Chemguide: Support for CIE A level Chemistry

Learning outcome 13

13.1: Formulae, functional groups and the naming of organic compounds

Statement 13.1.4

You should read this statement in your syllabus, along with the really useful examples in the two pages of tables beforehand.

Take care! This statement 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 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". 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 CnH2n+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 CnH2n. Alcohols are CnH2n+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.

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© Jim Clark 2020