Alkenes

The first three alkenes:

n	Name	Formula	Notes
2	ethene	C₂H₄	CH₂=CH₂ — simplest alkene; basis for polythene
3	propene	C₃H₆	CH₃CH=CH₂ — basis for polypropene
4	butene	C₄H₈	three isomers (but-1-ene, but-2-ene cis/trans, methylpropene)

The general formula CₙH₂ₙ has TWO LESS hydrogens than the equivalent alkane (CₙH₂ₙ₊₂), because the double bond 'uses up' two H atoms.

The first member is ethene (C₂H₄), NOT methene — because you'd need at least two carbons to make a C=C bond. (Methene would just be CH₂ which has unbonded valencies and doesn't exist as a stable molecule.)

Like alkanes, alkene boiling points increase with chain length. The first three are all gases at room temperature.

Unlike alkanes, alkenes react readily because the C=C double bond can OPEN UP and accept new atoms across it. These are addition reactions — two molecules combine to give one bigger one.

1. With bromine (Br₂) — the famous test: $\text{CH}_2{=}\text{CH}_2 + \text{Br}_2 \rightarrow \text{CH}_2\text{Br}{-}\text{CH}_2\text{Br}$ Ethene + bromine → 1,2-dibromoethane. The orange bromine water DECOLOURISES.

2. With hydrogen (H₂) — hydrogenation (catalyst: nickel at ~150 °C): $\text{CH}_2{=}\text{CH}_2 + \text{H}_2 \rightarrow \text{CH}_3{-}\text{CH}_3$ The alkene becomes an ALKANE. Used industrially to convert unsaturated vegetable oils into solid margarine (saturated fat).

3. With water (H₂O) — hydration (with phosphoric acid catalyst and high temperature): $\text{CH}_2{=}\text{CH}_2 + \text{H}_2\text{O} \rightarrow \text{CH}_3\text{CH}_2\text{OH}$ Produces ethanol. Industrial route to alcohol (alternative to fermentation).

The pattern in every addition reaction: the C=C becomes a single C-C, with new atoms added to the carbons that used to share the double bond.

Polymerisation is the big industrial use of alkenes. Many small alkene molecules (the monomers) join end-to-end to form a very long chain — the polymer.

Polythene (polyethene, polyethylene) — made from ethene:

$n\,\text{CH}_2{=}\text{CH}_2 \rightarrow {(\text{CH}_2{-}\text{CH}_2)_n}$

The C=C in each ethene OPENS UP and links to the next. The result: a long chain (typically n = 1 000 - 100 000). Used for shopping bags, bottles, food wrap, plastic toys.

Polypropene — made from propene. Used for ropes, carpets, packaging.

Polystyrene — made from styrene (an alkene with a benzene ring side group). Used for foam packaging, drink cups, insulation.

PVC (poly-vinyl-chloride) — made from chloroethene. Used for pipes, electrical insulation, vinyl flooring.

Modern life depends on polymers — they're cheap, light, durable, mouldable. The downside is that they're so durable they don't break down in the environment for hundreds of years.