Describe the alkenes as unsaturated hydrocarbons, explain their addition reactions including the bromine test for unsaturation, and describe addition polymerisation

What this dot point is asking

SEAB wants you to describe the alkenes as unsaturated hydrocarbons with a carbon-carbon double bond, explain their characteristic addition reactions (including the bromine-water test that distinguishes them from alkanes), and describe how alkene monomers join by addition polymerisation to form polymers such as poly(ethene). This builds directly on the alkanes and is a high-frequency exam topic.

The answer

The alkenes as unsaturated hydrocarbons

The alkenes are a homologous series of hydrocarbons that contain a carbon-to-carbon double bond ($\text{C}=\text{C}$). Because the double bond means the carbons are not bonded to the maximum number of hydrogen atoms, the alkenes are unsaturated. Their general formula is:

So ethene is $\text{C}_2\text{H}_4$ and propene is $\text{C}_3\text{H}_6$. Alkenes are made by cracking larger alkane molecules from crude oil, which produces smaller, more useful molecules including alkenes.

Why alkenes are more reactive

The double bond is the reactive part of an alkene. One of the two bonds can open up, allowing other atoms to add on. This makes alkenes much more reactive than the saturated alkanes, which have only single bonds. The characteristic reactions of alkenes are addition reactions.

Addition reactions

In an addition reaction, the double bond opens and atoms add across it, so two molecules become one with no atoms lost. Key examples:

• With bromine: ethene reacts with bromine to form a colourless dibromo compound. This is the basis of the bromine test.
• With hydrogen (and a catalyst): an alkene adds hydrogen to become the corresponding alkane (hydrogenation), used to harden vegetable oils into margarine.
• With steam (and a catalyst): ethene adds water to form ethanol, an industrial route to alcohol.

The bromine test for unsaturation

The test that distinguishes an alkene from an alkane uses bromine water (aqueous bromine), which is orange-brown:

• An alkene (unsaturated) decolourises bromine water (it turns colourless), because the double bond reacts with the bromine in an addition reaction.
• An alkane (saturated) does not decolourise it (it stays orange-brown).

So decolourising bromine water is the standard test for a carbon-carbon double bond.

Addition polymerisation

Many alkene molecules can join together to form a very long chain in addition polymerisation. The small starting molecules are called monomers (such as ethene), and the long molecule formed is the polymer (such as poly(ethene)). In the reaction, the double bond of each monomer opens and the molecules join end to end into a single chain, with no small molecule lost. Poly(ethene) is the familiar plastic used for bags and bottles.

Examples in context

Example 1. Hardening oils into margarine. Liquid vegetable oils contain carbon-carbon double bonds. Adding hydrogen across these bonds (hydrogenation, an addition reaction with a nickel catalyst) removes some double bonds and makes the oil more solid, turning it into margarine. This is the alkene addition reaction used in the food industry.

Example 2. Plastic from ethene. Ethene, obtained by cracking crude oil fractions, is polymerised into poly(ethene), the plastic in shopping bags and bottles. The huge scale of plastics manufacture rests on the simple idea of alkene double bonds opening and joining, linking organic chemistry to everyday materials.

Try this

Q1. State the general formula of the alkenes and the name of the bond that makes them unsaturated. [1 mark]

• Cue. $\text{C}_n\text{H}_{2n}$; the carbon-to-carbon double bond.

Q2. Describe the result of adding bromine water to an alkene and to an alkane. [2 marks]

• Cue. The alkene decolourises the orange-brown bromine water (turns it colourless); the alkane leaves it orange-brown (no change).

Q3. Name the small molecule and the large molecule in addition polymerisation, and state what happens to the double bond. [2 marks]

• Cue. The small molecule is the monomer and the large molecule is the polymer; the double bond opens to let the monomers join end to end.