Download Aromatic Compounds Early in the history of organic chemistry (late

Aromatic Compounds
Early in the history of organic chemistry (late 18th, early 19th century) chemists discovered a class of compounds which were unusually stable
A number of these compounds had a distinct odor
Hence these compounds were called “aromatic”
Today the term aromatic is used regardless of the odor of the compound
Some “aromatic” compounds have little to no odor
Benzene
The parent aromatic compound was discovered
to have a molecular formula of C6H6
This 1:1 ratio of carbon to hydrogen is extremely low compared to other known compounds
It was also quickly discovered that these aromatic compounds did not react like other alkene compounds
Structure
Before NMR and other spectroscopic tools it was hard to determine the structure of organic compounds
Ultimately the symmetry of the molecule revealed its structure
All carbon atoms, and all carbon-carbon bonds, are symmetrically equivalent
To account for these observations the proposed structure consisted of a cyclic compound stabilized by resonance
Each resonance structure is equal in energy and thus each contributes equally to the overall structure
Stability
The resonance structures imply an extra stability, but the amount of stability in benzene is much more than a typical resonance structure
Consider reactivity:
RCO3H
RCO3H
RCO3H
O
O
No reaction
The same reactivity behavior is observed for almost any alkene reaction
Aromatic Compounds Have More Stability than Conjugated Alkenes
Can measure stability by hydrogenation
H2
catalyst
The energy required for this hydrogenation indicates the stability of the alkene
2 Kcal/mol
Conjugation stability
55.4 Kcal/mol
57.4 Kcal/mol
Almost double in energy
49.8 Kcal/mol
28.6 Kcal/mol
How much energy should be in the hydrogenation of Benzene?
Have three double bonds in conjugation, so therefore should expect ~79 Kcal/mol
(~24 Kcal/mol more than 55 Kcal/mol for 1,3-cyclohexadiene)
Benzene is ~ 30 Kcal/mol more stable than predicted!!
Aromatic Stabilization
This ~30 Kcal/mol stabilization is called “aromatic stabilization”
It is the cause of the difference in reactivity between normal alkenes
It would cost ~30 Kcal/mol to break the aromaticity and thus the normal alkene reactions do not occur with benzene
Somehow having these three double bonds in resonance in a cyclic system offers a tremendous amount of energy
Cyclic system alone, however, is not sufficient for aromatic stabilization
Consider a four membered ring
Cyclobutadiene also has a ring structure with conjugated double bonds
This compound however is highly reactive and does not exist with equivalent single and double bonds
In solution it reacts with itself in a Diels-Alder reaction
Why the Difference in Stability?
Can already see in electron density maps that cyclobutadiene is not symmetric
Benzene
6-fold symmetry
Cyclobutadiene
Not symmetric
Consider the Molecular Orbitals for Benzene
For benzene there are 6 atomic p orbitals in conjugation therefore there will be 6 MO’s
As the number of nodes increase, the energy increases
For lowest energy MO there are zero nodes, therefore bonding interactions between each carbon-carbon bond
Benzene model
Top view with orbitals
Side view
Entire MO Picture for Benzene
6 nodes
4 nodes
4 nodes
E
2 nodes
2 nodes
Zero nodes
Notice all electrons are in bonding MO’s
All the antibonding MO’s are unfilled
With a cyclic system we obtain degenerate orbitals
(orbitals of the same energy)
Overall this electronic configuration is much more stable than the open chain analog
This is now the definition of an aromatic compound (not aroma),
Flat conjugated cyclic system is MORE stable than the open chain analog
Consider Cyclobutadiene
E
Unlike benzene, cyclobutadiene has two electrons at the nonbonding energy level
(these electrons do not stabilize the electronic structure)
Antiaromatic
Cyclobutadiene is less stable than butadiene
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If a cyclic conjugated system is less stable than the open chain analog
it is called antiaromatic
Part of the reason for cycobutadiene to be antiaromatic
is the presence of two MO’s at the nonbonding level
In butadiene all electrons are in bonding MO’s therefore the electrons are more stable in butadiene relative to cyclobutadiene
Hückel’s Rule
In order to determine if a system is aromatic or antiaromatic, without needing to determine the overall electronic energy of the closed form versus the open form, Hückel’s rule was developed
First the cyclic system must have a p orbital on all atoms in a continuous cyclic chain
(if there is an atom without a p orbital in the cycle then the system is nonaromatic)
In practice this means the cyclic system must be flat
(to allow overlap of p orbitals)
If these criteria are met then:
If the system has 4n+2 π electrons, it is aromatic
If the system has 4n π electrons, it is antiaromatic
Examples
6 π electrons, 4n+2 where n=1
Therefore aromatic
4 π electrons, 4n where n=1
Therefore antiaromatic
If there is an atom without a p orbital in the ring, the compound is nonaromatic
nonaromatic
Remember that the cyclic ring must have overlap of p orbitals to be considered aromatic or antiaromatic
Molecule adopts a non-flat low energy conformation top view
side view
Aromatic Ions
Benzene is a neutral aromatic compound
Any compound with 4n+2 electrons in a continuous loop is considered aromatic regardless of the number of carbons in the loop
There are many aromatic compounds with a different number of electrons than atoms in the loop
Due to this difference usually these compounds are ions, hence aromatic ions
Cyclopentadienyl Anion
Cyclopentadiene is nonaromatic since there is not a p orbital on one of the carbons in the ring
base
nonaromatic
pKa ~16
Upon removal of a proton, however, there is now a p orbital on each carbon
6 electrons in system, therefore according to Hückel this is aromatic
Due to this aromaticity cyclopentadienyl anion has unique properties
First, the compound is very acidic, pKa of ~16 compared to other allyl positions of ~45
Due to stability of anion once formed
Since it is aromatic it is more stable than the open chain anion analog
(pentadienyl anion)
It will still react with electrophiles in an SN2 reaction
According to Hückel’s rule, however, the carbocation should be antiaromatic
Cyclopentadienyl cation has only 4 electrons in the continuous loop of p orbitals,
therefore it is an antiaromatic compound
Since it is antiaromatic it will not form, too high in energy
Other Common Aromatic Ions
Any compound that will have 4n+2 electrons in a continuous loop
for planar conjugated compound will be favored due to aromatic nature
Nomenclature of Benzene Derivatives
The IUPAC name of 1,3,5-cyclohexatriene is never used
The common name of benzene dominates naming of these structures
In addition, another common naming tool for benzene derivates
is for disubstituted compounds (ortho, meta, para)
ortho-
dimethylbenzene
meta-
dimethylbenzene
para-
dimethylbenzene
Other naming follows rules already learned
Number along ring to give lowest number
First priority substituent is at the 1-position
Other common names
O
CH3
toluene
OH
phenol
OH
benzoic acid
If the benzene group is being considered as a substituent instead of as a root name, then it is given a phenyl prefix (reason for the word phenol)
Another common name is used for the substituted toluene
(called benzyl)
Heterocyclic Aromatic Compounds
Compounds that contain atoms besides carbon can also be aromatic
Need to have a continuous loop of orbital overlap and follow Hückel’s rule for the number of electrons in conjugation
Common noncarbon atoms to see in aromatic compounds include oxygen, nitrogen, and sulfur
Pyridine
One common aromatic compound with nitrogen is pyridine
N
One carbon atom of benzene has been replaced with nitrogen
Consider the placement of electrons
N
Lone pair is orthogonal to conjugated electrons in ring
The number of electrons in conjugation is 6
(don’t include lone pair that is orthogonal to ring)
therefore pyridine follows Hückel’s rule and is aromatic
Pyridine can be protonated in acidic conditions and it will still be aromatic, protonation occurs at lone pair
Pyrrole
A similar aromatic compound is pyrrole
H
N
N H
With pyrrole the lone pair is included in the conjugated ring
Have 6 electrons in loop and therefore this compound is aromatic
If protonated, however, pyrrole will become nonaromatic since the nitrogen would thus be sp3 hybridized without a p orbital for conjugation
Difference in electron placement affects properties
pyridine
pyrrole
Excess electron density of lone pair is localized orthogonal to ring in pyridine while the electron density is conjugated in ring with pyrrole
Some other common heterocyclic aromatic compounds
All of these compounds have 6 electrons conjugated in ring
Consider where the lone pair(s) are located for each heteroatom
O
S
furan
thiophene
N
N
pyrimidine
N
NH
imidazole
Fused Rings
Compounds with more than one fused ring can also be aromatic
The simplest two ring fused system is called naphthalene
Like benzene, naphthalene is an aromatic compound
with 10 electrons in a continuous ring around the cyclic system (one p orbital on each carbon is conjugated)
The reactivity of naphthalene is similar to benzene
It is unreactive toward normal alkene reactions because any addition would lower the aromatic stabilization
If it did react, however, there would still be one benzene ring intact
HBr
Br
Hypothetical reaction – does not occur
With larger fused ring systems normal alkene reactions start to occur
Anthracene
Br
Br2
Br
Two intact benzene rings
Reactions occur at central ring due to large aromatic stabilization remaining
NO2
NO2
Diels-Alder reactions can also occur about this central ring
Fused Heterocyclics
Fused ring systems with heterocyclics can also be aromatic
Extremely important compounds biologically and medicinally
Two of the four constituents of base pairs in DNA consist of fused aromatic rings, the other two bases, cytosine (C) and thymine (T), are one ring aromatic base pairs
Spectroscopy of Aromatic Compounds
We have already seen how aromatic benzene compounds have a relatively large downfield NMR shift due to aromatic ring current
Therefore any of these aromatic systems, which by definition have a ring current, have a large downfield shift
Can use as a characteristic of aromaticity
Mass Spectrometry
A characteristic peak in a MS for a benzenoid compound
is the presence of a peak at m/z 91 (if formation is possible)
Due to resonance stabilized benzyl cation