Organic Chemistry

Crude oil is a dark, smelly and sticky brown liquid. It is called crude because it is unrefined. Despite this, wars are fought over it,

people’s finances are severely affected by it and many other of the World’s crises are caused by oil or the lack of it. This mixture contains hydrocarbons (molecules made from hydrogen and carbon only) and the amount of carbon in each molecule affects its properties. The smallest is methane (CH4), then Ethane (C2H6), Propane (C3H8), Butane (C4H10) and so on. As the chain gets longer, the properties change and so does the way we use it. Crude oil, therefore, needs to be separated into different fractions (length of chains) and the physical property used to separate them is the boiling temperature.

The way to spot an alkane or to work out the formula of an alkane is to use the general formula:CnH2n+2 These are called saturated hydrocarbons as we cannot add any more hydrogens, the molecule is completely saturated with hydrogen.


Alkanes have simple properties that follow a pattern depending on the length of their chain:  Using the relationship between length of chain and boiling point, the crude oil enters this column and as it is cool at the top, only the shortest fractions are still a gas and can escape through the upper most exit to be used as bottled gas (LPG). On the other end of the spectrum, even at 350°C some fractions are still a very viscous liquid and are used as road tar or for sealing roofs.

Depending where in the world it comes from, crude oil is different in colour and this is determined by the proportion of long and short chain alkanes. Venezuelan crude is dark “heavy crude” and Saudi Arabian is pale “light crude”. We use these fuels to get energy from them. Burning oils mainly produces carbon dioxide and water. Below are the balanced equations for the combustion of 2 alkanes:


2C2H6 + 7O2 -> 4CO2 + 6H2O


2C4H10 + 13O2 -> 8CO2 + 10H2O

Properties of alkanes

Make sure that you understand these and note that in this case I have not allowed 3½O2. This does not always happen, if there is not enough oxygen, then carbon monoxide is produced CO. This is poisonous especially in your home which is why gas appliances should be checked annually and fitted by a professional.

Because now, more than ever, people have cars, use buses, have consumables that are shipped around the globe in lorries, aircraft and on ships and we travel by air more than ever...there is a problem with the amount of CO2 emitted due to combustion of hydrocarbons. CO2 is a greenhouse gas which means that it absorbs heat energy that would normally be bounced back into space. Without the greenhouse effect, Earth may well be too cold for us to live here but too much will make our climate change.

Other gases and particulates that escape cause harm also. Incomplete combustion (not enough oxygen) produces carbon monoxide CO which affects our blood's ability to carry oxygen and seriously affects those with heart complaints. Oxides of sulphur and nitrogen lower the pH of rain and can also aggravate asthma. Finally, unburned hydrocarbons float into the upper atmosphere and reflect sunlight causing global dimming.

We can reduce this impact by using a catalytic converter in a car which takes a chemical reaction on a stage to make some gases less harmful:

Carbon Monoxide + Nitrogen Oxides -> Carbon Dioxide + Nitrogen

FGD or Flue Gas Desulphurisation is used in industry and can react the emitted sulphur oxides with quicklime to prevent them escaping into our atmosphere.

Ironically, the most abundant hydrocarbons are some of the least in demand. After fractional distillation, the majority of products are the longer chains which we don't need as much of, we need fuels! These heavier fractions undergo a process called "Catalytic Cracking". Long chain alkanes are heated until they are gases and passed over a catalyst. They break down in a reaction called: Thermal Decomposition

C10H22 -----> C5H12 + C3H6+ C2H4

Or: Decane decomposes at 800°C with a catalyst to form Pentane, Propene and Ethene.
If you look at the general formula of an Alkane, only decane and pentane fit. Also, the last two hydrocarbons end in the letters "ene". As well as making smaller hydrocarbons, we have made a new species. These are called Alkenes and they have a general formula of:


Alkane and Alkene structural differences

Alkenes are very useful hydrocarbons as we will see later, however, you must remember the two ways to spot them. Firstly, look at the formula or count the number of carbon and hydrogen atoms and see which of the two general formulae it fits in. The second way is a chemical test. Carbon ALWAYS forms 4 bonds and hydrogen ALWAYS forms 1 bond, for these new species, there is a problem with adding up the bonds which is solved by forming a carbon to carbon double bond.

This allows alkenes to be described as unsaturated as they could still absorb some hydrogen and alkanes are called saturated. This links to vegetable oils. If you shake an alkene in bromine water, the bromine will bond on the double bond making it saturated, as it leaves the water, it decolourises. Simply put: Alkenes decolourise bromine water.

Chloroethene monomers

The most important use of alkenes is in plastics. An alkene is a monomer (mono meaning single). When we saw above, bromine can saturate the double bonds, they can saturate their own by converting from many monomers into a single polymer (poly meaning many). Right, there are lots of chloroethene molecules lined up:
If they are under the correct conditions, they will join in a reaction called: "addition polymerisation" Don't forget the individual repeating units:

These make plastics of countless different properties. The names are common to us all, as well as PVC, Ethene makes polyethene or polythene, Propene makes the plastic polypropene ect.
Plastics can be categorised into two main areas. These categories are based on the strength of their intermolecular forces or how well these polymer chains are stuck to each other.



he first type are Thermosoftening plastics. They have weak intermolecular forces and are made into items by melting pellets, heating them until molten and injecting them into a mould. Once cool, they retain their shape.

The second type are Thermosetting plastics such as HDPE which has much stronger intermolecular forces. The polymer chains are aligned with more structure rather than those of a thermosoftening plastic which lie like cooked spaghetti on a plate. These stronger plastics have a higher melting temperature which allows them to be used for kettles etc.

New and Useful Polymers:
This topic is very open-ended and so I suggest that you search the internet for some ideas. I will start you off with a few basics though. Rather than use a polymer for its properties, chemists can now make a polymer do more or less anything. They can design a polymer to have the properties that you want.

Think of these to start you off:
-PET plastic bottles, lighter than glass ones that withstand impact;
-Stitches that will not only dissolve so that you don't have to have further treatment but on warming from your body, they contract to hold a wound shut;
-If you wince when having a plaster ripped off, chemists have made them with a light sensitive polymer that can switch off their stickiness.

Key words and terms for this topic: mixture, distillation, fraction, hydrocarbon, alkane, saturated hydrocarbon, unsaturated hydrocarbon, fractional distillation, viscosity, flammable, oxidised, incomplete combustion, carbon monoxide, global warming, global dimming, particulate, biodiesel, biofuel, carbon neutral.

Science Department footer