THE NAMES OF AROMATIC COMPOUNDS

This page looks at the names of some simple aromatic compounds. An aromatic compound is one which contains a benzene ring. It assumes that you are reasonably confident about naming compounds containing chains of carbon atoms (aliphatic compounds).
	Note:  If you aren't sure about naming aliphatic compounds follow this link before you go on.
Naming aromatic compounds isn't quite so straightforward as naming chain compounds. Often, more than one name is acceptable and it's not uncommon to find the old names still in use as well. Background The benzene ring All aromatic compounds are based on benzene, C6H6, which has a ring of six carbon atoms and has the symbol: Each corner of the hexagon has a carbon atom with a hydrogen attached.
	Note:  If you don't understand this structure, it is explained in full in two pages on the structure of benzene elsewhere in this site. Following this link could well take you some time!
The phenyl group Remember that you get a methyl group, CH3, by removing a hydrogen from methane, CH4. You get a phenyl group, C6H5, by removing a hydrogen from a benzene ring, C6H6. Like a methyl or an ethyl group, a phenyl group is always attached to something else. Aromatic compounds with only one group attached to the benzene ring Cases where the name is based on benzene chlorobenzene This is a simple example of a halogen attached to the benzene ring. The name is self-obvious. The simplified formula for this is C6H5Cl. You could therefore (although you never do!) call it phenyl chloride. Whenever you draw a benzene ring with one other thing attached to it, you are in fact drawing a phenyl group. In order to attach something else, you have to remove one of the existing hydrogen atoms, and so automatically make a phenyl group. nitrobenzene The nitro group, NO2, is attached to a benzene ring. The simplified formula for this is C6H5NO2. methylbenzene Another obvious name - the benzene ring has a methyl group attached. Other alkyl side-chains would be named similarly - for example, ethylbenzene. The old name for methylbenzene is toluene, and you may still meet that. The simplified formula for this is C6H5CH3. (chloromethyl)benzene A variant on this which you may need to know about is where one of the hydrogens on the CH3 group is replaced by a chlorine atom. Notice the brackets around the (chloromethyl) in the name. This is so that you are sure that the chlorine is part of the methyl group and not somewhere else on the ring. If more than one of the hydrogens had been replaced by chlorine, the names would be (dichloromethyl)benzene or (trichloromethyl)benzene. Again, notice the importance of the brackets in showing that the chlorines are part of the side group and not directly attached to the ring. benzoic acid (benzenecarboxylic acid) Benzoic acid is the older name, but is still in common use - it's a lot easier to say and write than the modern alternative! Whatever you call it, it has a carboxylic acid group, -COOH, attached to the benzene ring. Cases where the name is based on phenyl Remember that the phenyl group is a benzene ring minus a hydrogen atom - C6H5. If you draw a benzene ring with one group attached, you have drawn a phenyl group. phenylamine Phenylamine is a primary amine and contains the -NH2 group attached to a benzene ring. The old name for phenylamine is aniline, and you could also reasonably call it aminobenzene. Phenylamine is what it is most commonly for UK-based exam purposes.
	Note:  In all cases where there is some possibility of alternative names, you need to know what your examiners are likely to call a particular compound. Refer to your syllabus and recent exam papers. If you are working to a UK-based syllabus for 16 - 8 year olds, and haven't got these, follow this link to find out how to get hold of them.
phenylethene This is an ethene molecule with a phenyl group attached. Ethene is a two carbon chain with a carbon-carbon double bond. Phenylethene is therefore: The old name for phenylethene is styrene - the monomer from which polystyrene is made. phenylethanone This is a slightly awkward name - take it to pieces. It consists of a two carbon chain with no carbon-carbon double bond. The one ending shows that it is a ketone, and so has a C=O group somewhere in the middle. Attached to the carbon chain is a phenyl group. Putting that together gives: phenyl ethanoate This is an ester based on ethanoic acid. The hydrogen atom in the -COOH group has been replaced by a phenyl group.
	Note:  If you aren't happy about naming esters, follow this link before you go on.
phenol Phenol has an -OH group attached to a benzene ring and so has a formula C6H5OH. Aromatic compounds with more than one group attached to the benzene ring Numbering the ring Any group already attached to the ring is given the number 1 position. Where you draw it on the ring (at the top or in any other position) doesn't matter - that's just a question of rotating the molecule a bit. It's much easier, though, to get in the habit of drawing your main group at the top. The other ring positions are then numbered from 2 to 6. You can number them either clockwise or anti-clockwise. As with chain compounds, you number the ring so that the name you end up with has the smallest possible numbers in it. Examples will make this clear. Some simple examples Substituting chlorine atoms on the ring Look at these compounds: All of these are based on methylbenzene and so the methyl group is given the number 1 position on the ring. Why is it 2-chloromethylbenzene rather than 6-chloromethylbenzene? The ring is numbered clockwise in this case because that produces a 2- in the name rather than a 6-. 2 is smaller than 6