AP | A-Level | IB | NCERT 11 + 12 – FREE NOTES, RESOURCES AND VIDEOS!
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 to Alkenes

Specification Reference R3.4.5

Quick Notes:

  • Alkenes contain a carbon–carbon double bond (C=C) with high electron density.
  • This makes alkenes susceptible to attack by electrophiles.
  • These reactions are called electrophilic addition reactions.
  • Common electrophilic additions:
    • Alkenes + H2O → alcohols
    • Alkenes + halogens (Cl2, Br2) → dihalogenoalkanes
    • Alkenes + hydrogen halides (HCl, HBr) → halogenoalkanes
  • Bromine water is decolourized by alkenes but not by alkanes.
  • Alkenes are often called “starting molecules” in industry due to their reactivity.

Full Notes:

Why Alkenes Are Reactive with Electrophiles?

Alkenes are susceptible to electrophilic attack because of the high electron density of the carbon–carbon double bond. These reactions lead to electrophilic addition.

IB Chemistry diagram showing electron density of the carbon–carbon double bond in alkenes, explaining susceptibility to electrophilic attack.

Electrophilic Addition Reactions

These reactions involve breaking the double bond and adding atoms across it. You need to know the following examples:

Reaction with steam:

Alkene + H2O (steam) → alcohol

IB Chemistry electrophilic addition of steam to alkenes forming alcohols, catalysed by phosphoric acid under high pressure and temperature.

Phosphoric acid catalyst, high pressure (60 to 70 atm) and temperature (300 °C)

Reaction with halogens (e.g. Br2):

Alkene + Br2 → dihalogenoalkane

IB Chemistry electrophilic addition of bromine to alkenes producing dihalogenoalkane, basis of bromine water test.

This is useful as it forms the basis of the bromine water test for alkenes (bromine water turns from orange to colourless when mixed with an alkene).

Reaction with hydrogen halides (e.g. HBr):

Alkene + HBr → halogenoalkane

IB Chemistry electrophilic addition of hydrogen bromide to alkenes forming halogenoalkane.

Unsymmetrical alkenes give major and minor products due to carbocation stability.

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: 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 diagram showing bromine electrophilic addition across a double bond, decolourising bromine water.

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.

Industrial Significance of Alkenes

Alkenes are readily reactive, especially in addition reactions making them useful in industry.

Summary