Annulenes and Heteroannulenes

1. Annulenes and
Heteroannulenes
-PREMIE FERNANDES
St Aloysius College Autonomous Mangaluru

2. ANNULENES
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Monocyclic conjugated systems
Represented as structures having alternating single and
double bonds
General formula: (CH)2m
Ring size indicated by number in square brackets [x]
E.G.
Benzene = [6]annulene cyclooctatetraene = [8]annulene

3. AROMATICITY
(‘aroma’= fragrance)
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Cyclic
Planar
Uninterrupted cloud of π-electrons
Huckel’s Rule - (4n+2)π electrons
Before 1960s, Benzene and Cyclooctatetraene were
the only two annulenes which were available for
verifying the predictions of Huckel’s rule.
During 1960s,F. Sodheimer synthesized a number of
large ring annulenes and the predictions of Huckel’s
rule were verified.

4. 
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Aromatic annulenes- obeys all laws of aromaticity
Anti-Aromatic annulenes- doesn’t follow Huckel’s
Rule
Non-Aromatic annulenes- follow Huckel’s Rule but
not any one of the other laws
TYPES OF ANNULENES

5. [18] Annulene
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Cyclic
Conjugated
Follows Huckel Rule
(4n+2)=18
n=4
Planar
It is Aromatic.

6. [14] Annulene
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Cyclic
Conjugated
Follows Huckel Rule
(4n+2) = 14
n=3
X-ray studies have shown that
the molecule is non-planar due
to interference of hydrogen
atoms.
It is Aromatic.

7. Cyclobutadiene = [4] Annulene
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Synthesized by R. Pettit and his
co-workers in 1965.
Cyclic
Planar
Conjugated
(4n+2)π = 4
Doesn’t follow Huckel Rule
It is Anti-aromatic.

8. [16] Annulene
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Cyclic
Planar
Conjugated
(4n+2)=16
Doesn’t follow Huckel’s
Rule
It is Anti-aromatic.

9. [10] Annulene
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Cyclic
Follows Huckel
Rule
(4n+2)=10
n=2
It is Non-aromatic.

10. [10] Annulene
a.
b.
All-Cis: Angle Strain as Bond Angle = 144˚
Di-Trans: Bond Angle = 120˚
No Angle Strain but the two hydrogen atoms
interfere with each other and carbon atoms of its
ring are thus, prevented from bring coplanar.
c. Mono-trans: Less Angle Strain

11. HETEROANNULENES
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Monocyclic conjugated systems
One or more heteroatoms are present in the ring system
e.g.
Pyridine Pyrrole Furan

12. Phosphorine Phosphole Silabenzene
Thiophene 2H-Pyran

13. AROMATICITY
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Heteroannulenes can be aromatic or non-aromatic.
Most of the heteroannulenes are aromatic in
nature but on protonating the heteroatom present
in them, some of them become non-aromatic.
E.g. Pyrrole is aromatic but when nitrogen atom of
pyrrole is protonated, it becomes non-aromatic.

14. Resonance structures of Pyrrole
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Lone pair of electrons present on the nitrogen atom
take part in resonance.
Planar
Cyclic
Follows Huckel Rule
(4n+2)π = 6
n=1
It is Aromatic.

15. Protonated Pyrrole
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Lone pair present on the nitrogen
atom is lost due to protonation of
nitrogen atom
No Conjugation
Doesn’t obey Huckel Rule
(4n+2)=4
It is non-aromatic.

16. Chemistry of heteroannulenes with a
Nitrogen heteroatom
Reviewed byAnastassion andSchroeder
aza [9]
annulene
aza [13] annulene
Aza [17] annulene

17. Aza [19] annulene

18. Oxa [13] annulene
Oxa [17] annuleneOxa [9] annulene
Chemistry of heteroannulenes
with a Oxygen heteroatom

19. References
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Pozharskii, A. F. (1985). Heteroaromaticity (Review)*. M.
A. Suslov Rustov State University. Ruston-on-Don:
Plenum Publishing Corporation.
Haddon, R. C. (1982). Molecular Orbital Studies of the
Hetero (A,B) polyenes and Hetero (A, B) annulenes.
Pure and Applied Chemistry, 54 (5), 1129-1142.
Singh, J., Y., L. (2017). Advanced Organic Chemistry (14th
Ed.). Meerut: Pragati Prakashan.
Solomons, G. T. W., Fryle, C. B. (2010). Organic
Chemistry (9th Ed.). New Delhi: Wiley India (P.) Ltd.
March, J., Smith, M. B. (2007). March’s Advanced
Organic Chemistry: Reactions, Mechanisms, and
Structure (6th Ed.). New Jersey: John Wiley & Sons, Inc.

20. THANK YOU