6.1.1 (c) Naming Aromatic Compunds
Syllabus
(c) use of IUPAC rules of nomenclature for systematically naming substituted aromatic compounds
{Use of locant numbers to identify positions of substitution e.g. 2,4-dinitromethylbenzene.}
What does this mean?
The Naming of Aromatic Compounds is a confusing matter.
Most students quickly come to recognise a Benzene ring.
Most then accept that Phenyl is the prefix that means "contains a Benzene ring" - even though it sounds like Phenol which is not the same thing at all.
The problem is that some compounds containing Benzene rings contain the name Benzene as a suffix.
This is similar to the problem of naming alcohols where we usually name them ending with the suffix -ol, but sometimes name them with the prefix Hydroxy-.
Benzene or Phenyl?
This is clearly an alcohol.
It has no other functional groups.
So we would use the suffix -ol.
Hence, it is ethanol.
This is also an alcohol.
But it contains a Nitrile functional group as well.
We're not taught a way to name Nitriles as side-chains so we use the prefix Hydroxy to name the alcohol as a sidechain instead.
Hence we could call it Hydroxymethanenitrile.
This is Benzene.
If we add functional groups that are usually suffixes then they continue to do so and the new compound will end with Benzene.
So these compounds are Bromobenzene and Chlorobenzene respectively.
But this Benzene ring is bonded to an Amine group.
Amine is (generally) used as a suffix.
So we can't also use Benzene as a suffix.
We're left with using Phenyl as a prefix.
So it is Phenylamine.
Without a way to name an NO2 group as a suffix we have to use Nitro as a prefix.
So this is Nitrobenzene.
This compound contains a Benzene ring bonded to Ethene.
We're not taught a suffix to name Ethene as a side-chain.
So we will have to end the name with Ethene and start with Phenyl.
Hence it is Phenylethene.
This compound is an important exception.
It might have been Hydroxybenzene or Phenylol.
Both would suggest it was the alcohol that it looks like - although it doesn't always behave like an alcohol.
Actually, we simplify the second option (because the OH is the main functional group & so should be a suffix) so that it is simply Phenol
The only significant exception to the above is Benzoic Acid.
which we might reasonably expect to start with the Phenyl prefix since the -oic acid part is a suffix.
Go figure!
Numbering.
If an aromatic compound contains more than one functional group then the suffix is automatically assumed to be on Carbon 1.
No numbering is required for methylbenzene since the Methyl group would be one C1 wherever we chose to draw it.
But for our trinitromethylbenzene we have to say where the Nitrogroups are.
Counting clockwise or anticlockwise gives us 2,4,6-trinitromethylbenzene.
And since all these are Chloromethylbenzenes we simply count to the lowest numbers for the Chloro groups, assuming that C1 is where the methyl group is found.
And given the choice between 2,4-dchloromethylbenzene and 3,6-dchloromethylbenzene we have to take the 2,4 option.