Alkenes
In alkenes, the high electron density of the carbon–carbon double bond leads to attack on these molecules by electrophiles.
Full topic guide: the detailed syllabus page with worked examples and common mistakes lives at studyvector.co.uk/a-level/chemistry/organic-chemistry/alkenes-a-level.
Topic preview: Alkenes
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Topic explanation
Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond (C=C), which consists of a sigma (σ) bond and a pi (π) bond. The π-bond is a region of high electron density, making alkenes much more reactive than alkanes. Their key reaction is electrophilic addition, where the double bond breaks and the electrophile adds across the two carbon atoms. This mechanism is used in reactions with hydrogen halides, halogens, and sulfuric acid.
Alkenes is easiest to revise when it is treated as a precise exam behaviour, not a loose note-taking category. In A-Level Chemistry, the goal is to recognise how the topic appears in a question, identify the command word, and decide what evidence, method, or vocabulary earns marks. StudyVector keeps this page tied to AQA · Edexcel · OCR language where coverage is available, then routes practice towards the same topic so revision moves from explanation into retrieval.
A strong revision session starts with a short recall check. Write down the rule, definition, process, or method linked to Alkenes before looking at any notes. Then answer one exam-style prompt and compare your answer with the mark-scheme logic: did you make a clear point, support it with the right step, and avoid drifting into a nearby topic? This matters because many lost marks come from almost-correct answers that do not match the expected structure.
Use this guide as the first layer: understand the topic, look at the worked examples, complete the mini quiz, then move into full practice. The full StudyVector practice loop is designed to capture whether mistakes are caused by knowledge, method, language, or timing. That distinction is important. If the error is factual, you need reteaching. If the error is method-based, you need a worked retry. If the error is wording, you need command-word calibration. That is how Alkenes becomes a controlled revision target rather than another page in a folder.
Lost marks → repair task
Why marks are usually lost here
These are the error patterns StudyVector looks for after an attempt. The goal is not a generic explanation; it is one repair move and one follow-up question.
Unit, formula, or method slip
Examiner move: Select the correct method and keep units, substitutions, signs, and rounding visible.
Repair drill: Redo the calculation or method line slowly, naming the formula before substituting values.
Missing chain of reasoning
Examiner move: Show the link between point, method, evidence, and conclusion instead of jumping to the final line.
Repair drill: Write the missing because/therefore step, then retry one isomorphic question.
Command-word miss
Examiner move: Answer the action in the command word before adding extra detail.
Repair drill: 60-second rewrite: start the answer with explain, compare, evaluate, state, or calculate in mind.
Mini quiz
Use these checks before full practice. They test topic recognition, exam technique, and whether you can connect the explanation to a marked response.
1. What should you check first when a Alkenes question appears in A-Level Chemistry?
- A.The command word and the exact topic focus
- B.The longest paragraph in your notes
- C.A memorised answer from a different topic
2. Which revision action gives the strongest evidence that Alkenes is improving?
- A.Rereading the explanation twice
- B.Answering a timed exam-style question and reviewing lost marks
- C.Highlighting every key phrase in the topic notes
Sample questions
Topic-specific public question previews are still being reviewed. We keep them off public pages until the topic match is safe.
Exam tips
- Read the command word carefully — "explain" needs reasons; "state" expects a short fact.
- For Alkenes, show structured working even when you are practising multiple choice — it builds accuracy under time pressure.
- Mark yourself against the mark scheme style: one clear point per mark, in logical order.
- Come back to this topic after a day or two; short spaced reviews beat one long cram.
Worked examples
Example 1
Modelled exam response
Show the mechanism for the reaction of propene with hydrogen bromide. Step 1: The H-Br bond is polar (Hδ+-Brδ-). The electron-rich C=C double bond attacks the Hδ+, and the H-Br bond breaks. This forms a bromide ion and a carbocation. Step 2: Two possible carbocations can be formed: a primary carbocation or a more stable secondary carbocation. The secondary carbocation (with the positive charge on the central carbon) is the major intermediate. Step 3: The bromide ion (Br-) then attacks the positive carbocation, forming the major product, 2-bromopropane.
Example 2
Identify the task before answering
Question type: a Alkenes prompt asks for a clear response in A-Level Chemistry. Step 1: underline the command word. Step 2: name the exact part of Alkenes being tested. Step 3: decide whether the mark scheme wants a definition, method, explanation, comparison, or calculation. Why it works: most weak answers fail before the content starts because they answer the topic generally rather than the exact exam task.
Example 3
Turn feedback into a repair task
Suppose your answer shows partial understanding but loses marks for precision. First, rewrite the missing mark as a short target: "I need to state the mechanism, unit, reason, or evidence explicitly." Then answer one similar question without notes. Finally, compare the second attempt with the first and check whether the same mark was recovered. Why it works: Alkenes improves faster when feedback creates a specific retry, not another passive reading session.
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Common mistakes
- Confusing the mechanism of electrophilic addition with free-radical substitution. Alkenes undergo addition reactions, while alkanes undergo substitution.
- Incorrectly predicting the major product in the addition of an unsymmetrical reagent (like HBr) to an unsymmetrical alkene. According to Markownikoff's rule, the hydrogen atom adds to the carbon atom that is already bonded to the most hydrogen atoms, because this forms the more stable carbocation intermediate.
- Forgetting the test for unsaturation. Shaking an alkene with bromine water (orange/brown) will decolourise it as the bromine adds across the double bond.
Exam board notes
All boards require a detailed understanding of the electrophilic addition mechanism. AQA often includes questions on E/Z isomerism and the Cahn-Ingold-Prelog priority rules. Edexcel may focus on the addition polymerisation of alkenes and the properties and uses of the resulting polymers. OCR frequently tests the mechanism including the formation of major and minor products according to Markownikoff's rule.
FAQs
What is a pi bond?
A pi (π) bond is formed by the sideways overlap of two p-orbitals, one from each carbon atom in the double bond. The electron density is concentrated above and below the plane of the sigma bond, and this exposed electron density is what makes alkenes susceptible to attack by electrophiles.
What is E/Z isomerism?
E/Z isomerism is a type of stereoisomerism that occurs in alkenes when there are two different groups attached to each carbon atom of the C=C double bond. The 'Z' isomer has the highest priority groups on the same side of the double bond, while the 'E' isomer has them on opposite sides. Priority is determined by atomic number using the Cahn-Ingold-Prelog rules.
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