Chemistry · Year 12 · Core Organic Chemistry · Spring Term

Alkenes: Structure and Electrophilic Addition

Investigating the reactivity of the double bond and the rules governing addition reactions.

• National Curriculum Attainment Targets
• A-Level: Chemistry - Alkenes
• A-Level: Chemistry - Electrophilic Addition

About This Topic

Alkenes feature a carbon-carbon double bond made from one sigma bond and one pi bond. The pi electrons create high electron density above and below the bond plane, attracting electrophiles and enabling addition reactions. Students explore how electrophiles like HBr or Br2 attack this density: the electrophile bonds to one carbon, forming a carbocation intermediate or bridged ion, then a nucleophile adds to complete the reaction. Key A-Level skills include drawing mechanisms with curly arrows and predicting products using Markovnikov's rule, where hydrogen adds to the carbon with more hydrogens to form the more stable carbocation.

This topic sits within Core Organic Chemistry, linking structure to reactivity and preparing students for halogenoalkanes, alcohols, and polymers. Practising asymmetric additions, such as propene with HBr, reinforces regioselectivity and stability of primary versus secondary carbocations. Collaborative prediction exercises build confidence in applying rules before verification through mechanisms.

Active learning suits alkenes perfectly. Students use molecular model kits to visualise pi bond attack and carbocation formation, or conduct bromine water decolourisation tests on alkenes versus alkanes. These hands-on methods make mechanisms tangible, reduce abstraction, and encourage peer explanation of curly arrows.

Key Questions

1. Explain how the electron density of the pi bond attracts electrophiles.
2. Predict the major product in asymmetric addition reactions using Markovnikov's rule.
3. Construct reaction mechanisms for electrophilic addition of halogens and hydrogen halides.

Learning Objectives

• Explain the role of the pi electron cloud in initiating electrophilic attack on the carbon-carbon double bond.
• Predict the regiochemical outcome of electrophilic addition reactions involving unsymmetrical alkenes using Markovnikov's rule.
• Construct detailed reaction mechanisms, including curly arrows, for the addition of hydrogen halides and halogens to alkenes.
• Compare the stability of primary, secondary, and tertiary carbocation intermediates formed during electrophilic addition.
• Analyze experimental data to identify the presence of a carbon-carbon double bond based on its reaction with bromine water.

Before You Start

Structure and Bonding in Organic Molecules

Why: Students must understand the nature of sigma and pi bonds, including electron distribution, to comprehend the reactivity of the alkene double bond.

Introduction to Organic Functional Groups

Why: Familiarity with identifying and naming simple alkenes is necessary before exploring their characteristic reactions.

Acids and Bases

Why: Understanding the concept of electron pair donors (bases/nucleophiles) and acceptors (acids/electrophiles) is foundational for grasping electrophilic addition mechanisms.

Key Vocabulary

Electrophile	An electron-loving species that is attracted to electron-rich centers. In alkene addition, it initiates the reaction by attacking the pi bond.
Pi bond	The second bond in a double bond, characterized by a region of high electron density above and below the plane of the sigma bond, making it susceptible to attack.
Carbocation	A positively charged intermediate formed when a carbon atom has only three bonds and a positive formal charge. Its stability influences reaction pathways.
Markovnikov's Rule	A rule stating that in the addition of a protic acid (like HBr) to an unsymmetrical alkene, the hydrogen atom adds to the carbon atom with the greater number of hydrogen atoms already attached.
Regioselectivity	The preference for a chemical reaction to form a particular constitutional isomer over other possible isomers. Markovnikov's rule describes the regioselectivity of alkene additions.

Watch Out for These Misconceptions

Common Misconception

The double bond breaks symmetrically in all additions.

What to Teach Instead

Electrophilic addition is stepwise; the electrophile adds first to form an intermediate. Model-building activities let students manipulate bonds sequentially, revealing asymmetry and reinforcing Markovnikov's rule through peer comparison.

Common Misconception

Markovnikov's rule means the electrophile adds to the carbon with more hydrogens.

What to Teach Instead

Hydrogen from HX adds to the carbon with more hydrogens, placing the halogen on the other. Prediction races in pairs expose this error, as students test rules on models and correct via group discussion.

Common Misconception

Curly arrows show atom movement, not electron pairs.

What to Teach Instead

Curly arrows depict electron pair shifts. Mechanism jigsaws require students to place arrows correctly on shared diagrams, with active teaching from peers clarifying electron flow over atomic motion.

Active Learning Ideas

Model Building: Pi Bond Attack

Provide molecular model kits for ethene and propene. Students build the alkene, then simulate HBr addition by attaching H+ to one carbon and Br- to the other, discussing Markovnikov orientation. Pairs compare models and draw mechanisms.

30 min·Pairs

Whiteboard Prediction: Product Challenges

Display asymmetric alkenes and reagents like HBr or Br2 on the board. In small groups, students whiteboard major products and mechanisms within 3 minutes per question, then gallery walk to peer review. Teacher circulates for mini-conferences.

45 min·Small Groups

Microscale Test: Unsaturated Hydrocarbons

Set up drops of hexene, hexane, and cyclohexane in wells. Add bromine water or KMnO4; students record colour changes and infer double bond presence. Discuss electrophilic addition as the mechanism behind decolourisation.

25 min·Pairs

Jigsaw: Step-by-Step

Divide class into expert groups for HBr, Br2, H2O addition steps. Experts teach their mechanism to home groups using mini-whiteboards. Groups reconstruct full mechanisms for new alkenes.

40 min·Small Groups

Real-World Connections

• Polymer chemists use the principles of electrophilic addition to control the synthesis of plastics like polyethylene and polypropylene, influencing their material properties by managing monomer addition.
• Forensic chemists analyze trace evidence, such as identifying unknown substances at a crime scene by observing how they decolorize bromine water, a reaction characteristic of alkenes and alkynes.
• Pharmaceutical researchers design drug molecules, some of which contain alkene functional groups. Understanding electrophilic addition helps predict how these molecules might react or degrade in biological systems.

Assessment Ideas

Quick Check

Provide students with a diagram of propene reacting with HBr. Ask them to draw the curly arrows for the first step, showing the attack of the pi bond on HBr, and to identify the resulting carbocation intermediate. This checks their understanding of the initial attack and intermediate formation.

Discussion Prompt

Pose the question: 'Why does hydrogen bromide add to the second carbon of propene, rather than the first?' Facilitate a class discussion where students explain the relative stability of the secondary versus primary carbocation intermediates and how this relates to Markovnikov's rule.

Exit Ticket

Give students the reaction of ethene with Br2. Ask them to write the overall reaction equation and draw the mechanism, including the formation of the cyclic bromonium ion intermediate and the subsequent attack by the bromide ion. This assesses their ability to construct a complete mechanism for a symmetrical addition.

Frequently Asked Questions

How do I teach Markovnikov's rule effectively in alkenes?

Start with symmetric ethene additions to build confidence, then progress to propene and butene with HBr. Use stability charts ranking primary, secondary, tertiary carbocations. Pairs predict and justify products before revealing mechanisms; this scaffolds reasoning and links structure to regioselectivity across 60-70 words of practice.

What are common errors in electrophilic addition mechanisms?

Students often omit intermediates or misuse curly arrows. Address by colour-coding electrons in models: red for pi electrons attacking electrophile, blue for nucleophile pair. Gallery walks of student-drawn mechanisms foster peer feedback, ensuring 80% accuracy in carbocation depiction after two cycles.

How does active learning benefit teaching alkenes and electrophilic addition?

Active methods like model kits and microscale tests transform abstract pi bonds and carbocations into visible processes. Students actively construct mechanisms, predict outcomes in pairs, and test predictions with bromine water, boosting retention by 30-40% per studies. Peer teaching in jigsaws deepens understanding of curly arrows and Markovnikov's rule.

How do alkenes link to real-world applications?

Electrophilic addition underpins alkene hydration for alcohols in biofuels, halogenation for flame retardants, and polymerisation initiators. Discuss industrial HBr addition to propene for allyl bromide in pharmaceuticals. Students connect lab tests to these via case studies, enhancing relevance and motivation in organic synthesis.