Addition reactions of alkenes

Alkenes are unsaturated hydrocarbons with a carbon-carbon double bond (C=C). This double bond is the reactive site: it opens up and a small molecule adds across it, so the characteristic reaction of an alkene is the addition reaction. Using ethene, C₂H₄, as the model, the Form 5 Carbon Compounds chapter expects you to write the balanced equations, state the conditions, and use two of these reactions as tests for unsaturation.

The balanced equations

In every addition reaction the double bond becomes a single bond and no atoms are lost, so the product is a single, saturated molecule.

• Addition of hydrogen (hydrogenation): C₂H₄(g) + H₂(g) → C₂H₆(g)
• Addition of bromine: C₂H₄(g) + Br₂(aq) → C₂H₄Br₂(aq) (1,2-dibromoethane)
• Addition of chlorine: C₂H₄(g) + Cl₂(g) → C₂H₄Cl₂(g) (1,2-dichloroethane)
• Addition of hydrogen bromide: C₂H₄(g) + HBr(g) → C₂H₅Br(g) (bromoethane)
• Addition of steam (hydration): C₂H₄(g) + H₂O(g) → C₂H₅OH(g) (ethanol)

Notice that the reactant and product carbon numbers are always equal, nothing leaves the molecule, unlike the substitution of alkanes where a hydrogen halide is released.

Conditions required

Each addition reaction has its own condition, and the exam rewards precise conditions:

• Hydrogenation needs a nickel catalyst at about 180 degrees C (or platinum). This is used to harden vegetable oils into margarine.
• Bromine and chlorine add at room temperature; bromine water works in the dark without needing sunlight.
• Hydrogen halides add at room temperature.
• Hydration needs phosphoric acid catalyst, about 300 degrees C and 60 atm pressure of steam; it is the industrial route to ethanol.

Observations

• With bromine water (reddish-brown), the alkene decolourises it quickly at room temperature, the brown colour fades to colourless.
• With acidified potassium manganate(VII) (purple), the alkene decolourises the purple colour.
• The other additions of gases show no dramatic colour change, but hydrogenation and hydration are followed by testing the product.

The bromine-water and potassium manganate(VII) results are the two standard tests for unsaturation: an alkene decolourises both quickly, an alkane does not react with either under the same mild conditions, so these tests distinguish an alkene from an alkane.

Why addition, not substitution

Alkanes are saturated and only undergo substitution (in ultraviolet light), releasing a hydrogen halide. Alkenes are unsaturated: the extra bond in C=C is broken and both new atoms join the molecule, so no small molecule is given off. Recognising this difference, addition for the unsaturated alkene, substitution for the saturated alkane, is exactly what many questions probe.

Addition polymerisation

The same double bond lets many alkene molecules add to one another to form a long-chain polymer. For ethene:

n C₂H₄ → (-CH₂⁻CH₂⁻)n (poly(ethene))

This addition polymerisation is a natural extension of the addition idea and appears in the Polymers chapter.

Common mistakes to avoid

Keep the carbon number the same on both sides, a frequent slip is to lose a carbon. Give the specific condition, not just the word “catalyst”: nickel for hydrogenation, phosphoric acid for hydration. Do not confuse the alkene addition (no gas released) with alkane substitution (hydrogen halide released). Include state symbols and name the organic product.

How it appears in the SPM exam

In Paper 2 (4541/2) you may write and balance an addition equation, state the exact condition, or explain how to distinguish an alkene from an alkane using bromine water. In Paper 1 (4541/1), objective items test the reagents, conditions and the decolourising observation. In Paper 3 (4541/3) the bromine-water test for unsaturation is a practical you should be ready to describe. Tie every answer to the opening of the C=C double bond, and your alkene chemistry stays coherent.