Chemguide: Support for CIE A level Chemistry

This statement covers the way you draw and name organic structures.

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

There is a long introduction to this organic section before you get to learning outcome 10.1(a), and it is important that you read it so that you know what it contains. It gives useful information about exactly what CIE mean by the various types of formula that you will come across. Start reading from the very beginning of the organic chemistry section.

You don't need to try to learn all of this introduction just now, but you should be aware that it exists so that you can come back and look at it again whenever you need to.

Take care! This statement 10.1(a) is one of the most important in the whole syllabus. Take your time over it, and make sure that you really understand how to draw and name organic molecules. If you rush it, you will find the rest of the organic chemistry a total nightmare! And if that happens, you might as well give up chemistry now, because you won't get a good A level result.

Drawing formulae - types of structures

Start by working through the page How to draw organic molecules. It is essential that you are happy about the difference between molecular formulae, simple structural formulae, displayed formulae (structural formulae where all the bonds are drawn), and skeletal formulae.

It is also important that you can understand the difference between the way the various types of structures are drawn and how the molecules would actually look in 3-dimensions. If you find this difficult, then get hold of some molecular models (or make your own - but you risk getting bond angles wrong), and play with them until you are happy about it.

When I was teaching, I would get students to play around with models for at least an hour in the early stages of the course. You will find organic chemistry a lot easier if you really understand this basic stuff.

The importance of understanding skeletal formulae for CIE exams

Some CIE questions about organic compounds are set using a skeletal formula. If you don't understand exactly how to read that formula, you can't do the question. Take some time to be sure that you understand formulae of this kind.

General formulae

The page I have suggested you read doesn't mention general formulae. You will meet these later on, but for completeness, I will talk about them here as well.

Organic compounds can be arranged in families, known as "homologous series" (although CIE don't use that term in the syllabus - this is what they mean by "classes of compounds"). You have probably come across the family of hydrocarbons known as the alkanes.

If you know how many carbon atoms there are in a particular alkane, you can easily work out the number of hydrogen atoms, because the family has a general formula. For the alkanes, this is C_nH_2n+2.

So an alkane which has 5 carbon atoms has a molecular formula C₅H₁₂.

An alkane which has 15 carbon atoms has a molecular formula C₁₅H₃₂.

Other families have different general formulae. Alkenes, for example are all C_nH_2n. Alcohols are C_nH_2n+1OH.

It is completely pointless learning all this now for compounds which mean absolutely nothing to you at the moment. You will simply get confused. Wait until you look in detail at particular classes of compounds - and even then, you are unlikely to need to learn general formulae for anything much beyond the simplest compounds.

Naming organic compounds

You should read the page Understanding the names of organic compounds as far as the end of the section about alcohols.

Take your time over this, and make sure that you understand it. This is one of the major hurdles in organic chemistry. It isn't difficult, but it takes time.

The page I have suggested is the first of three about organic names, and these will cover all the names that you will need for the CIE syllabus. But don't go beyond the end of the alcohols on the first page for now.

If you take your time over reading this first bit, you will have a good understanding of how organic names work.

If you try to work through all the names of all the different compounds that you will come across during the rest of the course, you will just get confused.

Once you have understood how naming works, it is easy to add another sort of compound as you come across it later on. You can always refer back to these pages again later if you need to. It is really important for your confidence that you don't try to do too much too soon.

A final comment

I have made no attempt here to cover exactly what statement 10.1(a) asks point-by-point. That is quite deliberate. What I have asked you to do is enough to give you confidence without risking confusing you.

You will find everything else covered a bit at a time as you go through the rest of the course.

For your own satisfaction, it might be worth printing this page of the syllabus (or copying it either onto paper or into another computer file), and ticking off the various classes of substance as you learn how to draw and name them later on. That way you can be sure that everything has been covered.

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