AROMATICITY AND RELATED
UG & PG
1Dr. Meenal Gupta, Vikram University, Ujjain
Benzenoid and non-benzenoid aromatic compounds
• Aromatic compounds are those compounds which are planar, cyclic conjugated
and follow Huckel’s aromaticity rule (4n+2) π electrons where n = 0, 1, 2…….
• They give substitution reactions easily rather than addition reactions and retained
• They are stable due to resonance and have high resonance energy.
• They show ring current effect in NMR spectrum, chemical shift values obtained at
highly deshield area δ>6.0 ppm.
• Give specific feature in UV, IR and 13C-NMR (δ value: 145-170 ppm) spectroscopy
• Three absorption bands observed in UV-Visible spectroscopy:
• λmax = 184 nm(intense), 203.5 nm(K band), 254 nm(B band)
2Dr. Meenal Gupta, Vikram University, Ujjain
Resonating structures of benzene
According to Huckel’s rule (4n+2π) e: n= 1 6 π electrons in benzene, gives
nitration, Friedel craft reaction etc, easily.
First three are Dewar structures and after that are Kekule structures and last is resonance
hybrid. All carbon-carbon bond lengths are equal 1.39 Ao in between double and single
bond, indicates resonance of it .
3Dr. Meenal Gupta, Vikram University, Ujjain
Distribution of pi-molecular orbital in benzene can be shown as:
According to Huckel’s rule (4n+2π) electrons: n = 2 10 π electrons, ring is
cyclic, coplanar, having conjugated pi- electrons and stabilized by resonance, shows
ring current effect, therefore aromatic.
4Dr. Meenal Gupta, Vikram University, Ujjain
Possibility of easy aromatization in dihydro derivatives - dihydropyridine is easily
oxidized to more stable pyridine.
5Dr. Meenal Gupta, Vikram University, Ujjain
Classification on basis of Aromaticity
6Dr. Meenal Gupta, Vikram University, Ujjain
 annulene is aromatic, monocyclic
ring is cyclic coplanar having conjugated double
bond which delocalized and follows Huckel's rule
It is cyclic, pi-bonds are conjugated and follows
Huckel's rule having 10 pi electrons but due to
junction of trans hydrogens having non-bonded
repulsion more therefore it is highly unstable, hence
it it not aromatic.
When interacting hydrogens of the ring is replaced by bridging
methylene(-CH2) group, having 10 pi-electrons and shows
resonance,highly stable and it is aromatic.
7Dr. Meenal Gupta, Vikram University, Ujjain
Tropone is aromatic due to electron withdrawing
nature of carbonyl group and system will be
conjugated with positive charge of carbonyl
carbon with 6 pi-electrons.
O O O
Resonating structure of tropone
It is cyclic coplanar having 4-pi electrons in the ring and it is not conjugated because
one carbon is SP3 hybridized therefore it is nonaromatic.
But it behaves as acid and form salt with reaction of base to form potassium salt of
cyclopentadienide which is aromatic and negative charge is involved in resonance
due to conjugation.
Follows Huckel’s rule of aromaticity where 4n+2 pi electrons (n=1 has 6 pi electrons).
8Dr. Meenal Gupta, Vikram University, Ujjain
Distribution of pi-electron density can be shown by resonating structure of pyridine
It is cyclic, coplanar, due to 6π electrons are conjugated in pyridine therefore it is
aromatic like benzene.
The lone pair of electron is free not involve in conjugation therefore can be easily
donate to an acid, hence it acts as Lewis base and form pyridinium salt, retained their
9Dr. Meenal Gupta, Vikram University, Ujjain
Resonating structures of five membered heterocycles
Five membered rings are coplanar, cyclic having 4π electrons but due to presence of lone
pair of electron on hetero atom of the ring which is conjugated with pi- bonds, they are
Lone pair is donated to the ring and showing resonance, follow Huckel’s aromaticity rule,
where total 6 π- electrons are involved in resonance.
X X X X X
Resonating structures in five membered aromatic heterocycles
(X=NH, O or S) Pyrrole,furan or thiophene, respectively
Like benzene they give electrophilic substitution reactions easily.
Benzofused heterocycles like –Benzofuran, benzopyrrole Indole are also aromatic
10Dr. Meenal Gupta, Vikram University, Ujjain
A monocyclic ring having conjugated double bonds is alled annulene.
Simplest annulene is Benzene; named as  annulene and 1,3,5,7-cyclooctatetraene
named as annulene respectively.
The number of pi-electrons is shown as prefix in square bracket and then written
For aromaticity , annulene must obey Huckel’s (4n+2) pi electrons where n=o.,1,2……along
with they must coplanar, conjugated system,
showing ring current effect in nmr spectrum and show resonance. They are highly stable.
Annulenes may be antiaromatic, they follow Huckel’s 4nPi electrons rule where n = 1,2,3,
….. and ring must be coplanar and conjugated.
They are highly unstable.
As shown in the following diagram, 1,3,5,7,9-cyclodecapentaene fails to adopt a planar
conformation, either in the all cis-configuration or
in its 1,5-trans-isomeric form. The transannular hydrogen crowding that destabilizes
the latter may be eliminated by replacing the interior
hydrogens with a bond or a short bridge (colored magenta in the diagram).
As expected, the resulting 10 π-electron annulene derivatives
exhibit aromatic stability and reactivity as well as characteristic ring current anisotropy
in the nmr.
11Dr. Meenal Gupta, Vikram University, Ujjain
Naphthalene and azulene are annulene analogs stabilized by a transannular bond.
the CH2 bridged structure to the right of naphthalene in the diagram is not exactly planar,
the conjugated 10 π-electron ring is sufficiently close to planarity to achieve aromatic
stabilization. The bridged annulene compound on the far right, also has aromatic
12Dr. Meenal Gupta, Vikram University, Ujjain
Conjugated ring systems having 4n π-electrons (e.g. 4, 8, 12 etc. electrons) not only fail to show any aromatic properties, but appear to be
stable and more reactive than expected. As noted above, 1,3,5,7-cyclooctatetraene is non-planar and adopts a tub-shaped conformation.
The compound is readily prepared, and undergoes addition reactions typical of alkenes. Catalytic hydrogenation of this tetraene produces
cyclooctane. Planar bridged annulenes having 4n π-electrons have proven to be relatively unstable. Examples of 8 and 12-π-electron
are shown below, together with a similar 10 π-electron aromatic compound.
Planar (antiaromatic) anti-aromatic aromatic anti-aromatic Non-planar (non-aromatic)
The simple C8H6 hydrocarbon pentalene does not exist as a stable compound, and its hexaphenyl derivative is air sensitive.
analog heptalene has been prepared, but is also extremely reactive (more so than cyclooctatetraene). On the other hand,
azulene is a stable
10-π-electron hydrocarbon that incorporates structural features of both pentalene and heptalene. Azulene is a stable blue
crystalline solid that
undergoes a number of typical aromatic substitution reactions. The unexpected instability of 4n π-electron annulenes has
Other examples may be cited. Thus, all attempts to isolate 1,3-cyclobutadiene have yielded its dimer, or products from
other compounds introduced into the reaction system. Similarly, cyclopentadienyl cation (4 π-electrons) and cycloheptatrienyl
anion (8 π-electrons)
show very high reactivity when forced to form. 13Dr. Meenal Gupta, Vikram University, Ujjain
Carbanions and carbocations may also show aromatic stabilization. Some
The three-membered ring cation has 2 π-electrons and is surprisingly stable,
considering its ring strain.
Cyclopentadiene is as acidic as ethanol, reflecting the stability of its 6 π-electron
Salts of cycloheptatrienyl cation (tropylium ion) are stable in water solution, again
the stability of this 6 π-electron cation.
14Dr. Meenal Gupta, Vikram University, Ujjain
1. To select order of basic strength in pyrrole, pyridine and piperidine.
2. Account for aromaticity observed in following:
(i) 1,3-cyclopentadienyl anion but not 1,3-cyclopentadiene.
(ii) 1,3,5,-cycloheptatrienyl cation but not for 1,3,5-cycloheptatriene.
(iii) Cyclophenyl cation.
(iv) The heterocycles pyrrole, furan and pyridine.
3. Why cyclooctatetraene decolorizes dil KMnO4 and Br2 in CCl4 both.
4. Deduce structure when a red compound formed by reaction of 2 mol of AgBF4 with 1
5. A stable compound from the reaction of 2 mol of K with 1 mol of 1,3,5,7-cycloocta-
tetraene with no liberation of H2.
6. Account that  annulene is aromatic but  and  annulenes are not.
7. Draw Pi molecular energy orbitals of cyclooctatetraene and cyclo butadiene and
8. The obs. Heat of combustion of benzene is -3301.5 KJ/mol. Experimentally obtained
theoretical value from other compound are (Kj/mol) C=C (-491.5); C-C (-206.5); C-H (-
By use these data to calculate heat of combustion for benzene and the difference between
this and experimental value.
9. Exp. Determined heat of combustion of cyclooctatetraene is -4580 KJ/mol, use above
calculate the resonance energy of it. 15Dr. Meenal Gupta, Vikram University, Ujjain