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Structure and electrophilic addition reactions of alkenesAQA A-Level Chemistry: Flashcards

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What is an unsaturated hydrocarbon?

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What is an unsaturated hydrocarbon?
A hydrocarbon containing at least one C=C double bond.
Why do alkenes react with electrophiles?
The C=C bond is a centre of high electron density.
Define an electrophile.
An electron pair acceptor.
What is the test for an alkene?
Shake with bromine water: orange to colourless.
What is the product of ethene and bromine?
1,2-dibromoethane, CH₂BrCH₂Br.
What is the product of ethene and HBr?
Bromoethane, CH₃CH₂Br.
What is the product of ethene and cold concentrated H₂SO₄?
Ethyl hydrogensulfate, CH₃CH₂OSO₂OH.
How is bromine able to act as an electrophile?
It is polarised by the electron-rich C=C bond, giving a δ+ bromine atom.
What is a carbocation?
An ion with a positively charged carbon atom, formed as an intermediate.
Place primary, secondary and tertiary carbocations in order of stability.
Tertiary > secondary > primary.
Why are alkyl groups stabilising to carbocations?
They release electron density (positive inductive effect) towards the positive carbon.
What are the major and minor products of HBr with propene?
Major: 2-bromopropane; minor: 1-bromopropane.

Exam questions on Structure and electrophilic addition reactions of alkenes

  1. Propene, CH₃CH=CH₂, is an unsaturated hydrocarbon. A student shakes a sample of propene gas with bromine water, and in a separate experiment reacts propene with bromine in an inert solvent.
    Write an equation for the reaction of propene with bromine, using structural formulae, and name the organic product.2 marks
  2. Carbocations are intermediates in electrophilic addition reactions of alkenes. They are classified as primary, secondary or tertiary according to the number of alkyl groups attached to the positively charged carbon atom.
    Explain why a tertiary carbocation is more stable than a primary carbocation.2 marks
  3. Alkenes react with electrophiles such as bromine and concentrated sulfuric acid. A student is asked to describe the mechanisms of these reactions using ethene as the example.
    Describe the mechanism for the reaction of ethene with bromine, referring to the movement of electron pairs.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).