1 | CHMSC CRIM
NAT SCI 11-GENERAL CHEMISTRY
Organic Chemistry Chapter 4
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 compoundsfollow
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.
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.
Background
The benzene ring
All aromatic compounds are based on benzene, C6H6, which has a ring
of six carbon atoms and has the symbol:
The simplified formula for this is C6H5NO2.
methylbenzene
Each corner of the hexagon has a carbon atom with a hydrogen
attached.
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.
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.
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.
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.
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
2 | CHMSC CRIM
NAT SCI 11-GENERAL CHEMISTRY
Organic Chemistry Chapter 4
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.
phenyl ethanoate
Cases where the name is based on phenyl
This is an ester based on ethanoic acid. The hydrogen atom in the COOH group has been replaced by a phenyl group.
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.
Note: If you aren't happy about naming esters, follow this link
before you go on.
phenol
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.
Phenol has an -OH group attached to a benzene ring and so has a
formula C6H5OH.
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 yoursyllabus 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.
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
phenylethanone
Substituting chlorine atoms on the ring
This is a slightly awkward name - take it to pieces. It consists of a two
carbon chain with no carbon-carbon double bond. The oneending
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:
Look at these compounds:
3 | CHMSC CRIM
NAT SCI 11-GENERAL CHEMISTRY
Organic Chemistry Chapter 4
2,4,6-trichlorophenol
All of these are based on methylbenzene and so the methyl group is
given the number 1 position on the ring.
This is based on phenol - with an -OH group attached in the number 1
position on the ring. There are 3 chlorine atoms substituted onto the
ring in the 2, 4 and 6 positions.
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.
Warning! You will find all sorts of variations on this depending on
the age of the book you look it up in, and where it was published.
What I have described above isn't in strict accordance with the most
modern interpretation of the IUPAC recommendations for naming
organic compounds.
methyl 3-nitrobenzoate
The names should actually be 1-chloro-2-methylbenzene, 1-chloro3-methylbenzene, and so on. The substituted groups are named in
alphabetical order, and the "1" position is assigned to the first of
these - rather than to the more logical methyl group.
This is a name you might come across as a part of a practical exercise
in nitrating benzene rings. It's included partly for that reason, and partly
because it is a relatively complicated name to finish with!
This produces some silly inconsistencies. For example, if you had
the exactly equivalent compounds containing nitro groups in place of
the chlorines, the names would change completely, to 1-methyl-2nitrobenzene, 1-methyl-3-nitrobenzene, etc. In this case, the normal
practice of naming the hydrocarbon first, and then attaching other
things to it has been completely wrecked.
The structure of the name shows that it is an ester. You can tell that
from the oate ending, and the methyl group floating separately from
the rest of the name at the beginning.
Do you need to worry about this? NO! It is extremely unlikely that
you would ever be asked to name these in an exam, and it is always
easy to write a structure from one of these names - however illogical
it may be! There is a simple rule for exam purposes. Unless you are
specifically asked for the name of anything remotely complicated,
don't give it. As long as you have got the structure right, that's all
that matters.
There will be a benzene ring with a -COOH group in the number 1
position and a nitro group, NO2, in the 3 position. The -COOH group is
modified to make an ester by replacing the hydrogen of the -COOH
group by a methyl group.
2-hydroxybenzoic acid
This might also be called 2-hydroxybenzenecarboxylic acid. There is a
-COOH group attached to the ring and, because the name is based on
benzoic acid, that group is assigned the number 1 position. Next door
to it in the 2 position is a hydroxy group, -OH.
benzene-1,4-dicarboxylic acid
The di shows that there are two carboxylic acid groups, -COOH, one of
them in the 1 position and the other opposite it in the 4 position.
The ester is based on the acid, 3-nitrobenzoic acid - so start with that.
Methyl 3-nitrobenzoate is therefore: