Alkenes
Unsaturated hydrocarbons, cracking, the bromine water test, and addition reactions.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 11.5.1State the general formula of the alkenes and describe them as unsaturated hydrocarbons.
- 11.5.2Describe the manufacture of alkenes by cracking.
- 11.5.3Describe the addition reactions of alkenes, including with bromine and steam.Supplement
6 minute read
Alkenes
Alkenes are alkanes' more reactive cousin: swap one of the alkane's single C–C bonds for a C=C double bond and everything about the chemistry changes, because that double bond is a site where new atoms can be added.
General formula and unsaturation
The general formula is CₙH₂ₙ. Having a C=C double bond makes alkenes unsaturated — able to react by opening the double bond and adding atoms across it, something saturated alkanes cannot do.
The bromine water test
Shaking an alkene with orange/brown bromine water decolourises it, because the double bond opens and bromine atoms add across it: CH₂=CH₂ + Br₂ → CH₂BrCH₂Br. Alkanes leave bromine water unchanged, which is exactly why this is the standard test to distinguish the two series.
Addition reactions
Because they are unsaturated, alkenes undergo addition reactions (nothing is lost, everything from both reactants ends up in one product):
- + hydrogen (nickel catalyst) → alkane (hydrogenation, used to harden vegetable oils into margarine).
- + steam (phosphoric acid catalyst, high temperature/pressure) → alcohol, e.g. ethene + steam → ethanol.
- + bromine → dibromoalkane (the test above).
Cracking: where alkenes come from
Long-chain alkanes from crude oil are less useful than short-chain ones. Cracking breaks long alkane molecules into a mixture of shorter alkanes and alkenes using heat and a catalyst (or just very high temperature): a long alkane → a shorter alkane + an alkene. This is how the alkenes needed for polymers and other chemicals are manufactured, since crude oil itself contains very few alkenes.
Think of it like this
A C=C double bond is like a folded coupon: while folded (double bond) it can't accept anything new, but as soon as it "unfolds" into two single bonds, there is a free hand on each carbon ready to grab a new atom — that's an addition reaction.
Worked examples
Method, step by step
Decane, C₁₀H₂₂, is cracked to produce octane, C₈H₁₈, and one other product. Identify the other product and state its general formula.
- 1Balance carbons: 10 = 8 + 2, so the other product has 2 carbons.
- 2Balance hydrogens: 22 = 18 + 4, so the other product has 4 hydrogens.
- 3C₂H₄ fits the alkene general formula CₙH₂ₙ (n = 2).
The other product is ethene, C₂H₄, an alkene (general formula CₙH₂ₙ).
Common misconceptions
- Forgetting that addition reactions conserve every atom — nothing is released as a by-product, unlike substitution reactions in alkanes which release HCl or similar.
- Thinking cracking is just heating an alkane until it "boils apart" — it specifically breaks C–C bonds to form new, shorter molecules, at least one of which is unsaturated; this is a chemical change, not a physical separation like fractional distillation.
In the exam
- Bromine water questions expect the specific colour change "orange/brown to colourless" — just saying "changes colour" loses the mark.
- When writing a cracking equation, check the atom count balances and that at least one product is an alkene (CₙH₂ₙ) — a common error is producing only alkanes.