Aromatic Nomenclature
Naming substituted benzenes
Monosubstituted Benzenes
• Most monosubstituted aromatics are named using
-benzene as the parent name preceded by the
substituent name (as a prefix; all one word):
fluoro
nitro
ethyl
F
NO2
CH2CH3
fluorobenzene
nitrobenzene
ethylbenzene
Alkyl-substituted Benzenes
• Alkyl substituted benzenes are named according
to the length of the carbon chain of the alkyl group.
• With six carbons or fewer in the alkyl chain, they
are named as ‘alkylbenzene.’
• e.g., propylbenzene:
CH2CH2CH3
Alkyl-substituted Benzenes
• With more than six carbons in the alkyl chain, they
are named as a ‘phenylalkane,’ where the benzene
ring is named as a substituent (phenyl) on the
alkane chain
• e.g., 4-phenylnonane
CH 3CH 2CH 2
CH2CH2CH3
CHCH2CH2CH2CH2CH3
=
CHCH 2CH 2CH 2CH 2CH 3
CH3CH2CH2CHCH2CH2CH2CH2CH3
4-phenylnonane
Common Names of Subs. Benzenes
• There are a number of nonsystematic
(common) names commonly used for certain
monosubstituted benzenes (see next slide)
• These ten common names should be
memorized.
• These common names are used as base
names when naming more their more highly
substituted derivatives. Examples of these
will be given later.
Common Names of Subs. Benzenes
CH3
NH2
OH
toluene
OCH3
O
C
C
O
anisole
C
H
benzaldehyde
benzonitrile
CH2
styrene
O
C
OH
benzoic acid
C N
aniline
H
phenol
CH3
CHCH3
cumene
CH3
acetophenone
The Benzyl Group
• The benzyl group is a common name for a methyl
substituted benzene (toluene) having substitution
for one of the hydrogens on the methyl group.
CH2
the benzyl group
CH 2Br
CH2Br
CH2OH
benzyl bromide
benzyl alcohol
Disubstituted Benzenes
• Disubstituted benzenes can be named in one of two
ways. Each method describes the relative positions
of the two groups on the benzene ring.
• Systematic numbering of the aromatic ring.
• Using the prefixes ortho-, meta-, or para-.
• When numbering the ring carbons, carbon # 1 is
always a substituted carbon.
• The substituents are listed alphabetically.
Disubstituted Benzenes
ortho- (abbreviated o- ) = 1,2-disubstituted
(two groups on adjacent carbons on the ring)
F
F
CH2CH3
Br
o-difluorobenzene
or
1,2-difluorobenzene
o-bromoethylbenzene
or
1-bromo-2-ethylbenzene
Disubstituted Benzenes
meta- (abbreviated m- ) = 1,3-disubstituted
(two groups having one unsubstituted carbon
between them)
Br
Br
m-dibromobenzene
or
1,3-dibromobenzene
Br
NO2
m-bromonitrobenzene
or
1-bromo-3-nitrobenzene
Disubstituted Benzenes
para- (abbreviated p- ) = 1,4-disubstituted
(two groups on opposite sides of the ring)
Br
Cl
Cl
p-dichlorobenzene
or
1,4-dichlorobenzene
Cl
p-bromochlorobenzene
or
1-bromo-4-chlorobenzene
Disubstituted Benzenes
• When one of the substituents changes the base
name, either o-, m-, and p- or numbers may be used
to indicate the position of the other substituent.
• Carbon # 1 is always the carbon bearing the
substituent that changes the base name.
Br
4
1
OH
3
1 2 Cl
2
NH2
p-bromoaniline
or
4-bromoaniline
o-chlorophenol
or
2-chlorophenol
Common Names of Disubs. Benzenes
• There are a few nonsystematic (common) names
for disubstituted benzenes that you should be
familiar with:
CH3
CH3
CH3
CH3
CH3
m-xylene
o-xylene
CH3
p-xylene
CH3
OH
o-cresol
CH3
CH3
OH
m-cresol
OH
p-cresol
Polysubstituted Benzenes
• Polysubstituted benzenes must be named by
numbering the position of each substituent on the
ring (with more than two substituents, o-, m-, and pcan NOT be used.)
• The numbering is carried out to give the substituents
the lowest possible numbers. Carbon #1 always has
a substituent.
CH2CH3
• List the substitutents
2
F 1
3
alphabetically with
4
their appropriate #s.
NO2
2-ethyl-1-fluoro-4-nitrobenzene
Polysubstituted Aromatics
having a Common base name
• Common names of the monosubstituted benzenes
are used as parent names for polysubstituted
aromatics when one of the substituents changes
the base name.
• For such rings with common names, the carbon
bearing the substituent responsible for the common
name is always carbon #1.
toluene
CH3
• The substitutents are
1
Cl chloro
listed in alphabetical
2
order.
bromo
Br 5
3
4
5-bromo-2-chlorotoluene
Polysubstituted Benzenes
Br
OH
4
1
1
3
2 CH2CH3
NO2
4-bromo-2-ethyl-1-nitrobenzene
Cl
2
Br 5
3
4
5-bromo-2-chlorophenol
Polysubstituted Benzenes
Br
2
3
O2N 4
Br
1 CH3
6
5
6 Cl
2-bromo-6-chloro-4-nitrotoluene
O2N 5
1 2 CH2CH3
3
4
Cl
1-bromo-3-chloro-2-ethyl-5-nitrobenzene
Polycyclic Aromatic
Hydrocarbons (PAH)
anthracene
naphthalene
phenanthrene
pyrene
benzo [a] pyrene
Metabolic byproducts of benzo [a] pyrene react with
DNA to form adducts, leading to carcinogenesis (cancer).