Characteristic organic reactions
Cambridge A-Level Chemistry (9701) · Unit 29: An introduction to A Level organic chemistry · 9 flashcards
Characteristic organic reactions is topic 29.2 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 29 — An introduction to A Level organic chemistry , alongside Formulas, functional groups and the naming of organic compounds. In one line: Electrophilic substitution is a reaction where an electrophile (electron-seeking species) replaces an atom or group in a molecule. The electrophile is attracted to an area of high electron density and forms a covalent bond, displacing another group.
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 — 3 definitions and 6 key concepts — covering the precise wording mark schemes reward. Use the 3 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.
Some cards in this deck are on other syllabus topics: 29.3 Shapes of aromatic organic molecules; σ and π bonds and 29.4 Isomerism: optical. Each card is labelled with its topic.
Electrophilic substitution, including the role of the electrophile
Electrophilic substitution is a reaction where an electrophile (electron-seeking species) replaces an atom or group in a molecule. The electrophile is attracted to an area of high electron density and forms a covalent bond, displacing another group.
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).
- Understand and use the mechanism terms (a) electrophilic substitution and (b) addition-elimination.
Also in this deck: 29.3 Shapes of aromatic organic molecules; σ and π bonds
- Describe how benzene and other aromatic molecules are shaped, and explain that shape using sp²-hybridised carbon atoms, the σ bonds between them and the delocalised π system.
Also in this deck: 29.4 Isomerism: optical
- Understand that the two enantiomers of a compound share the same physical and chemical properties, except that they rotate plane-polarised light differently and may differ in biological activity.
- Understand what is meant by an optically active substance and by a racemic mixture, and use both terms.
- Describe what each of a substance's two optical isomers does to plane-polarised light.
- Explain why chirality matters when synthesising drug molecules, including: (a) the two enantiomers may differ in biological activity; (b) the need to split a racemic mixture into its two pure enantiomers; (c) chiral catalysts can be used to make just one pure optical isomer. Appreciate that a compound can have more than one chiral centre; meso compounds and terms such as diastereoisomers are not required.
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.
Topic 29.2 · Characteristic organic reactions
Define electrophilic substitution, including the role of the electrophile.
Electrophilic substitution is a reaction where an electrophile (electron-seeking species) replaces an atom or group in a molecule. The electrophile is attracted to an area of high electron density and forms a covalent bond, displacing another group.
Topic 29.3 · Shapes of aromatic organic molecules; σ and π bonds
Describe the structure and bonding in benzene, including reference to sp² hybridisation and the delocalised π system.
Benzene has a planar, cyclic structure with each carbon atom sp² hybridised. Each carbon forms three sigma (σ) bonds (two to adjacent carbons and one to a hydrogen). The remaining p-orbital on each carbon overlaps sideways, forming a delocalised π system above and below the plane of the ring.
Topic 29.4 · Isomerism: optical
What are enantiomers, and what is their key characteristic regarding plane-polarised light?
Enantiomers are stereoisomers that are non-superimposable mirror images of each other. They have identical physical and chemical properties, except for their ability to rotate plane-polarised light in opposite directions.
Topic 29.4 · Isomerism: optical
Define 'optically active' and 'racemic mixture'.
A substance is optically active if it rotates the plane of polarised light. A racemic mixture is an equimolar mixture of two enantiomers; it shows no overall rotation of plane-polarised light because the rotations cancel each other out.
Topic 29.4 · Isomerism: optical
Describe the effect of individual optical isomers on plane polarised light.
Each optical isomer of a single substance rotates plane polarised light. One enantiomer rotates the light clockwise (dextrorotatory, +), while the other rotates it anticlockwise (laevorotatory, -) by the same amount.
Topic 29.4 · Isomerism: optical
Explain why the different enantiomers of a drug molecule might exhibit different biological activity.
Enantiomers interact differently with chiral biological receptors (
Topic 29.4 · Isomerism: optical
Why is it sometimes necessary to separate a racemic mixture of a drug into pure enantiomers?
Separating a racemic mixture is important if only one enantiomer has the desired therapeutic effect and the other is inactive or harmful. Administering only the active enantiomer reduces dosage and minimises potential side effects.
Topic 29.4 · Isomerism: optical
Explain how chiral catalysts can be used to produce a single pure optical isomer.
Chiral catalysts have a chiral environment that selectively favours the formation of one enantiomer over the other during a reaction. This results in the synthesis of a single, pure optical isomer, rather than a racemic mixture, improving drug efficacy and safety.
Topic 29.2 · Characteristic organic reactions
Explain the difference between addition-elimination and electrophilic substitution mechanisms.
Electrophilic substitution: an electrophile replaces an atom (usually H) on an electron-rich system, typically an arene, e.g. nitration of benzene. Addition-elimination: a nucleophile adds to the carbonyl carbon to form a tetrahedral intermediate, then a leaving group (e.g. Cl⁻) is eliminated and C=O reforms.
More Chemistry flashcards
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All Chemistry FlashcardsMore topics in Unit 29 — An introduction to A Level organic chemistry
Characteristic organic reactions 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 Characteristic organic reactions deck
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