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S1.1 - Introduction to the particulate nature of matter S1.2 - The nuclear atom S1.3 - Electron configurations S1.4 - Counting particles by mass - The mole S1.5 - Ideal gases S2.1 - The ionic model S2.2 - The covalent model S2.3 - The metallic model S2.4 - From models to materials S3.1 - The periodic table - Classification of elements S3.2 - Functional groups - Classification of organic compounds R1.1 - Measuring enthalpy changes R1.2 - Energy cycles in reactions R1.3 - Energy from fuels R1.4 - Entropy and spontaneity AHL R2.1 - How much? The amount of chemical change R2.2 - How fast? The rate of chemical change R2.3 - How far? The extent of chemical change R3.1 - Proton transfer reactions R3.2 - Electron transfer reactions R3.3 - Electron sharing reactions R3.4 - Electron-pair sharing reactions

R3.4 - Electron-pair sharing reactions

3.4.1 Nucleophilic 3.4.2 Nucleophilic Substitution Reaction 3.4.3 Electrolytic Fission and Ionic Formation 3.4.4 Electrophilic 3.4.5 Electrophilic Addition to Alkenes 3.4.6 Lewis Acids and Bases (AHL) 3.4.7 Lewis Acid-Base Reaction and Co-ordinate Bonds (AHL) 3.4.8 Complex Ions and Ligand Co-coordination (AHL) 3.4.9 SN1 and SN2 Reaction (AHL) 3.4.10 Leaving Group and Substitution (AHL) 3.4.11 Electrophilic Addition of Alkenes (AHL) 3.4.12 Major Product of Addition Reaction (AHL) 3.4.13 Electrophilic Substitution of Benzene (AHL)

Electrophilic Addition Reactions of Alkenes HL Only

Specification Reference R3.4.11

Quick Notes

  • Alkenes have a π bond (from the double bond) that is electron-rich and attracts electrophiles.
  • Electrophilic addition = addition of an electrophile across the C=C double bond.
  • Typical reactions include:
    • Alkene + halogen (e.g. Br2) to form a dihalogenoalkane
    • Alkene + hydrogen halide (e.g. HBr) to form a halogenoalkane
    • Alkene + water (via H+ catalyst) to form an alcohol
  • For symmetrical alkenes (e.g. ethene), the same product forms regardless of which carbon is attacked.
  • The mechanism typically involves: IB Chemistry overview of electrophilic addition to alkenes showing electrophile attack, carbocation formation, and nucleophilic capture.
    • Electrophile attacking the π bond
    • Formation of a carbocation intermediate
    • Nucleophile attacking the carbocation

Full Notes:

Alkenes react with electrophiles because the C=C double bond is electron-rich.

IB Chemistry diagram showing electron density in a C=C double bond of an alkene attracting electrophiles.

In electrophilic addition reactions, heterolytic fission occurs – the covalent bond breaks unevenly, forming ions. One atom takes both electrons, leaving a carbocation (ion with a positively charged carbon) and an anion.

The more substituted the carbocation (i.e., the more alkyl groups attached), the more stable it is. Stability order: tertiary > secondary > primary.

Electrophilic Addition Mechanisms

The high electron density within a carbon–carbon double bond attracts electrophiles and an addition reaction mechanism follows three basic steps:

IB Chemistry overview of electrophilic addition to alkenes showing electrophile attack, carbocation formation, and nucleophilic capture.

Example Electrophilic Addition mechanisms

You need to know the following mechanisms:

Bromine + Ethene

IB Chemistry electrophilic addition mechanism of Br2 to ethene forming 1,2-dibromoethane via a carbocation intermediate.
  1. Br2 molecule approaches C=C (polarised by electron density)
  2. Double bond breaks, Br+ forms bond giving a carbocation intermediate
  3. Br ion attacks carbocation to form 1,2-dibromoethane

HBr + Ethene

IB Chemistry electrophilic addition mechanism of HBr to ethene with carbocation intermediate and bromide attack.
  1. HBr is polar (Hδ+—Brδ), and H+ acts as the electrophile
  2. H+ bonds to one carbon of the C=C bond, forming a carbocation
  3. Br attacks the carbocation, forming a halogenoalkane

H2O + Ethene (in presence of acid)

IB Chemistry acid-catalysed hydration of ethene showing electrophilic addition mechanism to form ethanol.
  1. π electrons attack H+ from H3O+ (formed when H2O accepts H+ from the acid catalyst), giving a carbocation
  2. H2O then attacks the carbocation to form an oxonium ion
  3. Deprotonation gives the alcohol

Notes on Symmetrical Alkenes

A symmetrical alkene (like ethene or but-2-ene) gives the same carbocation regardless of which carbon atom from the double bond the electrophile first bonds to, so only one product is formed.

This is unlike unsymmetrical alkenes which can form two products in unequal amounts (major and minor products). This is covered in R3.4.12 ‘Major Product of Addition Reaction’.

Linked Course Question

Reactivity 3.3 — Linked Course Question

Why is bromine water decolourised by alkenes in the dark, but not by alkanes?

Alkenes decolourise bromine water in the dark
(As outlined above) Alkenes contain a C=C double bond with a region of high electron density. This can attract electrophilic Br2 molecules, causing an electrophilic addition reaction. The Br–Br bond breaks, and each Br atom adds across the double bond — forming a colourless dibromoalkane and the orange colour of bromine disappears.

IB Chemistry schematic showing electron-rich C=C double bond polarising Br2 leading to addition and loss of bromine colour.

Alkanes do not react in the dark
Alkanes lack a C=C bond and are relatively unreactive. They only react with bromine via a radical substitution reaction, which requires UV light to initiate homolytic fission of Br2. No reaction occurs in the dark, so the orange colour remains.

Summary