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.
Fruit ripening gas
Alkenes are a family of hydrocarbons (compounds containing only carbon and hydrogen) that all contain a C=C double bond. The simplest example is a gas called ethene (or ethylene), with the formula H2C=CH2. During ripening, fruits including bananas produce ethene and this gas can help to ripen a number of other fruits, including apples. That is why putting a banana that is going brown next to apples in a fruit bowl will greatly speed up their ripening. A great example of a chemical reaction in action!
Small errors can have big consequences
Accuracy is absolutely crucial when it comes to drawing chemical structures and writing chemical names. This is because even a very small error can have a profound effect. As an example, let’s look at the consequences of changing from the plasticiser 1,5-pentanediol, HOCH2CH2CH2CH2CH2OH, to the very similar compound 1,4-butanediol, HOCH2CH2CH2CH2OH (count the carbons).
A plasticiser is used in plastic products to soften and make them more pliable, and 1,5-pentanediol was used as a plasticiser in a children’s arts and crafts toy, called Aqua Dots. In 2007, 4.2 million units of the toy were recalled after five children were hospitalised after swallowing the beads. The manufacturer had used 1,4-butanediol in place of nontoxic 1,5-pentanediol, perhaps because it was a much cheaper chemical. The problem with 1,4-butanediol is that when it is ingested in the body it is converted into 4-hydroxybutanoic acid (or GHB), HOCH2CH2CH2CO2H. Unfortunately, GHB has a very similar structure to GABA,
H2NCH2CH2CH2CO2H, which is our main inhibitory neurotransmitter (it transmits an impulse from a nerve cell to another nerve and it has a relaxation-like effect), and our bodies cannot tell the two molecules apart. Consequently, GHB acts as a neurotransmitter and in low doses it causes for example, drowsiness and nausea, while at higher doses it can cause unconsciousness and seizures. In contrast, when 1,5-pentanediol enters the body it is converted into HOCH2CH2CH2CH2CO2H, which, because of the slightly longer chain does not mimic GABA. So, we can see that changing the carbon chain by just one carbon atom has a remarkable effect.
If you would like to learn more about recognising functional groups you may find this YouTube clip of use.
This is an additional video, hosted on YouTube.
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