AlkenesCambridge IGCSE Chemistry: Revision notes

Section 1

What Defines an Alkene?

Alkenes are a homologous series of hydrocarbons with the general formula $C_nH_{2n}$ .

The bonding in alkenes includes at least one carbon-carbon double bond (C=C), which makes alkenes unsaturated hydrocarbons. You should be able to name and draw the structural and displayed formulae of alkenes up to four carbons, including but-1-ene and but-2-ene (the number shows where the double bond is).

Key termsalkeneunsaturated hydrocarbon

Section 2

How Are Alkenes Made from Alkanes?

Alkenes (and hydrogen) are manufactured by cracking larger alkane molecules, using a high temperature and a catalyst.

Cracking is carried out because:

Long-chain hydrocarbons from crude oil are in lower demand than shorter ones
Cracking produces more of the shorter, more useful hydrocarbons (fuels) that are in high demand
It also produces alkenes, which are needed as starting materials (feedstock) for making polymers and other chemicals

Key termscracking

Section 3

How Do You Test for an Alkene (Unsaturation)?

To distinguish between saturated and unsaturated hydrocarbons, shake the substance with aqueous bromine:

Alkene (unsaturated): aqueous bromine is decolourised (orange/brown to colourless) — the double bond reacts with the bromine
Alkane (saturated): aqueous bromine stays orange/brown — no reaction occurs

This is a key practical test to identify the presence of a C=C double bond.

Key termsdecolourise

Exam tip

If a substance decolourises bromine water, it must contain a C=C double bond — this is one of the most commonly tested practical chemistry facts.

Section 4

What Addition Reactions Do Alkenes Undergo?

Because of their reactive double bond, alkenes readily take part in addition reactions, where two molecules join to make one, with nothing given off:

Reactant added	Conditions	Product type
Bromine or aqueous bromine	Room temperature	Dibromoalkane
Hydrogen	Nickel catalyst	Alkane
Steam	Acid catalyst	Alcohol

You should be able to draw the structural or displayed formulae of the products of each of these addition reactions.

Key termsaddition reaction

Example

Ethene + steam --acid catalyst--> ethanol (CH2=CH2 + H2O -> CH3CH2OH).

Common mistake

Don't confuse alkene addition reactions with alkane substitution reactions — alkenes add across the double bond; alkanes swap one atom for another.

Must Know

Alkenes have general formula $C_nH_{2n}$ and contain a C=C double bond, making them unsaturated
Alkenes (and hydrogen) are made by cracking larger alkanes at high temperature with a catalyst
Aqueous bromine is decolourised by alkenes but stays orange/brown with alkanes — the standard test for unsaturation
Alkenes undergo addition reactions with bromine/aqueous bromine, hydrogen (nickel catalyst) and steam (acid catalyst)
Steam + ethene (acid catalyst) produces ethanol
Number the double bond position in names, e.g. but-1-ene vs but-2-ene

That's the notes covered.

Carry on to the next subtopic.