Alkenes

Unsaturated. Each alkene contains at least one C=C double bond. Carbons in the double bond have only TWO other bonds each (vs four for alkanes). They can ADD more atoms across the double bond — unlike alkanes which are "saturated" (full).

General formula: $\text{C}_{n}\text{H}_{2n}$ (two fewer hydrogens than alkanes).

First three alkenes.

Name	Formula	Structural
Ethene	$\text{C}_{2}\text{H}_{4}$	$\text{CH}_{2}=\text{CH}_{2}$
Propene	$\text{C}_{3}\text{H}_{6}$	$\text{CH}_{2}=\text{CHCH}_{3}$
Butene (1- or 2-)	$\text{C}_{4}\text{H}_{8}$	$\text{CH}_{2}=\text{CHCH}_{2}\text{CH}_{3}$ (but-1-ene)

Note. No "methene" — there's only one carbon to bond to itself, can't form a C=C. Alkenes start at 2 carbons.

Why are alkenes more reactive than alkanes? The C=C double bond consists of one strong $\sigma$ bond + one weaker $\pi$ bond (Cambridge IGCSE doesn't go this deep). The $\pi$ bond breaks easily during addition reactions, allowing new atoms to add to the carbons.

The test. Add a few drops of orange/yellow-brown bromine water to a sample.

Result.

• Alkene present: bromine water RAPIDLY DECOLOURISES (turns colourless).
• Alkane present: bromine water STAYS orange/brown — no reaction (under normal conditions; substitution needs UV).

Why? The alkene undergoes addition with Br₂: $\text{CH}_{2}=\text{CH}_{2} + \text{Br}_{2} \to \text{CH}_{2}\text{Br}\text{CH}_{2}\text{Br}.$

The product 1,2-dibromoethane is colourless → solution clears.

Worked qualitative. A student is given two unknown liquids labelled A and B, both colourless. Add a few drops of bromine water to each. A turns the bromine colourless quickly; B keeps the orange colour. A is unsaturated (alkene-like); B is saturated (alkane-like).

Cambridge tip. Always state BOTH the colour change AND the conclusion explicitly. "The bromine water decolourises FROM ORANGE TO COLOURLESS, indicating an alkene."

General. A small molecule X-Y adds across the C=C double bond. Each carbon gains a new bond. $\text{C}=\text{C} + \text{X-Y} \to \text{C(X)-C(Y)}.$

1. Addition of hydrogen (hydrogenation). Catalyst: nickel. Conditions: heat, $\sim 200\degree\text{C}$. $\text{CH}_{2}=\text{CH}_{2} + \text{H}_{2} \xrightarrow{\text{Ni}} \text{CH}_{3}\text{CH}_{3}.$

Industrial use: turning vegetable oil (unsaturated) into margarine (more saturated, solid at room T).

2. Addition of halogens. No catalyst needed at room T (with Br₂ in solution, the bromine water test). $\text{CH}_{2}=\text{CH}_{2} + \text{Br}_{2} \to \text{CH}_{2}\text{Br}\text{CH}_{2}\text{Br}.$

3. Addition of water (hydration). Catalyst: phosphoric or sulfuric acid. Conditions: high T ($\sim 300\degree\text{C}$) and pressure ($\sim 60\,\text{atm}$). $\text{CH}_{2}=\text{CH}_{2} + \text{H}_{2}\text{O} \xrightarrow{\text{H}_{3}\text{PO}_{4}} \text{CH}_{3}\text{CH}_{2}\text{OH}.$

Industrial use: making ethanol for industrial / fuel use (NOT food/drink — for those, fermentation is used).

Cambridge tip. Memorise three additions and their conditions:

• H₂: Ni catalyst, $200\degree\text{C}$.
• Br₂: no catalyst, room T.
• H₂O: H₃PO₄ catalyst (or H₂SO₄), high T and P.

Cracking. Heat long-chain alkanes (from heavy crude-oil fractions) at high T ($\sim 600-700\degree\text{C}$) with a catalyst (e.g. Al₂O₃, SiO₂, or zeolite). $\text{C}_{12}\text{H}_{26} \xrightarrow{\text{cat, heat}} \text{C}_{8}\text{H}_{18} + \text{C}_{4}\text{H}_{8}.$

(One short alkane + one short alkene from each long-chain.)

Why crack?

• Long-chain alkanes are LESS USEFUL (high b.p., bad for petrol engines).
• Short-chain alkanes are MORE USEFUL (gasoline range).
• Alkenes are needed for the petrochemical industry (plastics, ethanol, etc.).

Polymerisation. Alkenes can join end-to-end to form polymers — long chains of repeating units. $n\text{CH}_{2}=\text{CH}_{2} \to (\text{-CH}_{2}\text{-CH}_{2}\text{-})_{n}.$

This is poly(ethene), the most-produced plastic. Conditions: high T, high P, catalyst.

See the Polymers study note for full details on polymerisation, plastics, and biodegradability.

Worked. What useful products come from cracking the dodecane?

• Octane (gasoline component).
• Butene (alkene → polymerisation feedstock).