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
Official 2025-2027 spec · A2 LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- recall the reactions (including reagents and conditions) by which alkenes can be produced: (a) elimination of HX from a halogenoalkane by ethanolic NaOH and heat (b) dehydration of an alcohol, by using a heated catalyst (e.g. Al 2O3) or a concentrated acid (e.g. concentrated H2SO4) (c) cracking of a longer chain alkane
- describe the following reactions of alkenes: (a) the electrophilic addition of (i) hydrogen in a hydrogenation reaction, H2(g) and Pt/Ni catalyst and heat (ii) steam, H2O(g) and H3PO4 catalyst (iii) a hydrogen halide, HX(g), at room temperature (iv) a halogen, X2 (b) the oxidation by cold dilute acidified KMnO4 to form the diol (c) the oxidation by hot concentrated acidified KMnO4 leading to the rupture of the carbon–carbon double bond and the identities of the subsequent products to determine the position of alkene linkages in larger molecules (d) addition polymerisation exemplified by the reactions of ethene and propene
- describe the use of aqueous bromine to show the presence of a C=C bond
- describe the mechanism of electrophilic addition in alkenes, using bromine/ethene and hydrogen bromide/propene as examples
- describe and explain the inductive effects of alkyl groups on the stability of primary, secondary and tertiary cations formed during electrophilic addition (this should be used 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.
Tips to avoid common mistakes in Alkenes
- › Check if elimination products form a side chain or ring C=C bond, as this determines the optical isomerism of subsequent reaction products.
- › Treat the cyclohexene ring as non-planar and ensure straight-chain alkenes with the formula C6H10 have two C=C double bonds.
- › Identify every C=C double bond in the molecule and determine if cleavage results in a ketone, aldehyde, or carboxylic acid.
- › Only count hybridised orbitals belonging to carbon atoms; internal carbons in alkenes are sp2 hybridised, while terminal methyl carbons are sp3.
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.
Bromine becomes polarised as it approaches the alkene. The π electrons of the C=C bond attack the partially positive bromine, forming a cyclic bromonium ion intermediate. A bromide ion then attacks the carbon, breaking the bromonium ion and forming a dibromoalkane.
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.
More Chemistry flashcards
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All Chemistry FlashcardsMore topics in Unit 14 — Hydrocarbons
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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