Alkenes
Cambridge A-Level Chemistry (9701) · Unit 14: Hydrocarbons · 9 flashcards
Alkenes is topic 14.2 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 14 — Hydrocarbons , alongside Alkanes. In one line: When an alkene reacts with aqueous bromine, the bromine water decolourises from orange/brown to colourless. This indicates the presence of a carbon-carbon double bond (C=C).
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 9 flashcards — 2 definitions and 7 key concepts — covering the precise wording mark schemes reward. Use the 2 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Observed when an alkene reacts with aqueous bromine, and what does this indicate
When an alkene reacts with aqueous bromine, the bromine water decolourises from orange/brown to colourless. This indicates the presence of a carbon-carbon double bond (C=C).
What the Cambridge 9701 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Recall how alkenes can be made, including reagents and conditions: (a) eliminating HX from a halogenoalkane by heating with NaOH in ethanol; (b) dehydrating an alcohol over a heated catalyst such as Al₂O₃, or with a concentrated acid such as concentrated H₂SO₄; (c) cracking a longer-chain alkane.
- Describe these alkene reactions: (a) electrophilic addition of (i) hydrogen (hydrogenation), using H₂(g) with a Pt or Ni catalyst and heat, (ii) steam, H₂O(g), with an H₃PO₄ catalyst, (iii) HX(g), a hydrogen halide, at room temperature, (iv) X₂, a halogen; (b) oxidation by cold, dilute, acidified KMnO₄ to give a diol; (c) oxidation by hot, concentrated, acidified KMnO₄, which breaks the C=C double bond, and using the products formed to locate the double bond in larger molecules; (d) addition polymerisation, using ethene and propene as examples.
- Describe how aqueous bromine is used as a test for a C=C bond.
- Describe the electrophilic addition mechanism for alkenes, using bromine with ethene and hydrogen bromide with propene as examples.
- Describe and explain how the inductive effect of alkyl groups affects the stability of the primary, secondary and tertiary carbocations formed in electrophilic addition, and use this to explain Markovnikov addition.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Outline the reagents and conditions required for the elimination of HX from a halogenoalkane to produce an alkene.
Elimination of HX from a halogenoalkane requires ethanolic NaOH and heat. The reaction produces an alkene, NaX, and water.
Describe the two methods for dehydrating an alcohol to form an alkene, including reagents and conditions.
Alcohols can be dehydrated using either a heated catalyst (
What are the reagents and conditions for the hydrogenation of an alkene?
The hydrogenation of an alkene requires hydrogen gas (H₂(g)) and a platinum or nickel catalyst (Pt/Ni) with heat. This reaction converts the alkene into an alkane.
Outline the reagents and conditions for the addition of steam to an alkene to form an alcohol.
The addition of steam to an alkene requires water as steam (H₂O(g)) and a phosphoric acid catalyst (H₃PO₄). This reaction converts the alkene into an alcohol.
What is observed when an alkene reacts with aqueous bromine, and what does this indicate?
When an alkene reacts with aqueous bromine, the bromine water decolourises from orange/brown to colourless. This indicates the presence of a carbon-carbon double bond (C=C).
Describe the mechanism of electrophilic addition of bromine (Br₂) to ethene.
As Br₂ approaches the C=C, it is polarised (Brδ+–Brδ−). The π electrons of the C=C attack the Brδ+, and the Br–Br bond breaks heterolytically, giving a carbocation (CH₂Br–CH₂⁺) and a bromide ion. The Br⁻ then donates a lone pair to the positive carbon, forming 1,2-dibromoethane, CH₂BrCH₂Br.
Explain Markovnikov's rule and how it relates to the stability of carbocations.
Markovnikov's rule states that in the addition of HX to an alkene, the hydrogen atom adds to the carbon with more hydrogen atoms already attached. This is because the more substituted carbocation intermediate (tertiary > secondary > primary) is more stable due to the inductive effect of alkyl groups donating electron density.
Outline the products formed when an alkene is oxidised by hot concentrated acidified KMnO₄.
Oxidation by hot concentrated acidified KMnO₄ leads to the rupture of the carbon-carbon double bond. Depending on the alkene, the products can include ketones, carboxylic acids, or carbon dioxide, which depend on the substituents on the C=C.
Describe the reaction of alkenes in addition polymerisation, giving an example.
Alkenes undergo addition polymerisation where many alkene monomers join together to form a long chain polymer.
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Alkenes sits alongside these A-Level Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Alkenes deck
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