Organic Chemistry - 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 C_nH_2n+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 first few members of the alkanes are shown below:

		Here you see methane (with one carbon atom) and ethane (with two). Methane is the gas that is supplied to domestic heating systems through the gas mains, and for this reason it is often called natural gas or town gas.
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):

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.

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