Chemguide: Support for CIE A level Chemistry

This statement tests your ability to draw isomers from a given molecular formula.

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

You need to be able to draw all the isomers from a given molecular formula. Most people can do this reasonably well for structural isomers - although you will find this easier after you have done some more organic chemistry. If you have never come across, say, esters, you might well not consider the possibility of drawing isomers which contain an ester linkage. Don't worry about this for now.

The first thing to do is to draw as many structural isomers as you can - or at least, as many as you can given any restrictions that the question might impose. For example, it might ask you just to draw isomers which contain an -OH group, or a carbon-oxygen double bond, or whatever.

Now look at your isomers again to see whether any of them might have stereoisomers - either optical or cis-trans isomers.

• If one of the structures you have drawn has a carbon-carbon double bond, look for the possibility of cis-trans isomers.

  Have you got a double bond with two different groups at both ends? If so, draw the cis and trans forms of this carefully, showing the proper bond angles of about 120° either end of the double bond.

• And then look for chiral centres in the structure.

  Have you got one or more carbon atoms with four different groups attached? If so, you will have optical isomers. Draw both forms in the way that I have used on the page about optical isomerism.

Take your time over this. It is very easy to miss cis-trans and optical isomers if you try to rush it.

The best way to practise all this is to work through past exam questions looking for isomerism questions, and referring to the mark schemes and examiner's reports to see what CIE expect. That way, you will get used to the sorts of structures that CIE use, and won't be frightened by them when you come to your real exam.

Unfortunately, most past papers aren't available from the part of the CIE website that is open to students. Teachers, however, can access all the past papers, mark schemes, etc. Talk nicely to your teacher, and see if they will get you at least some past papers.

You can find what is available from the CIE A level Chemistry page. Teachers can get to more papers by following the "Teacher Support" link on that page.

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