Alkenes — AQA A-Level Chemistry Revision

    Alkenes are unsaturated hydrocarbons characterized by a carbon-carbon double bond, which acts as a center of high electron density. This structural feature

    Topic Synopsis

    Alkenes are unsaturated hydrocarbons characterized by a carbon-carbon double bond, which acts as a center of high electron density. This structural feature makes them susceptible to electrophilic attack, leading to various addition reactions and the formation of commercially important addition polymers.

    Key Concepts & Core Principles

    Exam Tips & Revision Strategies

    Common Misconceptions & Mistakes to Avoid

    Examiner Marking Points

    Alkenes

    AQA
    A-Level

    Alkenes are unsaturated hydrocarbons characterized by a carbon-carbon double bond, which acts as a center of high electron density. This structural feature makes them susceptible to electrophilic attack, leading to various addition reactions and the formation of commercially important addition polymers.

    0
    Objectives
    4
    Exam Tips
    4
    Pitfalls
    5
    Key Terms
    10
    Mark Points

    Topic Overview

    Alkenes represent a fundamental class of organic compounds in A-Level Chemistry, distinguished by the presence of at least one carbon-carbon double bond (C=C). Unlike their saturated counterparts, alkanes, alkenes are unsaturated hydrocarbons. This crucial C=C bond consists of one strong sigma (σ) bond and one weaker pi (π) bond, with the pi bond being a region of high electron density located above and below the plane of the sigma bond. This unique bonding arrangement dictates the characteristic reactivity of alkenes, making them far more reactive than alkanes and central to many organic synthesis pathways.

    The study of alkenes is vital for understanding how organic molecules react and transform. Their high reactivity, particularly towards electrophilic addition reactions, allows for the synthesis of a wide array of functional groups, including haloalkanes, alcohols, and polymers. Industrially, alkenes are indispensable; ethene (ethylene) and propene (propylene) are key starting materials for producing plastics (addition polymers) and other bulk chemicals. Mastering alkenes provides a strong foundation for understanding more complex organic reactions and mechanisms, including those involving aromatic compounds and carbonyls later in your studies.

    Within the AQA A-Level Chemistry curriculum, the 'Alkenes' topic builds directly upon your knowledge of alkanes, isomerism, and fundamental bonding principles. It introduces the concept of electrophilic addition as a major reaction pathway, requiring a deep understanding of curly arrow mechanisms, carbocation stability, and stereoisomerism (specifically E/Z isomerism). This topic not only tests your recall of reactions and conditions but also your ability to apply mechanistic principles and predict products, often involving multiple steps and considerations like Markovnikov's rule. It's a cornerstone for appreciating the logic and predictability of organic chemistry.

    Key Concepts

    Core ideas you must understand for this topic

    • →Structure and Bonding: Alkenes contain a C=C double bond, comprising a sigma (σ) bond and a pi (π) bond. The pi bond, formed by the sideways overlap of p-orbitals, is a region of high electron density and is responsible for the characteristic reactivity of alkenes.
    • →Nomenclature and Isomerism: Systematically naming alkenes using IUPAC rules, including the identification of the longest carbon chain containing the double bond. Crucially, understanding E/Z isomerism (geometric isomerism) which arises due to the restricted rotation around the C=C double bond and the presence of two different groups on each carbon of the double bond.
    • →Electrophilic Addition Reactions: The defining reaction type for alkenes. The electron-rich pi bond attracts electrophiles (electron-pair acceptors), leading to the breaking of the pi bond and the formation of new sigma bonds. Key examples include reactions with hydrogen (hydrogenation), halogens (halogenation), hydrogen halides, and steam (hydration).
    • →Reaction Mechanisms and Carbocation Stability: Detailed understanding of the step-by-step mechanisms for electrophilic addition, using curly arrows to show electron movement. This includes the formation of carbocation intermediates and applying Markovnikov's rule to predict the major product based on carbocation stability (tertiary > secondary > primary).
    • →Addition Polymerisation: Alkenes undergo addition polymerisation, where many monomer units (alkenes) add together across their double bonds to form long-chain polymers. Understanding how to draw repeat units and identify the monomer from a given polymer structure is essential.

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Alkenes are unsaturated hydrocarbons
    • Carbon-carbon double bond is a center of high electron density
    • Electrophilic addition reactions with HBr, H2SO4, and Br2
    • Use of bromine water to test for unsaturation
    • Formation of major and minor products in unsymmetrical alkenes
    • Relative stabilities of primary, secondary, and tertiary carbocation intermediates
    • Repeating units of addition polymers
    • Unreactive nature of addition polymers

    Marking Points

    Key points examiners look for in your answers

    • Alkenes are unsaturated hydrocarbons
    • Carbon-carbon double bond is a center of high electron density
    • Electrophilic addition reactions with HBr, H2SO4, and Br2
    • Use of bromine water to test for unsaturation
    • Formation of major and minor products in unsymmetrical alkenes
    • Relative stabilities of primary, secondary, and tertiary carbocation intermediates
    • Repeating units of addition polymers
    • Unreactive nature of addition polymers
    • Intermolecular forces between polyalkene molecules
    • Uses of PVC and the role of plasticisers

    Examiner Tips

    Expert advice for maximising your marks

    • 💡Always show curly arrows starting from the double bond in electrophilic addition mechanisms
    • 💡Clearly label the major and minor products based on the stability of the carbocation intermediate
    • 💡Practice drawing repeating units from monomers and vice versa
    • 💡Be prepared to explain why addition polymers are unreactive
    • 💡Master Curly Arrow Mechanisms: Practice drawing all electrophilic addition mechanisms repeatedly. Pay meticulous attention to the origin and destination of each curly arrow, the charges on intermediates (carbocations), and the lone pairs of electrons. A single misplaced arrow or missing charge can lose significant marks.
    • 💡Understand E/Z Isomerism Systematically: Don't just memorise 'same side/opposite side'. Learn and apply the Cahn-Ingold-Prelog priority rules rigorously to each carbon of the double bond. Clearly identify the highest priority group on each carbon, then determine if they are on the 'Z' (zusammen - together) or 'E' (entgegen - opposite) side.
    • 💡Connect Structure to Reactivity: Always link the presence of the C=C double bond, particularly the electron-rich pi bond, to the characteristic electrophilic addition reactions. When explaining reactivity, explicitly mention the high electron density of the pi bond making it susceptible to attack by electrophiles. This demonstrates a deeper understanding beyond just recalling reactions.

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Confusing electrophilic addition mechanisms with nucleophilic substitution
    • Incorrectly identifying the major product in addition reactions by ignoring carbocation stability
    • Failing to draw the correct repeating unit for addition polymers
    • Misunderstanding the role of the plasticiser in modifying polymer properties
    • Confusing E/Z isomerism with cis/trans: While cis/trans is a simpler form of geometric isomerism, E/Z is the systematic IUPAC method. Students often incorrectly apply cis/trans to more complex alkenes where it doesn't clearly define the relative positions of the highest priority groups. Remember, E/Z uses Cahn-Ingold-Prelog priority rules, which must be applied to each carbon of the double bond independently.
    • Incorrect curly arrow mechanisms: A common error is drawing curly arrows originating from the wrong place or pointing to the wrong place. In electrophilic addition, the first curly arrow MUST start from the centre of the pi bond and point towards the electrophile. The second curly arrow, if forming a carbocation, must show the bond breaking and electrons moving to form the new bond with the electrophile, while the other carbon becomes the carbocation centre.
    • Misapplication of Markovnikov's Rule: Students sometimes forget the 'why' behind Markovnikov's rule. It's not just about 'the rich get richer' (hydrogen adds to the carbon with more hydrogens); it's fundamentally about the formation of the most stable carbocation intermediate. Always consider the stability of the intermediate carbocations (tertiary > secondary > primary) when predicting major and minor products, especially with unsymmetrical alkenes and hydrogen halides.

    Revision Plan

    How to revise this topic in 1–2 weeks

    1. 1Week 1 - Day 1-2: Review prerequisites (alkanes, bonding, isomerism). Focus on alkene structure, bonding (sigma and pi bonds, sp2 hybridisation), and systematic nomenclature. Practice drawing and naming a variety of alkenes. Introduce E/Z isomerism and practice assigning E or Z configurations using Cahn-Ingold-Prelog rules.
    2. 2Week 1 - Day 3-4: Dive into electrophilic addition. Start with the general mechanism, then apply it to specific reactions: hydrogenation (H2/Ni), halogenation (X2), and reaction with hydrogen halides (HX). Pay close attention to the formation of carbocations and the application of Markovnikov's rule for unsymmetrical alkenes.
    3. 3Week 2 - Day 1-2: Continue with electrophilic addition reactions: hydration (steam/H3PO4 catalyst) and oxidation (cold, dilute KMnO4 for diols; hot, concentrated KMnO4 for cleavage products). Understand the different conditions and products for these reactions. Practice drawing full mechanisms for each.
    4. 4Week 2 - Day 3-4: Focus on addition polymerisation. Understand how monomers link to form polymers, how to draw repeat units, and how to identify the monomer from a given polymer structure. Review all alkene reactions and their conditions, creating a summary table or flashcards.
    5. 5Week 2 - Day 5-7: Consolidate learning by working through a wide range of past paper questions. Focus on mechanism drawing, predicting products, explaining E/Z isomerism, and applying Markovnikov's rule. Identify weak areas and revisit relevant sections of your notes or textbook. Practice explaining concepts concisely and accurately.

    Exam Question Types

    How this topic typically appears in the exam

    • 📋Mechanism Drawing Questions: These are very common. You'll be asked to draw the full mechanism for an electrophilic addition reaction (e.g., alkene + HBr, alkene + H2SO4 then H2O), showing all curly arrows, intermediate structures, and charges. Advice: Practice these until they are second nature. Every arrow and charge matters for marks.
    • 📋Predicting Products and Reagents: Given an alkene and reagents, you might need to predict the major and minor products, or given a product, identify the starting alkene and reagents. Advice: Understand Markovnikov's rule and carbocation stability. Be aware of stereoisomerism if applicable.
    • 📋Explaining E/Z Isomerism: Questions will ask you to explain why a particular alkene exhibits E/Z isomerism, or to draw and label the E and Z isomers. Advice: Clearly state the conditions for E/Z isomerism (restricted rotation, two different groups on each carbon of the double bond) and apply Cahn-Ingold-Prelog rules correctly.
    • 📋Polymerisation Questions: You could be asked to draw the repeat unit of an addition polymer given its monomer, or to identify the monomer from a section of a polymer chain. Advice: Remember that the double bond in the monomer breaks to form two new single bonds in the polymer chain; the repeat unit is simply the monomer without the double bond.

    Frequently Asked Questions

    Common questions students ask about this topic

    Before You Start

    Prior knowledge that will help with this topic

    • •Alkanes and Organic Nomenclature: A solid understanding of basic organic structures, naming conventions for simple alkanes, and the concept of homologous series is essential before tackling alkenes.
    • •Covalent Bonding and Hybridisation: Knowledge of sigma (σ) and pi (π) bonds, sp2 hybridisation in alkenes, and the concept of electron density is crucial for understanding alkene structure and reactivity.
    • •Basic Reaction Mechanisms: Familiarity with curly arrows to represent electron movement and the general idea of nucleophiles (electron-rich) and electrophiles (electron-deficient) will make learning alkene mechanisms much easier.

    Study Guide Available

    Comprehensive revision notes & examples

    Read Study Guide

    Key Terminology

    Essential terms to know

    • Structure and Bonding (Sigma and Pi bonds, planar geometry)
    • Electrophilic Addition Mechanisms (Reaction with Br2, HBr, H2SO4, H2O)
    • Addition Polymerisation (Monomers to polymers, repeating units)
    • Stereoisomerism (E/Z isomerism due to restricted rotation)
    • Chemical Tests for Unsaturation (Bromine water decolourisation)

    Likely Command Words

    How questions on this topic are typically asked

    Outline
    Explain
    Draw
    Describe
    Predict

    Ready to test yourself?

    Practice questions tailored to this topic

    Related Topics in AQA A-Level Chemistry

    Acids and bases (A-level only)

    View Topic

    Alcohols

    View Topic

    Aldehydes and ketones (A-level only)

    View Topic

    Alkanes

    View Topic