Whenever we talk about aromatic compounds, the first thing which comes in our mind is BENZENE. Yes ofcourse being a chemistry student, the first aromatic compund which we come across is benzene only. I still remember when i was in class 10th and my teacher taught us about structure of benzene I was so amused with the beauty of it's structure. I mean how can a molecule look so beautiful with conjugate double bonds and a cute hexagonal shape. Well I'm sure most of us must have thought the same way.
Okay so without wasting time lets come upon our topic of Aromaticity. Aromaticity was basically derived from the term Aroma [which means sweet smell], so initially it was considered that aromatic compounds are all those compounds having a pleasant smell.
For a chemist, an aromatic compound must have following properties:
- have an extremely high resonance energy (36 kcal/mol for benzene)
- undergo substitution rather than addition reactions
- have delocalized pi-electrons
So the Question arises how can we conclude whether a molecule is aromatic or not?
I've tried to make this simple for you,
here's the answer; For a molecule to be regarded as Aromatic, it must meet following 4 conditions. If any of these conditions are violated, no aromaticity is possible.
Condition 1: The Molecule Must Be Cyclic
It's very simple if there's a Ring move to condition 2. If there’s no ring, forget it.
for e.g.: (
Z)-1,3,5 hexatriene has the same number of pi bonds (and pi electrons) as benzene, but isn’t aromatic. No ring, no aromaticity.
Condition 2: Every atom in the ring must be conjugated
In order for aromaticty to exist, there must also be a continuous
ring of p-orbitals around the ring that build up into a larger cyclic
“pi system”.
One way of saying this is that every atom around the ring must be capable of
conjugation with each other.
Remember
that the “available p orbital” condition applies not just to atoms that
are part of a pi bond, but also atoms bearing a lone pair, a radical,
or an empty p orbital (e.g. Carbocation)
The key thing that “kills” conjugation is an
sp3 hybridized atom with four bonds to atoms. Such an atom cannot participate in resonance.
Condition 3: The Molecule Must Have [4n+2] Pi Electrons {The Huckel Rule}
The third condition is that the cyclic, conjugated molecule must have [4n+2] pi electrons. Benzene and cyclooctatetraene are
both cyclic and conjugated, but benzene is aromatic and
cyclooctatetraene is not. The difference is that benzene has 6 pi
electrons, and cyclooctatetraene has 8.
However, this term [4n+2] causes a lot of confusion in organic chemistry. Many students stare at a molecule and try to figure out what “n” is.
“n” is not a property of the molecule!
“4n+2 is not a formula that you apply to see if your
molecule is aromatic. It is a formula that tells you what numbers are in
the magic series. If your pi electron value matches any number in this
series then you have the capacity for aromaticity.”
The “magic series” is: 2, 6, 10, 14, 18, 22….. (and so on).
[4n+2] is just a mathematical shorthand for writing out the series [2, 6, 10, 14, 18, 22…] .
The numbers in this “magic series” are sometimes referred to as
“Hückel Numbers” after Erich Hückel, who proposed this rule back in
1931.
The condition that aromatic molecules must have [4n+2] pi electrons is sometimes called “Hückel’s rule”.
In the figure below, molecules which fulfil Hückel’s rule are in green; those which do not fulfill Hückel’s rule are in red.
Note that we can count electrons in pi bonds as well as electrons
from lone pairs (so long as the carbon isn’t already participating in a
pi bond). So the cyclopentadiene anion has six pi electrons –
4 from the two double bonds, and two from the lone pair on carbon.
Here Question Arises-Which Electrons Count As “Pi Electrons”?
That seems easy. However, complications may arise
when we have atoms in the ring which both participate in pi bonding and
also have a lone pair. For example, pyridine, pyrrole etc.
Some Examples With 5-Membered Rings
Some molecules with five-membered rings can also present ambiguities.
The cyclopentadiene anion (below) has a lone pair on one of the carbons. Can this lone pair contribute to the pi system?
Since that carbon is not involved in any pi-bonding, the answer is
yes.
The total number of pi electrons for the cyclopentadiene anion equals
2 (from the lone pair) plus the 4 electrons in the two pi bonds, giving
us a total of 6. This is a Hückel number and the cyclopentadiene anion
is in fact aromatic.
A similar situation arises for
pyrrole.
The nitrogen bears a lone pair but is not involved in a pi bond
(unlike pyridine, above). Therefore it can contribute to the pi system
and this gives us a total of 6 pi electrons once we account for the 4
electrons from the two pi bonds.
A curious case is
furan, where the oxygen bears
two lone pairs. Does this mean that furan has 8 pi electrons?
No!
Why not? Because as we noted above, each atom can contribute a
maximum
of one p-orbital towards the pi system. In furan, one lone pair is in a
p orbital, contributing to the pi system; the other is in the plane of
the ring. This gives us a total of 6 pi electrons. Furan is aromatic.
(So is thiophene, the sulfur analog of furan).
Finally there is
imidazole, which has two nitrogens. One nitrogen (the N-H)
is not involved in a pi bond, and thus can contribute a full lone pair; the other
is
involved in a pi bond, and the lone pair is in the plane of the ring.
This also gives us a total of 6 pi electrons once we account for the two
pi bonds.
Condition 4: The Molecule Must Be Flat
The fourth condition for aromaticity is that the molecule must be planar.
As with certain vertebrates, the only thing that preventing a
molecule that fulfills the first three conditions from being flat is if
the flat conformation is incredibly strained.
One example in this
category is the molecule known as [10]-annulene, an isomer of which is
drawn below left. In the
trans,
cis,
trans, cis, cis
isomer, the molecule is cyclic, conjugated, and has 10 pi electrons,
but the two marked hydrogens bump into one another when attempting to
adopt a flat conformation.
The molecule is prevented from adopting planarity due to this punitive Van Der Waals strain , and is therefore
not aromatic.
Interestingly, if the hydrogens are removed and replaced with a bridging CH
2
group, the strain is relieved and the pi bonds can adopt a planar
conformation. The molecule below right shows the expected properties of
an aromatic molecule.
References:
- Wikipedia
- Google image search
- masterorganic chemistry
You may also like:
Questions Asked about Aromaticity in CSIR in Last 10 years
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