Chemguide: Support for CIE A level Chemistry

Learning outcome 14: Hydrocarbons

14.1: Alkanes

Statement 14.1.2

This statement looks at the reactions of alkanes with oxygen, chlorine and bromine.

Before you go on, you should find and read the statement in your copy of the syllabus.

If you have just read the syllabus statement, you will find that it refers specifically to the reactions of ethane as a typical alkane. It would, however, be a big mistake to restrict yourself to ethane, and ignore the other alkanes. It would be fairly typical of CIE to ask you about almost any alkane apart from ethane.

Statement 14.1.2(a)

First read the page about the combustion of alkanes.

Make sure that you can work out an equation for whatever alkane you happen to be asked in an exam. Trying to learn equations would be madness. In an exam, you could be asked for virtually any combustion reaction, and you have to be able to work them out as necessary.

Statement 14.1.2(b)

Read the page about the halogenation of alkanes. You can probably safely ignore the bit at the bottom about the bromination of cyclopropane.

You can't, though, ignore the bit at the top of the page about the reactions with fluorine and iodine.

In the November 2009 exam, CIE asked you to calculate enthalpy changes for the possible reactions between methane and chlorine or iodine. The answer to the iodine part was +22 kJ mol-1 for the methane-iodine reaction - a mildly endothermic change.

The question told you that this reaction didn't happen, and asked you to suggest why it didn't happen. The answer they gave was that it must have a very high activation energy. The Examiner's Report said that few students were able to deduce this. That doesn't surprise me! Anyway, if it comes up again, you now know what they want.

Note:  I'm not convinced this is a valid answer. If a reaction doesn't happen, it may be that the activation is very high, or it may be more fundamental than that - but you would need to know about entropy and free energy changes before you could understand it. However, the answer about activation energy is the only one that you could give for a Paper 2 question.

The reactions between ethane and chlorine or bromine

You may have spotted that the page you have read has nothing whatsoever about the reactions of ethane with chlorine or bromine.

The great thing about organic chemistry is that once you understand a reaction involving one member of a family of compounds (a homologous series), you can easily go on to work out how another member would react.

You have to do this, because there are vast numbers of organic compounds - you can't possibly learn each one separately. And in the case of CIE, they rarely seem to ask you questions about the simple compound mentioned in the syllabus - preferring to use something more complicated.

So, if you know about the reaction between methane and, say, chlorine, what can you deduce about the reaction between ethane and chlorine? (What I say about chlorine applies equally to bromine.)

Here is the equation for the first stage of the reaction between methane and chlorine in the presence of UV light.

You can see that a hydrogen atom has been removed and replaced by a chlorine atom to give chloromethane. This is a substitution reaction. Hydrogen chloride is also formed.

The reaction doesn't stop here. As long as there are hydrogens attached to the carbon, they can be replaced in a similar way to give dichloromethane, trichloromethane and tetrachloromethane. And that is true in any alkane or alkyl group.

Note:  Remember that an alkyl group is what you get if you remove a hydrogen from an alkane. The CH3 group is a methyl group, for example.

So, what happens with ethane? Exactly the same thing.

A hydrogen is replaced by a chlorine to give chloroethane, and a molecule of hydrogen chloride is formed as well.

And the reaction will continue as long as there are hydrogens to be replaced. Unfortunately, the situation is more complicated here than with methane, because different isomers can be formed.

Suppose you replace a second hydrogen by a chlorine. Which hydrogen will be replaced in CH3CH2Cl?

It could be one on the carbon which has already got a chlorine on it, or one on the other carbon. That means that you could get either CH3CHCl2 or CH2ClCH2Cl formed.

And, of course, the situation is just as complicated as you replace the rest of the hydrogen atoms. I'm not going to give you the structures of all the possible compounds formed, because some of you will then feel that you have to learn them. That is totally pointless. As long as you understand what is happening, you can work it out if you need to.

Note:  As far as I can tell, in their questions, CIE tend to focus on the first step of the substitution process to give mono-substitution - where only one hydrogen has been replaced by a halogen atom. That doesn't mean to say that they can't or won't ask about multiple substitution in the future.

My advice would be to concentrate on understanding the methane substitution reactions. If you can understand these, then you can always work out what is likely to happen in more complicated cases.

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© Jim Clark 2020