Topic 10:  Basic Concepts in Organic Chemistry 

This page explains how to write the formula for an organic compound given its name - and vice versa. It covers alkanes, cycloalkanes, alkenes, simple compounds containing halogens, alcohols, aldehydes and ketones. At the bottom of the page, you will find links to other types of compound.


How this page is going to tackle the problem

There are two skills you have to develop in this area:

  • You need to be able to translate the name of an organic compound into its structural formula.

  • You need to be able to name a compound from its given formula.

The first of these is more important (and also easier!) than the second. In an exam, if you can't write a formula for a given compound, you aren't going to know what the examiner is talking about and could lose lots of marks. However, you might only be asked to write a name for a given formula once in a whole exam - in which case you only risk 1 mark.

So, we're going to look mainly at how you decode names and turn them into formulae. In the process you will also pick up tips about how to produce names yourself.

 In the early stages of an organic chemistry course people frequently get confused and daunted by the names because they try to do too much at once. Don't try to read all these pages in one go. Just go as far as the compounds you are interested in at the moment and ignore the rest. Come back to them as they arise during the natural flow of your course.

Cracking the code

A modern organic name is simply a code. Each part of the name gives you some useful information about the compound.

For example, to understand the name 2-methylpropan-1-ol you need to take the name to pieces.

The prop in the middle tells you how many carbon atoms there are in the longest chain (in this case, 3). The an which follows the "prop" tells you that there aren't any carbon-carbon double bonds.

The other two parts of the name tell you about interesting things which are happening on the first and second carbon atom in the chain. Any name you are likely to come across can be broken up in this same way.

Counting the carbon atoms

You will need to remember the codes for the number of carbon atoms in a chain up to 6 carbons. There is no easy way around this - you have got to learn them. If you don't do this properly, you won't be able to name anything!

Alkyl groups

Compounds like methane, CH4, and ethane, CH3CH3, are members of a family of compounds called alkanes. If you remove a hydrogen atom from one of these you get an alkyl group.

For example:

  • A methyl group is CH3.

  • An ethyl group is CH3CH2.

These groups must, of course, always be attached to something else.

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