Tools of the trade: understanding functional groups and naming compounds
To categorise the vast number of organic compounds we divide them into functional groups – these are groups of atoms that give the molecule characteristic chemical properties. For example, in ethanol (CH3CH2OH), the –OH group is called a hydroxyl functional group, which is present in a family of compounds called alcohols. The carbonyl group between two carbons is called a ketone, whereas when it is between a carbon atom and hydrogen it is an aldehyde.
Naming organic compounds
When naming compounds, chemists can use common (or trivial) names, which are often based on their source, or use. A classic example is formic acid, HCO2H, which was first isolated from ants. The name comes from the Latin word for ant, which is formica. However these names do not help us to work out the structure of the organic compound. For this, we use a systematic method called IUPAC naming – this helps chemists to be able to draw a compound from its name, without having to look up the structure.
IUPAC names for compounds are typically made up of three parts:
The ‘prefix’ gives the name and position of any substituents on the main carbon chain, ‘parent’ identifies the name of the longest carbon chain (for compounds with a single carbon, the parent alkane is methane, for two linked carbons it is ethane, and for three linked carbons it is propane), and ‘suffix’ indicates the main functional group.
For example, the IUPAC name for formic acid is methanoic acid – the ‘methan’ specifies that there is one carbon (it is a derivative of methane, CH4) and the ‘oic acid’ tell us that the functional group is a carboxylic acid (i.e. that it contains a –CO2H group). For propanone (CH3COCH3), the ‘propan’ specifies the length of the carbon chain is 3 (it comes from propane, CH3CH2CH3) and the ‘one’ shows it is a ketone. In contrast, the related 3-carbon compound, CH3CH2CHO, is called propanal, the suffix ‘al’ indicating that this compound is an aldehyde.
Groups that are attached to the main carbon chain are called substituents - those containing only carbon and hydrogen, that are linked together by single bonds, are called alkyl groups. Shown below are the IUPAC names for alkyl groups, from a one-carbon chain to a four-carbon chain.
Alkyl groups are identified by the prefix in a IUPAC name. For example, 2-methylpropane has a CH3 substituent joined to the middle carbon of a three-carbon chain i.e. CH3CH(CH3)CH3. The prefix ‘2-methyl’ is used to indicate the CH3 substituent and where it is attached to the main chain (which is numbered from 1 to 3). Sometimes, for convenience, these groups are represented by abbreviations, for example, Me3CH for 2-methylpropane and Me2CO (or MeCOMe) for propanone.
Compounds with similar structures and the same functional group, or groups, have similar properties. Organic chemists study the structure and reactivity of organic compounds, so drawing accurate structures and recognising functional groups is crucially important.
As well as the images that appear on this page, see below for some functional group posters that we hope will be useful to you as you work through the course content.
If you would like some further information on recognising functional groups you may find this YouTube clip of use.
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