The bad news is that organic chemistry is kind of a big topic! So we’re going to split it up into the main topics you need to learn for GCSE, namely alkanes, alkenes and and the cracking of crude oil.
So let’s get cracking (get it) with alkenes and crude oil. In case you didn’t know, crude oil is pretty important stuff! It is the main source of many of the world’s organic chemicals and we use products made with crude oil every day.
Crude oil is formed from sea creatures’ remains that have been compressed over millions of years and turn into crude oil.
In fact this process is so slow that it means crude oil is effectively non-renewable – unless you’ve got a few million years to spare!
Crude oil is a mixture of compounds called hydrocarbons. Many useful materials can be produced from crude oil.
It can be separated into different fractions using fractional distillation, and some of these can be used as fuels.
The process involves heating the oil at the bottom of a large tower. Smaller hydrocarbons have a lower boiling point than large ones.
As the tower gets cooler the higher up the hydrocarbons get, the heavier chains condense near the bottom, while lighter ones rise further before condensing.
This separates out the hydrocarbons into fractions – mixtures of hydrocarbons with similar boiling points.
We know that alkanes are compounds containing only hydrogen and carbon but now let’s explore how alkenes combust (or burn in non-chemistry lingo).
The combustion of hydrocarbons is an exothermic reaction, obviously, as it would be pretty pointless burning for heat if it wasn’t going to produce any!
Unfortunately the combustion of hydrocarbons isn’t all that great for the planet. Burning fossil fuels produces carbon dioxide which is a major contributor to the greenhouse effect.
Carbon dioxide acts like an insulator, trapping infrared radiation (heat) on the planet’s surface.
Their burning can also produce sulfur dioxide which is a poisonous gas responsible for acid rain.
This is because hydrocarbon comes from living matter which contains amino acids including sulfur.
If combustion of hydrocarbons is also incomplete carbon monoxide, a poisonous gas, is produced.
A catalytic converter ensures we can remove some of the dangerous products of petrol fumes like NO and CO, by converting them into N2 and CO2.
In this process the metals are catalysts, and need a ceramic honeycomb support as it maximises the surface area of the catalyst, this is typically made from platinum and rhodium.
Lastly, we’re going to look at the modification of alkanes by cracking.
In oil refineries, heavy fractions of crude oil are vaporised and cracked in a steel vessel called a cracker. The large molecules split apart into smaller, more useful, molecules in a thermal decomposition reaction.
Long chain hydrocarbons are split into a mixture of smaller alkanes and alkenes. The small alkanes can be used as fuel (such as pentane) whilst the alkenes (such as ethene), can be used to make plastics and many other useful chemicals.
The length of the hydrocarbon chain determines its properties. A shorter hydrocarbon is less viscous, more flammable, more volatile and has a lower boiling point than a long chain hydrocarbon.
The long chain hydrocarbons which are split up during cracking form the residue fraction during fractional distillation of crude oil.
The residue is a thick, sticky liquid with a very high boiling point. It can be used to make road surfaces and coat flat roofs. The value of the crude oil residue is much smaller than that of the smaller products obtained from a cracking reaction.
And that’s alkanes mastered! Next: alkenes.
Alkenes are hydrocarbons that contain a carbon-carbon double bond. The number of hydrogen atoms in an alkene is double the number of carbon atoms.
Alkenes are unsaturated, meaning they contain a double bond. The presence of this double bond allows alkenes to react in ways that alkanes cannot. This allows us to distinguish alkenes from alkanes using a simple chemical test.
And this test involves using bromine (which is orange). This will become colourless when it is shaken with an alkene – nice and straight forward.
Alkanes on the other hand would not decolourise bromine water, making it easy to tell the difference.
However, we can turn alkenes into alkanes through the process of cracking by breaking down the long alkene chains.
The strongest intermolecular forces between alkene molecules are Van der Waals’ forces (remember him?).
And the longer the alkene chain, the higher the boiling point becomes.