Love it or hate it you won’t be able to ignore it – that’s organic chemistry. If this is a new topic to you, I hope that this blog posting will act as a ‘heads up’ something to help you to get ready for the course or to serve as a reminder / revision  of organic chemistry.

I think it is important to stress that this is an introduction – the rules I write about below do get more complicated – even at IB, so in the near future you may see another blog posting picking up where this one left off.

All organic substances contain carbon and hydrogen (some also contain on or more of the atoms oxygen, sulphur and nitrogen)


First of all – nomenclature of naming rules. These rules are set by IUPAC or the International Union of Pure and Applied Chemistry.

This is all to do with the number of carbon atoms.

Number of carbons Word used to describe this
1 Meth
2 Eth
3 Prop
4 But (pronounced ‘Bute’)
5 Pent
6 Hex
7 Sept


So if you have an organic substance with five carbon atoms you are going to need to word ‘pent’ somewhere.

The second this to identify is the family that the organic substance belongs to – this is referred to as a ‘homologous’ series. All molecules in a family (or homologous series) have the same general formula.

Some examples of a homologous series include

Name General formula Word used in naming
Alkanes CnH2n+2 -ane
Alkenes CnH 2n -ene
Alcohols Cn H2n+1OH -anol
Carboxylic acids CnH2n+1COOH -oic acid


Molecules within a homologous series will have similar chemical properties (ie, they will react in a similar manner) and a slow change (gradation) in physical properties (eg, as the number of carbon atoms increases, so does the melting point).

The homologous series contain functional groups (with the exception of the alkanes that do not have a functional group). I like to think of the functional group as being the part of the molecule that makes it different. In reality, it is the part of the molecule that reacts.

The alkene functional group will be a C==C double bond, the alcohol functional group will be ‘OH’ and carboxylic acid functional group COOH.

When it comes to naming an organic compound, you need to add the name given to the number of carbons to the name associated with the functional group.

So for example, the alkane C3H8 would be named prop+ane = propane or the alcohol CH3CH2CH2OH would be prop+anol = propanol or even CH3CH2CH2COOH would become but+anoic acid = butanoic acid.

As I wrote at the start, this is just an introduction and you will see that numbers need to be added to explain the position in the molecule of the functional group, a well as the prefix di, tri, tetra, etc to describe the number of functional groups.

Why don’t you try the following?

Name these:

1, C3H8

2, C3H6

3, C3H5OH


What are the formula of the following?

5, Ethanol

6, Butane

7, Pentanoic acid

8, Ethene

9, Propanol

10, Pentanoic Acid