This is the study of one particular element - carbon. It's often called Carbon Chemistry. Carbon forms an almost infinite number of compounds. All life forms are based on a huge number of carbon compounds.
Carbon is element number 6 in the Periodic Table and has 4 electrons in its outer shell, as you can see from this diagram.
Because there are 4 electrons in the outer shell, carbon always forms compounds with 4 covalent bonds in them. These are normally drawn using 4 straight lines. Some examples of carbon compounds are given below. I have represented the atoms by their chemical symbols (C for carbon, H for hydrogen etc.), which is very common when drawing organic molecules.
This is methane, the first of a family of organic compounds called alkanes. Its formula is CH4 and it is the gas that is used to power most household gas cookers. It is also used to fire Bunsen Burners. Methane is also known as as Town Gas or Natural Gas. Methane is a natural product formed by many biological process such as rotting rubbish. It was thought to be present on Earth before life began, and is found on Titan, the largest moon of Saturn.
This is Carbon Dioxide, CO2. The carbon is joined to two oxygen atoms, each with a double covalent bond. If you count the number of lines coming out of the carbon atom, it still comes to 4 lines. (Double bonds count as 2).
This is the next alkane in the series, ethane, formula C2H6. It shows that carbon atoms can form bonds with other carbon atoms. The formula for ethane is C2H6. It is a gas at room temperature and pressure.
This molecule is a type of organic compound called an Organic Acid. In fact, it is ethanoic acid, (formerly called acetic acid), which is found in vinegar.
Alkanes
Alkanes are a family of chemicals which are compounds of carbon and hydrogen only. They occur naturally - in fact, crude oil is a mixture of a large number of alkanes (and a few other chemicals).
The general formula for any alkane is CnH2n+2, where n is any whole number. This means that the number of hydrogen atoms in an alkane is always 2 more than double the number of carbon atoms. The smallest two alkanes are shown above, methane (with only one carbon atom) and ethane (with 2). The next three alkanes are shown below:
Propane, the third lightest alkane, has the formula C3H8. Like all alkanes, it is a fuel. In fact it is the gas found in those blue Calor Gas containers.
Butane has the formula C4H10. As alkanes get larger, they stop being gases at room temperature and pressure, and become liquids. Butane is the first to be liquid at room temperature, although it is volatile (i.e. turns into a gas very easily). For this reason, butane is used as lighter fuel.
Pentane, formula C5H12, is used in petrol (gasoline).
What do alkanes do?
They burn, which is why they make superb fuels. All small alkane molecules will burn in oxygen to form carbon dioxide and steam (which condenses to form water):
Alkane + Oxygen ® Carbon dioxide + water
Larger alkane molecules, with more than about 15 carbon atoms in them, burn less readily.
Alkanes can also burn incompletely. If there is not enough oxygen, then the reaction produces carbon monoxide instead of carbon dioxide:
Alkane
+
(Limited) oxygen
®
Carbon monoxide
+
Steam
2 CH4
+
3 O2
®
2 CO
+
4 H2O
This is the main reason that methane-burning boilers must be serviced regularly to ensure that they burn the methane "cleanly". Carbon monoxide is highly toxic, and an unserviced boiler can produce this "silent killer".
Fair enough, they burn. What else do they do?
When mixed with halogens (group 7), alkanes undergo a type of reaction called substitution. As you may have guessed, this involves a pair of atoms swapping their positions. Halogen elements exist in the form of diatomic molecules, i.e. molelcues containing two atoms. A molecule of chlorine, for example, contains two chlorine atoms, Cl2.
What happens is as follows: One of the hydrogens on the alkane detaches, as does one of the halogen atoms from the molecule. The loose halogen atom attaches to the alkane where the hydrogen was, forming a haloalkane, and the loose hydrogen attaches to the remaining halogen atom, forming a hydogen halide.
Alkenes
There is another family of organic chemicals, called alkenes, which consist of just carbon and hydrogen atoms. The crucial difference between alkanes and alkenes is that alkanes contain two carbon atoms joined together with a double bond.
You will recall that a single bond actually consists of two electrons. When two atoms (not just carbon but any atom capable of forming a covalent bond) form a single bond between them, both atoms contribute one electron to the bond, and these electrons then orbit the pair of atoms.
Now extend the logic, when atoms form a double bond, each atom contributes two electrons to the bond. This means a total of four electrons orbitting both atoms. In fact, a double bond is not simply the equivalent of two single bonds. Two of the four electrons form a strong "s"-type bond, whereas the other two form a weaker "p"-type bond. The concept of "s" for spherical and "p" for parabolic is one you will meet at A-level, and I don't want to go into it here, but it does mean that double bonds are quite weak, and have a tendency to "open up". You will meet this later on in this lesson.
Ethene is the smallest alkene. It has two carbon atoms. There is no alkene equivalent of methane as all alkenes contain a double bond between carbon atoms.
The next alkene is propene. Note that the molecule does not consist of double bonds all the way along. There is only one double bond.
As you may have guessed, the next family member is butene, with four carbon atoms.
The molecules look a little odd the way they have been drawn here. This is my attempt to "squash" 3-dimensional molecules into a flat diagram for the computer screen. If you count the number of hydrogen and carbon atoms, you will find that there are twice as many hydrogen atoms as carbon atoms. The general formula for alkenes is therefore CnH2n. Also, count the number of lines that emerge from each carbon atom (where a double bond counts as 2) - you will find that each carbon atom has four bonds, just as with all carbon compounds.
One difference between alkanes and alkenes - indeed, a difference that is used to distinguish between them - is the way in which they treat halogens. You will recall that alkanes undergo a substitution reaction with halogens, in which one halogen atom replaces one hydorgen atom on the alkane. The crucial aspect of this is that a hydrogen halide molecule (e.g. hydrogen chloride or hydrogen bromide) is produced, and that this will dissociate in any liquid water to produce an acid.
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