4/12/2011
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
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 p electrons, where n is an integer (n = 0, 1,
2, 3,…)
Only molecules with 2, 6, 10, 14, 18,… p electrons can
be aromatic
Molecules with 4n p electrons (4, 8, 12, 16,…) can not
be aromatic, said to be antiaromatic because
delocalization of their p electrons would lead to their
destabilization
Aromaticity and the Hückel 4n + 2 Rule
Examples of the Hückel 4n + 2 rule
• Cyclobutadiene
• Contains four p 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
1
4/12/2011
Aromaticity and the Hückel 4n + 2 Rule
• Benzene
•
•
Contains six p electrons (4n + 2 = 6 when n = 1)
Aromatic
Aromaticity and the Hückel 4n + 2 Rule
• Cyclooctatetraene
• Contains eight p electrons
• The p 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
2
4/12/2011
Aromaticity and the Hückel 4n + 2 Rule
• Energy levels of the six benzene p molecular orbitals
• The lowest-energy MO, y1, occurs single and contains a pair
of electrons
• y2 and y3, are degenerate, and it takes two pairs of electrons
to fill them
• The result is a stable six-p-electron aromatic molecule with
filled bonding orbitals
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
3
4/12/2011
Aromatic Ions and Aromatic Heterocycles
4n + 2 rule predicts cyclopentadienyl anion and
cycloheptatrienyl cation to be aromatic
Aromatic Ions and Aromatic Heterocycles
Aromatic cyclopentadienyl anion, showing the cyclic conjugation and six
p electrons in five p orbitals
Aromatic cycloheptatrienyl cation, showing the cyclic conjugation and
six p 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 p 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 p electron
4
4/12/2011
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 p
electrons
•
The nitrogen lone pair electrons are in an sp2 orbital in
the plane of the ring and are not involved with the
aromatic p system
Aromatic Ions and Aromatic Heterocycles
• Pyrimidine is much like benzene in its p 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 p system
Aromatic Ions and Aromatic Heterocycles
• Pyrrole is a five membered heterocycle with six p electrons
• Aromatic
• Each of the sp2-hybridized carbons contributes one p
electron
• The sp2-hybridized nitrogen atom contributes the two
electrons from its lone pair, which occupies a p orbital
5
4/12/2011
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 p system
The other nitrogen is not in a double bond and contributes
two from its lone pair
Aromatic Ions and Aromatic Heterocycles
Nitrogen atoms have different roles depending
on the structure of the molecule
• In pyridine and pyrimidine, the nitrogen atoms are
both in double bonds and contribute only one p
electron to the aromatic sextet, like a carbon atom
in benzene does
• In pyrrole, the nitrogen atom is not in a double
bond and contributes two p electrons (the lone
pair) to the aromatic sextet
• In imidazole, both a double-bonded “pyridine-like”
nitrogen that contributes one p electron and a
“pyrrole-like” nitrogen that contributes two p
electrons are present in the same molecule
Aromatic Ions and Aromatic Heterocycles
Pyrimidine and imidazole rings are important in biological
chemistry
•
Pyrimidine is the parent ring system in cytosine, thymine, and
uracil, three of the five heterocycle amine bases found in
nucleic acids
• An aromatic imidazole ring is present in histidine, one of the
twenty amino acids found in proteins
6
4/12/2011
Worked Example 9.1
Accounting for the Aromaticity of a Heterocycle
Thiophene, a sulfur-containing heterocycle, undergoes
typical aromatic substitution reaction rather than addition
reactions. Why is thiophene aromatic?
7