Alkanes and alkenes
A homologous series is a family of organic compounds with the same general formula and similar chemical properties. The first two families you need are alkanes and alkenes.
Alkanes have the general formula CnH2n+2. Every carbon-carbon bond is a single bond, so the molecule holds as much hydrogen as possible: we call this saturated. Examples: methane CH4, ethane C2H6, propane C3H8. They are fairly unreactive but burn well as fuels.
Alkenes have the general formula CnH2n and contain a carbon-carbon double bond (C=C). Because that double bond could take on more atoms, alkenes are unsaturated and much more reactive. Examples: ethene C2H4, propene C3H6.
- Bromine water test
- The test for a C=C double bond. Orange bromine water stays orange with a saturated alkane, but is decolourised (turns colourless) by an unsaturated alkene.
The alkene decolourises bromine water because the reactive double bond opens up and adds the bromine across it, an addition reaction. This simple colour change is how you tell saturated from unsaturated.
Two unlabelled gases, ethane and ethene, are each bubbled through orange bromine water. How do you tell which is which?
Cracking and addition
Crude oil gives us plenty of long-chain alkanes, but industry needs more short-chain molecules and alkenes. Cracking solves this: long saturated hydrocarbon molecules are broken into shorter ones using heat and a catalyst.
Cracking always produces a mix of a smaller alkane and at least one alkene, for example:
The small alkanes are useful fuels (like petrol), and the alkenes are used to make polymers. Alkenes then take part in addition reactions, where the double bond opens and atoms add across it, leaving a single saturated product with nothing else made. For instance, ethene plus bromine gives dibromoethane, and ethene plus steam (with a catalyst) gives ethanol.
Balance the carbons and hydrogens
When you write a cracking equation, count atoms on both sides. The carbons and hydrogens in the long molecule must all reappear in the shorter products, none are created or lost.
Alcohols and carboxylic acids
A functional group is the reactive part of a molecule that gives a family its characteristic reactions. Two more families are defined by their groups.
Alcohols contain the hydroxyl group, -OH. The general formula is CnH2n+1OH; ethanol is C2H5OH. Alcohols are made by fermenting sugars or by adding steam to ethene, and they burn as fuels.
Carboxylic acids contain the carboxyl group, -COOH. Ethanoic acid, CH3COOH, is the acid in vinegar. They are weak acids: they turn litmus red and react with carbonates to give a salt, water and carbon dioxide. When an alcohol is oxidised (for example, ethanol left open to air), it forms the matching carboxylic acid, which is why old wine turns sour.
| Family | Functional group | Example |
|---|---|---|
| Alkane | only C-C single bonds | ethane, C2H6 |
| Alkene | C=C double bond | ethene, C2H4 |
| Alcohol | -OH (hydroxyl) | ethanol, C2H5OH |
| Carboxylic acid | -COOH (carboxyl) | ethanoic acid, CH3COOH |
Polymerisation
A polymer is a very long molecule built by joining many small molecules (monomers) together. There are two ways this happens.
Addition polymerisation. Many alkene monomers add together, their double bonds opening to form a single long chain, with no other product. Ethene forms poly(ethene):
The monomer must be unsaturated (have a C=C), and the whole monomer ends up in the polymer.
Condensation polymerisation. Here two different monomers join, each with two reactive groups, and a small molecule (usually water) is lost each time a link forms. Nylon is a condensation polymer made this way. The defining difference: addition loses nothing, condensation always releases a small molecule such as water.
Common slip
Do not confuse the two. Addition polymerisation makes one product only from unsaturated monomers; condensation polymerisation makes the polymer plus a small molecule such as water.
Where this is assessed
Naming families, groups and reaction types is Criterion A. Using a test such as bromine water to identify an unknown, and justifying your conclusion from the observation, is Criterion B thinking.
Check yourself
1. What is the general formula of the alkenes, and what makes them unsaturated? +
Alkenes have the general formula CnH2n. They are unsaturated because they contain a carbon-carbon double bond (C=C), which could add more atoms across it.
2. A student adds bromine water to an unknown liquid and it turns colourless. What does this show? +
Decolourising bromine water shows the liquid is unsaturated: it contains a C=C double bond, so it is an alkene (an alkane would leave the bromine water orange).
3. Give one key difference between addition and condensation polymerisation. +
In addition polymerisation the only product is the polymer, made from unsaturated monomers. In condensation polymerisation a small molecule (usually water) is released each time monomers join, as when nylon is made.
Part of the Chemistry library. Spotted an error or want a topic added? That feedback makes the notes better.