Alkenes, Addition Reactions and the Bromine Test
Alkenes contain a carbon–carbon double bond, which makes them reactive. One test — bromine water — identifies them, and it appears in every paper.
1. The homologous series
Alkenes have the general formula CₙH₂ₙ and contain at least one C=C double bond.
| Alkene | Formula |
|---|---|
| Ethene | C₂H₄ |
| Propene | C₃H₆ |
| Butene | C₄H₈ |
There is no “methene” — you need at least two carbons for a C=C.
Alkanes are CₙH₂ₙ₊₂; alkenes are CₙH₂ₙ. Getting the general formula wrong was recorded — the alkene has two fewer hydrogens because of the double bond.
Ethane is an ALKANE; ethene is an ALKENE. Confusing them was recorded. The -ane ending means saturated; -ene means it has a double bond.
2. Saturated and unsaturated
SATURATED — contains only SINGLE carbon–carbon bonds (alkanes) UNSATURATED — contains at least one carbon–carbon DOUBLE bond (alkenes)
Define these in terms of C–C bonds, not “full of hydrogen”. Confusing the two definitions was recorded.
Unsaturated compounds are MORE REACTIVE, because the double bond can open to allow an addition reaction. Uncertainty about the relative reactivity was recorded.
3. The bromine water test
The single most examined test in organic chemistry.
Add bromine water (orange/yellow-brown) and shake.
| Result | Conclusion |
|---|---|
| Orange → COLOURLESS (decolourised) | UNSATURATED (alkene present) |
| Stays orange | SATURATED (alkane) |
The change is orange to COLOURLESS — not “clear” and not “white”. “Colourless” is the word the mark scheme wants; “clear” describes transparency, not colour. Uncertainty about the observation was recorded.
Bromine water is DECOLOURISED by an alkene, because the bromine adds across the double bond.
4. Addition reactions
In an ADDITION reaction, the C=C double bond OPENS and atoms add on to the two carbons, forming a saturated product.
With bromine:
C₂H₄ + Br₂ → C₂H₄Br₂ (1,2-dibromoethane)
With hydrogen (hydrogenation) — needs a nickel catalyst, ~150 °C:
C₂H₄ + H₂ → C₂H₆ (ethane)
Used to turn unsaturated vegetable oils into margarine.
With steam (hydration) — needs phosphoric acid catalyst, ~300 °C and 60 atm:
C₂H₄ + H₂O → C₂H₅OH (ethanol)
The double bond becomes a single bond in every addition product. Confusion about the product formed when butene reacts with bromine was recorded — the bromines attach to the two carbons that shared the double bond, one on each.
Addition is not substitution. Alkenes undergo addition (the double bond opens, nothing is lost); alkanes undergo substitution (an atom is swapped out and HBr or HCl is released). Confusing addition with other reaction types was recorded.
5. Cracking
CRACKING breaks long-chain alkanes into shorter, more useful alkanes and alkenes.
Conditions: a catalyst (silica/alumina) at high temperature (~600–700 °C), or high temperature and pressure.
Example: C₁₀H₂₂ → C₈H₁₈ + C₂H₄
Why it is done:
There is more demand for short-chain fractions (petrol) than the supply from crude oil, and cracking also produces alkenes, which are needed to make polymers and ethanol.
An alkene is always among the products — that is what makes cracking so valuable. Not knowing the uses of the alkenes produced was recorded: they are used to make polymers (poly(ethene)) and ethanol.
Check the temperature figures against your syllabus — a recorded error misremembered the catalytic cracking temperature, and different sources quote slightly different ranges.
6. Physical properties
Alkenes follow the same trends as alkanes:
As the chain gets longer: boiling point increases, volatility decreases, and viscosity increases.
VOLATILITY means how easily a liquid evaporates. A student was recorded as unfamiliar with the term — high volatility means it evaporates easily (low boiling point).
At room temperature: the first few alkenes are gases; longer chains are liquids.
7. Mistakes that cost marks
Wrong general formula — CₙH₂ₙ for alkenes.
Confusing ethane with ethene.
Saying bromine water goes “clear” or “white” instead of colourless.
Saying the alkane decolourises bromine water.
Confusing addition with substitution.
Leaving the double bond in an addition product.
Defining saturated as “full of hydrogen” rather than by single bonds.
Not knowing what the alkenes from cracking are used for.
Frequently asked questions
What is the general formula for alkenes? CₙH₂ₙ.
What makes a compound unsaturated? It contains at least one carbon–carbon double bond.
What is the test for an alkene? Bromine water — it turns from orange to colourless.
What does an alkane do to bromine water? Nothing — it stays orange.
What is an addition reaction? The double bond opens and atoms add on, giving a saturated product.
What forms when ethene reacts with bromine? 1,2-dibromoethane, C₂H₄Br₂.
What is hydrogenation used for? Turning unsaturated oils into margarine, using a nickel catalyst.
How is ethanol made from ethene? Hydration with steam, using a phosphoric acid catalyst.
What is cracking? Breaking long-chain alkanes into shorter alkanes and alkenes.
Why is cracking done? To meet demand for short-chain fuels and to produce alkenes for polymers and ethanol.
Quick revision checklist
- I know the general formula CₙH₂ₙ
- I can name and give formulae for the first few alkenes
- I can define saturated and unsaturated by C–C bonds
- I know unsaturated compounds are more reactive, and why
- I know the bromine water test and say colourless
- I know what an addition reaction is
- I can write addition with bromine, hydrogen and steam
- I know the catalysts and conditions
- I don’t confuse addition with substitution
- I know what cracking is, its conditions and why it’s done
- I know alkenes are used for polymers and ethanol
- I know what volatility means
These notes cover alkenes and addition reactions in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus for reaction conditions in your own exam series.
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