Alkenes and their general formula; the test for alkenes with bromine water; addition polymerisation; and the difference from alkanes.

What this dot point is asking

AQA wants you to describe alkenes as unsaturated hydrocarbons with a carbon to carbon double bond, give their general formula, describe the bromine water test, explain addition polymerisation, and contrast alkenes with alkanes. The double bond is the central idea: it is the reason alkenes are reactive, decolourise bromine water and can polymerise.

Alkenes

The first alkenes are ethene ($C_2H_4$), propene ($C_3H_6$) and butene ($C_4H_8$). The double bond is the functional group, the part of the molecule where reactions take place. Because the double bond can open up and react, alkenes take part in addition reactions where atoms add across the double bond.

Testing for alkenes

Addition polymerisation

For example, many ethene molecules join to make poly(ethene). The name of the polymer is poly followed by the monomer name in brackets, so propene makes poly(propene). The repeating unit is the monomer with its double bond opened to single bonds that extend to the next unit on each side.

Alkenes versus alkanes

• Alkanes: saturated (only single bonds), general formula $C_nH_{2n+2}$, less reactive, do not decolourise bromine water.
• Alkenes: unsaturated (a double bond), general formula $C_nH_{2n}$, more reactive, decolourise bromine water.

This single difference, a double bond versus only single bonds, explains every contrast between the two families.

Why alkenes are more reactive

The carbon to carbon double bond is a region of high electron density that other atoms are attracted to, so the double bond can open up and let atoms add across it in an addition reaction. This is exactly what happens in the bromine water test (bromine adds across the double bond) and in addition polymerisation (the double bonds open so monomers join). Alkanes have no double bond, so they cannot do this; they are saturated and far less reactive, taking part only in combustion and substitution reactions. So the reactivity, the bromine water result and the ability to polymerise all trace back to the one functional group.

Properties and uses of addition polymers

Addition polymers such as poly(ethene) and poly(propene) are made of very long chains held to each other by intermolecular forces, which makes them solid, flexible and durable at room temperature, useful for plastic bags, bottles and containers. Because their chains are unreactive and do not break down easily, many addition polymers are not biodegradable, which is why their disposal is an environmental concern and recycling matters.

Try this

Q1. State the general formula of an alkene. [1 mark]

• Cue. $C_nH_{2n}$.

Q2. Describe the result of adding an alkene to bromine water. [1 mark]

• Cue. The orange bromine water turns colourless.

Q3. Name the polymer made from the monomer ethene. [1 mark]

• Cue. Poly(ethene).