11/17/2010
John E. McMurry
http://www.cengage.com/chemistry/mcmurry
Chapter 9
Aromatic Compounds
Richard Morrison • University of Georgia, Athens
Early Days of Organic Chemistry
Aromatic Compounds
• Formerly used to describe fragrant substances such as
benzaldehyde (from cherries, peaches, and almonds),
toluene (from Tolu balsam), and benzene (from coal
distillate)
• Now used to refer to the class of compounds that contain
six-membered benzene-like rings with three double bonds
Present Days of Organic Chemistry
Aromatic Compounds
•
•
•
Many naturally occurring compounds are aromatic in part
• Steroidal hormone estrone
• Analgesic morphine
Many synthetic drugs are aromatic in part
• Antidepressant fluoxetine (Prozac)
Benzene
• Found to cause bone marrow depression
• Leads to leukopenia, or lowered white blood cell count, on
prolonged exposure
1
11/17/2010
9.1 Naming Aromatic Compounds
Aromatic substances have acquired nonsystematic names
•
Nonsystematic names are discouraged but allowed by IUPAC
•
Common name for methylbenzene is toluene
Common name for hydroxybenzene is phenol
•
Common name for aminobenzene is aniline
•
Naming Aromatic Compounds
Naming Aromatic Compounds
Monosubstituted Benzenes
•
Systematically named in same manner as other hydrocarbons
• – benzene used as parent name
•
•
•
C6H5Br is bromobenzene
C6H5NO2 is nitrobenzene
C6H5CH2CH2CH3 is propylbenzene
2
11/17/2010
Naming Aromatic Compounds
Arenes
• Alkyl-substituted benzenes
• Named depending on the size of the alkyl group
• Alkyl substituent smaller than the ring (6 or fewer carbons),
named as an alkyl substituted benzene
• Alkyl substituent larger than the ring (7 or more carbons),
named as a phenyl-substituted alkane
Phenyl
• Derived from the Greek pheno (“I bear light”)
• Michael Faraday discovered benzene in 1825 from the oily
residue left by illuminating gas used in London street lamps
• Used for the –C6H5 unit when the benzene ring is
considered as a substituent
(Greek phi)
• Abbreviated as Ph or
Naming Aromatic Compounds
Benzyl
• Used for the C6H5CH2– group
Naming Aromatic Compounds
Disubstituted benzenes
•
Named using one of the prefixes
ortho- (o-)
1.
•
2.
meta- (m-)
•
3.
Ortho-disubstituted benzene has
two substituents in a 1,2
relationship
Meta-disubstituted benzene has
its substituents in a 1,3
relationship
para- (p-)
•
Para-disubstituted benzene has
its substituents in a 1,4
relationship
3
11/17/2010
Naming Aromatic Compounds
Benzenes with more than two substituents
•
•
Named by numbering the position of each so that the lowest
possible numbers are used
The substituents are listed alphabetically when writing the name
Any of the monosubstituted aromatic compounds in Table 8.1
can serve as a parent name, with the principal substituent
(-OH in phenol or –CH3 in toluene) attached to C1 on the ring
9.2 Structure and Stability of Benzene
Benzene
• Benzene is unsaturated
• Benzene is much less reactive than typical alkene and
fails to undergo the usual alkene reactions
•
•
Cyclohexene reacts rapidly with Br2 and gives the addition
product 1,2-dibromocyclohexane
Benzene reacts only slowly with Br2 and gives the
substitution product C6H5Br
Structure and Stability of Benzene
A quantitative idea of benzene’s stability is obtained from
heats of hydrogenation
• Benzene is 150 kJ/mol (36 kcal/mol) more stable than
might be expected for “cyclohexatriene”
4
11/17/2010
Structure and Stability of Benzene
Carbon-carbon bond lengths and angles in benzene
•
•
All carbon-carbon bonds are 139 pm in length
• Intermediate between typical C-C single bond (154 pm) and typical
double bond (134 pm)
• Electrostatic potential map shows that the electron density in all six
carbon-carbon bonds is identical
Benzene is planar
• All C-C-C bond angles are 120
• All six carbon atoms are sp2-hybridized with p orbital perpendicular
to the plane of the ring
Structure and Stability of Benzene
All six carbon atoms and all six p orbitals in benzene are
equivalent
• Each p orbital overlaps equally well with both neighboring p
orbitals, leading to a picture of benzene in which the six
electrons are completely delocalized around the ring
• Benzene is a hybrid of two equivalent resonance forms
•
•
Neither form is correct by itself
The true structure of benzene is somewhere in between the
two resonance forms
Structure and Stability of Benzene
Six p atomic orbitals combine in a cyclic manner, six
benzene molecular orbitals result
•
The three lower-energy molecular orbitals, denoted
3, are bonding combinations
•
2
and
3
1,
2, and
have the same energy and are said to be degenerate
5
11/17/2010
Structure and Stability of Benzene
• The three higher-energy molecular orbitals, denoted
5
•
, and
6
4
,
, are antibonding combinations
4 and 5 have the same energy and are said to be
degenerate
Structure and Stability of Benzene
•
3 and 4 have nodes passing through ring carbon atoms,
therefore no electron density on these carbons
• The six p electrons occupy the three bonding molecular orbitals
and are delocalized over the entire conjugated system
9.3 Aromaticity and the Hückel 4n + 2 Rule
Benzene and other benzene-like aromatic molecules
share similar characteristics:
• Benzene is cyclic and conjugated
• Benzene is unusually stable, it is 150 kJ/mol (36 kcal/mol)
more stable than might be expected
• Benzene is planar and has the shape of a regular hexagon.
All bond angles are 120º, all carbon atoms are sp2hybridized, and all carbon-carbon bond lengths are 139 pm
• Benzene undergoes substitution reactions that retain the
cyclic conjugation rather than electrophilic addition reactions
that would destroy the conjugation
• Benzene is a resonance hybrid whose structure is
intermediate between two line-bond structures
6
11/17/2010
Aromaticity and the Hückel 4n + 2 Rule
The Hückel 4n + 2 rule
• Theory devised in 1931 by the German physicist
Erich Hückel
•
•
•
A molecule is aromatic only if it has a planar,
monocyclic system of conjugation and contains a total
of 4n + 2 electrons, where n is an integer (n = 0, 1,
2, 3,…)
Only molecules with 2, 6, 10, 14, 18,… electrons can
be aromatic
Molecules with 4n electrons (4, 8, 12, 16,…) can not
be aromatic, said to be antiaromatic because
delocalization of their electrons would lead to their
destabilization
Aromaticity and the Hückel 4n + 2 Rule
Examples of the Hückel 4n + 2 rule
• Cyclobutadiene
• Contains four electrons localized into two double bonds
rather than delocalized around the ring
• Antiaromatic
• Highly reactive
• Shows none of the properties associated with aromaticity
• Not prepared until 1965
Aromaticity and the Hückel 4n + 2 Rule
• Benzene
•
•
Contains six
Aromatic
electrons (4n + 2 = 6 when n = 1)
7
11/17/2010
Aromaticity and the Hückel 4n + 2 Rule
• Cyclooctatetraene
• Contains eight
electrons
The electrons are localized
onto four double bonds rather
than delocalized around the
ring
Not aromatic
•
•
•
•
•
The molecule is tub-shaped
rather than planar
It has no cyclic conjugation
because neighboring p orbitals
do not have the necessary
parallel alignment for overlap
Resembles an open-chain
polyene in its reactivity
Aromaticity and the Hückel 4n + 2 Rule
Energy Levels of Cyclic Conjugated Molecules (4n + 2
Electrons)
• There is always a single lowest-lying MO, above which the
MOs come in degenerate pairs
• When electrons fill the various molecular orbitals, one pair
of electrons fills the lowest-lying orbital and two pairs of
electrons fill each of the n successive energy levels – a total
of 4n + 2. Any other number would leave a bonding energy
level partially unfilled
Aromaticity and the Hückel 4n + 2 Rule
• Energy levels of the six benzene molecular orbitals
• The lowest-energy MO, 1, occurs single and contains a pair
of electrons
•
2 and 3, are degenerate, and it takes two pairs of electrons
to fill them
• The result is a stable six- -electron aromatic molecule with
filled bonding orbitals
8
11/17/2010
9.4
Aromatic Ions and Aromatic Heterocycles
Ions and heterocyclic compounds can also be aromatic
Aromatic Ions and Aromatic Heterocycles
Aromatic Ions
• There are three ways in which the hydrogen might be
removed from cyclopenta-1,3-diene and cyclohepta-1,3,5triene
•
•
•
The hydrogen can be removed with both electrons (H:-)
leaving a carbocation
The hydrogen can be removed with one electron (H.)
leaving a carbon radical
The hydrogen can be removed with no electrons (H+)
leaving a carbon anion, or carbanion
Aromatic Ions and Aromatic Heterocycles
4n + 2 rule predicts cyclopentadienyl anion and
cycloheptatrienyl cation to be aromatic
9
11/17/2010
Aromatic Ions and Aromatic Heterocycles
Aromatic cyclopentadienyl anion, showing the cyclic conjugation and six
electrons in five p orbitals
Aromatic cycloheptatrienyl cation, showing the cyclic conjugation and
six electrons in seven p orbitals
a)
b)
Aromatic Ions and Aromatic Heterocycles
Aromatic Heterocycles
• A cyclic compound that contains atoms of two or more
different elements in its ring, usually carbon along with
nitrogen, oxygen, or sulfur
• Pyridine is much like benzene in its
electron structure
•
•
A six-membered heterocycle with nitrogen in its ring
Each of the five sp2-hybridized carbons has a p orbital
perpendicular to the plane of the ring and each p orbital
contains one electron
Aromatic Ions and Aromatic Heterocycles
•
The nitrogen atom is also sp2-hybridized and has one
electron in a p orbital, bringing the total to six
electrons
•
The nitrogen lone pair electrons are in an sp2 orbital in
the plane of the ring and are not involved with the
aromatic system
10
11/17/2010
Aromatic Ions and Aromatic Heterocycles
• Pyrimidine is much like benzene in its
electron structure
• Has two nitrogen atoms in a six-membered, unsaturated ring
2
• Both nitrogens are sp -hybridized, and each contributes
one electron to the aromatic system
Aromatic Ions and Aromatic Heterocycles
• Pyrrole is a five membered heterocycle with six electrons
• Aromatic
• Each of the sp2-hybridized carbons contributes one
electron
• The sp2-hybridized nitrogen atom contributes the two
electrons from its lone pair, which occupies a p orbital
Aromatic Ions and Aromatic Heterocycles
• Imidazole is an analog of pyrrole that has two nitrogen
atoms in a five-membered, unsaturated ring
•
Both nitrogens are sp2-hybridized
•
•
One nitrogen is in a double bond and contributes only one
electron to the aromatic system
The other nitrogen is not in a double bond and contributes
two from its lone pair
11