SCH 4U
Name: ____________________
Unit: Organic Chemistry
Date: ____________________
Aromatic Hydrocarbons
Alkanes, alkenes and alkynes (whether straight chains, branched chains or cyclic structures) are all hydrocarbons.
Together they are also classified as "aliphatic compounds". A fourth class of hydrocarbons exists. Instead of
belonging to the aliphatic group, however, substances in this group are called "aromatic" compounds. They are
easily recognized in the lab by their “aromatic” odour, which is often (but not always) sweet and enticing.
Aromatic compounds are mostly derivatives of benzene (C6H6) (cyclohexatri– 1,3,5 – ene)
The logical structure for benzene would be a hybrid (or combination) of the two resonance structures shown
below. We will return to the idea of resonance structures in Unit 2… For now…
…Suffice to say that it does not make chemical sense for the single and double bonds in benzene to shift back and
forth. Here is some evidence to support the notion that benzene must have a very stable 6 carbon ring structure:
1) Benzene undergoes substitution reactions rather than addition reactions (ie. instead of adding reactants
onto the ring directly by breaking π bonds between carbon atoms, the C – C “double bonds” in the ring are
found to remain intact and hydrogen atoms outside the ring are removed and substituted for by the
reactant(s)). (We’ll come back to this…The bottom line is that alkene and alkyne multiple bonds undergo
addition not substitution reactions. Benzene “double bonds” do the opposite – so they must not be your
standard multiple bonds...)
2) Because all of the carbon atoms in the benzene ring molecule react in the same way, all of the carbon –
carbon bonds must be chemically equivalent to each other. This is a bit weird too because single and
double bonds are typically different from one another in terms of bond length and chemical reactivity.
3) The carbon-carbon bonds in benzene are all of equivalent lengths. All are 139 pm long. This value is
intermediate between the length of a carbon-carbon single bond (148 pm) and a carbon-carbon double
bond (134 pm).
These characteristics have led scientists to conclude that the p orbitals involved in creating the π bonds between
the carbons overlap all around the benzene ring structure to give a continuous pi bond around the molecule. We
indicate this arrangement by placing a circle inside the centre of the structural formula for the benzene ring. The
condensed structural formula for benzene is a hexagon with a circle inside it. (see below)
Right now, we do not know much about “p orbitals”.
We will in the second unit. For now, it will suffice to
just accept the condensed structural formula for
benzene shown to the left. Alternatively, you will see
it drawn as one of the resonance structures shown
above with alternating double and single bonds.
NOMENCLATURE FOR SIMPLE AROMATIC COMPOUNDS
The following chart indicates that the IUPAC naming system for benzene derivatives is quite straight forward and
follows established rules. The ortho-, meta- and para- designations indicated are still used today and you should
be familiar with them. Similarly, you need to know the structures and names of toluene and xylene.
Condensed
Structural Formula
IUPAC Name
Methylbenzene
1, 2 – dimethylbenzene
1, 3 – dimethylbenzene
1, 4 – dimethylbenzene
Common Name
Toluene
ortho-xylene
meta-xylene
para-xylene
Condensed
Structural Formula
Br
Br
Br
Br
Br
Br
Br
IUPAC Name
Bromobenzene
1, 2 – dibromobenzene
1, 3 – dibromobenzene
1, 4 - dibromobenzene
Common Name
bromobenzene
ortho - dibromobenzene
meta- dibromobenzene
para - dibromobenzene
Other benzene derivatives you should be aware of:
phenol
aniline
benzoic acid
Note also that benzene can be an alkyl branch on a long or complicated carbon parent chain. When this
happens, the branch name is phenyl.
Practice
1. Draw structural formulas for each of the following aromatic compounds.
a) 2,4,6-trichlorotoluene
b) 1-fluoro-2-ethylbenzene
c) 1,3,5-tribromobenzene
d) para-ethyltoluene
e) 1, 2 – diethylbenzene
2. Give the correct IUPAC name for each of the following compounds. (When benzene is named as an alkyl branch
it has the name phenyl.)
a)
b)
F
More Practice
Page 21 #7 and 8.
c)