Structure of Alkenes T
Combined Science students met alkenes only as “the reactive products of cracking”. Triple Chemistry students study them in detail: names, formulae and structures. Alkenes are hydrocarbons with a carbon–carbon double bond, written C=C. That double bond is the functional group — the part of the molecule where the chemistry happens — and it defines the homologous series:
CnH2n
Because of the double bond, an alkene carries two fewer hydrogen atoms than the alkane with the same number of carbons. The molecule is unsaturated — it does not contain the maximum possible hydrogen, and the spare bonding capacity at the C=C is exactly what makes alkenes reactive (section 6). The first four alkenes — the only names you need — are ethene (C2H4), propene (C3H6), butene (C4H8) and pentene (C5H10). There is no “methene” — a double bond needs two carbons, so the series starts at ethene.
The first four alkenes. One C=C double bond each (in red); every formula fits CnH2n. Adding one CH2 each time steps along the homologous series.
| Alkanes | Alkenes | |
|---|---|---|
| General formula | CnH2n+2 | CnH2n |
| Bonds | All single bonds | One C=C double bond |
| Saturation | Saturated | Unsaturated |
| First member | Methane, CH4 | Ethene, C2H4 |
| Reactivity | Less reactive | More reactive (at the C=C) |
| Bromine water | Stays orange | Orange → colourless |
A functional group is the atom or group of atoms that controls how a compound reacts — the “business end” of the molecule. It comes down to one sentence worth understanding: it is the generality of reactions of functional groups that determines the reactions of organic compounds. Every member of a homologous series carries the same functional group, so every member reacts the same way — learn what C=C does once, and you know the chemistry of every alkene from ethene to pentene. The same logic powers the alcohols (–OH, section 7) and carboxylic acids (–COOH, section 8).
🧪 Exam-style questions
Hexene is an alkene with six carbon atoms. Complete the formula: C6H? — how many hydrogen atoms does hexene have?
Show answer
- Alkenes fit CnH2n, so hexene is C6H12. 1 mark
Why are alkenes described as unsaturated hydrocarbons? Tick (✓) one box.
A hydrocarbon comes from the homologous series with general formula CnH2n. Which feature must its displayed formula show? Tick (✓) one box.
Addition Reactions of Alkenes T
Alkenes do burn — they react with oxygen like any hydrocarbon — but they burn badly: in air they tend to undergo incomplete combustion and produce smoky flames, which is one reason they’re used as chemical building blocks rather than fuels. Their useful chemistry happens at the C=C functional group instead, and it is all one type of reaction: addition.
In an addition reaction, a small molecule adds across the carbon–carbon double bond: the double bond opens to a single C–C bond, and one new atom (or group) attaches to each of the two carbons. Everything ends up in one product — nothing else is made. Alkenes add hydrogen, water (as steam) and the halogens (chlorine, bromine, iodine).
Ethene’s three addition reactions. The C=C opens, and one atom or group joins each carbon: H2 → ethane; steam → ethanol; bromine → dibromoethane.
| Added molecule | Conditions | Product type | Example (from ethene) |
|---|---|---|---|
| Hydrogen, H2 | Nickel catalyst, heat (≈150 °C) | Alkane (“hydrogenation”) | Ethane, C2H6 |
| Water, H2O (as steam) | Catalyst, high temperature & pressure | Alcohol | Ethanol, C2H5OH |
| Halogens (Cl2, Br2, I2) | Room temperature — no catalyst needed | Di-halo compound | Dibromoethane, C2H4Br2 |
The steam reaction is industrially important — it’s one of the two ways ethanol is manufactured (the other is fermentation, section 7) — and the bromine reaction is the chemistry behind the bromine water test: the orange bromine is used up as it adds across the double bond, leaving a colourless product.
Drawing the products
You need to be able to draw fully displayed formulae for the first four alkenes reacting with hydrogen, water, chlorine, bromine and iodine. One method covers every case:
- Draw the alkene’s carbon skeleton and replace the double bond with a single bond.
- Split the added molecule in two: H–H gives H and H; H–OH gives H and OH; Br–Br gives Br and Br.
- Attach one piece to each of the two carbons that used to share the double bond. Every other atom stays exactly where it was.
- Sanity-check by counting: propene + Br2 → C3H6Br2 — all six hydrogens kept, two bromines gained, no atoms lost.
- Leaving the double bond in the product. After addition the molecule is saturated — if your drawing still shows C=C, an atom count will catch it.
- Adding both halogen atoms to the same carbon. One goes on each carbon of the old double bond.
- Writing H2O as a product of the steam reaction. Addition reactions have one product only — the water is absorbed into the alcohol molecule.
🧪 Exam-style questions
Propene reacts with hydrogen in the presence of a nickel catalyst. What is the product? Tick (✓) one box.
Ethanol can be manufactured from ethene. Which reagent and conditions are used? Tick (✓) one box.
Which is the correct formula of the product when butene (C4H8) reacts with bromine (Br2)? Tick (✓) one box.
Why do alkenes tend to burn with smoky flames? Tick (✓) one box.
A few drops of bromine water are added to an alkene and the tube is shaken. What colour change is seen? Tick (✓) one box.