AS-Level Organic Chemistry (Introduction)

  • Prefixes are used when naming organic molecules to show how many carbon atoms are in a chain.

  • Meth = 1 carbon, eth = 2 carbons, pro = 3 carbons, but = 4 carbons, pent = 5 carbons, hex = 6 carbons, hept = 7 carbons, oct = 8 carbons, non = 9 carbons and dec = 10 carbons 


Carbon Chains


Within organic molecules, carbon atoms can be bonded to other carbon atoms. This creates a ‘chain’ of carbon within a molecule. The properties of an organic molecule can alter depending on the size of this chain, so it is important to be able to recognise and name the carbon chain present within an organic molecule.


If there are only carbon and hydrogen atoms in a molecule, it is called a hydrocarbon. When the carbon atoms are all bonded with single carbon-carbon bonds, it is called an alkane.


We name alkanes based on the length of the carbon chain.  The prefix (start!) of the name is unique to the carbon chain and the suffix (end!) of the name is simply –ane. The -ane just tells us that the molecule is an alkane. The suffix can and does change depending on what else is bonded to the chain.


1 carbon = meth-   


2 carbons = eth-


3 carbons = prop-


4 carbons = but-


5 carbons = pent-


6 carbons = hex-


7 carbons = hept-


8 carbons = oct-


9 carbons = non-


10 carbons = dec-



Most A-level specifications require you to know and memorise the prefix for molecules containing up to 6 carbons.


For alkanes, each carbon atom is bonded to another carbon atom and any remaining available bonds are made to hydrogen atoms. The alkane is described as a ‘saturated’.


If there is a double bond between two carbon atoms within the molecule, the hydrocarbon becomes an alkene and is described as ‘un-saturated’.

A carbon chain group bonded to another carbon chain is called an alkyl group and follows the same prefix as usual.