How is ethanol made from ethene?: Ethene is reacted with steam using a phosphoric acid catalyst (about 300 °C, 60 to 70 atm).
Equation for hydration of ethene: C₂H₄ + H₂O → C₂H₅OH
Product of ethene with cold acidified KMnO₄: Ethane-1,2-diol; the purple solution turns colourless.
Equation for oxidation of ethene to a diol: CH₂=CH₂ + [O] + H₂O → CH₂(OH)CH₂(OH)
Test for a C=C double bond: Shake with bromine water: orange to colourless.
Why is the atom economy of addition 100%?: All the reactant atoms end up in the single product.
What is hydrogenation used for in industry?: Hardening vegetable oils to make margarine.
Why are alkenes more reactive than alkanes?: The C=C has a pi bond with high electron density above and below the plane, which is easily attacked.
Why say 'colourless' rather than 'clear'?: A solution can be clear but still coloured; the bromine colour disappears, so it is colourless.
Which reagent gives a mono-substituted halogenoalkane from an alkene?: A hydrogen halide such as HBr.
What does [O] represent in an oxidation equation?: Oxygen supplied by the oxidising agent.

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).