Basic Concepts of Aromaticity

1. POWER POINT PRESENTATION
0N
STRUCTURE AND REACTIVITY
BY
DR. GOPINATH SHIROLE (M.Sc. SET. Ph.D.)
ASSISTANT PROFESSOR
DEPARTMENT OF CHEMISTRY
ARTS, SCIENCE & COMMERECE COLLEGE, RAHATA
A/P/TAL-RAHATA, DIST.-AHMEDNAGAR

2.  Aromaticity :
● The name “aromatic” was originated from the “aroma” of benzene like
compounds.
 Conditions for Aromaticity :
1) The molecule(system) must be cyclic.
2) The molecule(system) must be planar
(all atoms in the molecule must lie in the same plane)
3) The molecule(system) must be fully conjugated.
4) The molecule(system) has (4n+2) πe- (n=Any integer= 0,1,2,3...etc.)
01 Aromatic Obey (4n+2) rule + Resonance (Conjugation)
02 Anti-aromatic Obey (4n) rule + Resonance(Conjugation)
03 Non-aromatic Obey (4n+2) rule or (4n) rule
but no resonance (Non-conjugation)

3. (4n+2) Rule (or) Huckel Rule
Aromatic (n=0,1,2,3…)
(4n) Rule
Anti-aromatic (n=0,1,2,3…)
n=0 2πe- n=0 0πe-
n=1 6πe- n=1 4πe-
n=2 10πe- n=2 8πe-
n=3 14πe- n=3 12πe-
n=4 18πe- n=4 16πe-
n=5 22πe- n=5 20πe-
n=6 26πe- n=6 24πe-

4. 1) Cyclopropene 2) Cyclopropenyl cation 3) Cyclopropenyl anion
● Planer
● Cyclic
● Planer
● Cyclic
● Planer
● Cyclic
● Non-conjugated
system
● Fully conjugated system ● Fully conjugated
system
● Obey (4n+2) rule
● n=0, 4n+2= 2πe-
● Obey (4n+2) rule
● n=0, 4n+2= 2πe-
● Obey (4n) rule
● n=1, 4n= 4πe-
● Non-aromatic ● Aromatic ● Anti-aromatic
+ -

5. 4) Cyclobutadiene 5) Cyclobutadienyl dication
● Planer
● Cyclic
● Planer
● Cyclic
● Fully conjugated system ● Fully conjugated system
● Obey (4n) rule
● n=1, 4n= 4πe-
● Obey (4n+2) rule
● n=0, 4n+2= 2πe-
● Anti-aromatic ● Aromatic
+
+

6. 6) Cyclopentadiene 7) Cyclopentadienyl cation 8) Cyclopentadienyl
anion
● Planer
● Cyclic
● Planer
● Cyclic
● Planer
● Cyclic
● Non-conjugated
system
● Fully conjugated system ● Fully conjugated
system
● Obey (4n) rule
● n=1, 4n= 4πe-
● Obey (4n) rule
● n=1, 4n= 4πe-
● Obey (4n+2) rule
● n=1, 4n+2= 6πe-
● Non-aromatic ● Anti-aromatic ● Aromatic
+ -

7. 9) Cycloheptatriene 10) Cycloheptatrienyl
cation
11) Cycloheptatrienyl
anion
● Planer
● Cyclic
● Planer
● Cyclic
● Planer
● Cyclic
● Non-conjugated
system
● Fully conjugated system ● Fully conjugated
system
● Obey (4n+2) rule
● n=1, 4n+2= 6πe-
● Obey (4n+2) rule
● n=1, 4n+2= 6πe-
● Obey (4n) rule
● n=2, 4n= 8πe-
● Non-aromatic ● Aromatic ● Anti-aromatic
1
2
3
4
5
6
7 +
1
2
3
4
5
6
7 -
1
2
3
4
5
6
7

8. 12) Cyclo-octatetrene
● Cyclo-octatetrene looks Planer but in actual
structure it is non-planer
● Cyclic
● Fully conjugated system
● Obey (4n) rule
● n=2, 4n= 8πe-
● Anti-aromatic But Non-aromatic

9. AROMATICITY
IN
BENZENOID
COMPOUNDS
 The benzenoids are a class of chemical compounds with at least
one benzene ring.
OR
 The aromatic compound which has only benzene rings in their
structure is known as benzenoid aromatic compound.

10. 13) Cyclohexatriene
i.e. Benzene
14) Naphthalene 15) Anthracene
● Planer
● Cyclic
● Planer
● Cyclic
● Planer
● Cyclic
● Fully conjugated
system
● Fully conjugated
system
● Fully conjugated system
● Obey (4n+2) rule
● n=1, 4n+2= 6πe-
● Obey (4n+2) rule
● n=2, 4n+2= 10πe-
● Obey (4n+2) rule
● n=3, 4n+2= 14πe-
● Aromatic ● Aromatic ● Aromatic

11. 16) Phenanthrene 17) Chrysene
● Planer
● Cyclic
● Planer
● Cyclic
● Fully conjugated system ● Fully conjugated system
● Obey (4n+2) rule
● n=3, 4n+2= 14πe-
● Obey (4n+2) rule
● n=4, 4n+2= 18πe-
● Aromatic ● Aromatic

12. 18) Pyrene 19) Benzopyrene
● Planer
● Cyclic
● Planer
● Cyclic
● Fully conjugated system ● Fully conjugated system
● Obey (4n) rule
● n=4, 4n= 16πe-
● Obey (4n) rule
● n=5, 4n= 20πe-
● It disobey Huckel's rule as it has 16 πe-s
instead of 14 to obey Huckel's rule. But
one of the pi bond is not participating in
delocalisation so it cannot be considered
in the (4n+2) e-s. So it has only 14
delocalised πe-s and its aromatic.
● It disobey Huckel's rule as it has 20 πe-s
instead of 18 to obey Huckel's rule. But
one of the pi bond is not participating in
delocalisation so it cannot be considered
in the (4n+2) e-s. So it has only 18
delocalised πe-s and its aromatic.
● Aromatic ● Aromatic
1
2
3
4
5
6
7 1
2
3
4
5
6
7
8
9

13. AROMATICITY
IN
NON-BENZENOID
COMPOUNDS
 The compound which exhibits an aromatic behaviour but does not
contain any benzene nucleus .
 The non benzenoid aromatic compound have one or more rings fused
but none of the rings is a benzene ring.

14. 20) Pyridine 21) Pyrrole 22) Furan 23) Thiophene
● Planer
● Cyclic
● Planer
● Cyclic
● Planer
● Cyclic
● Planer
● Cyclic
● Don't consider
lone pair of N
atom
● consider lone
pair of N atom
● consider lone
pair of O atom
● consider lone
pair of S atom
● Fully
conjugated
system
● Fully
conjugated
system
● Fully
conjugated
system
● Fully
conjugated
system
● Obey (4n+2)
rule
● n=1, 4n+2=
6πe-
● Obey (4n+2)
rule
● n=1, 4n+2=
6πe-
● Obey (4n+2)
rule
● n=1, 4n+2=
6πe-
● Obey (4n+2)
rule
● n=1, 4n+2=
6πe-
● Aromatic ● Aromatic ● Aromatic ● Aromatic
N
N
H
O S

15.  The fully conjugated monocyclic hydrocarbons are called
annulenes.
 They can be represented by a general formula ( -CH=CH- )n
 They may be called cyclic polyenes and they are named by
writing the ring size followed by the word “annulene”.
 4,6,8,10,12,14,16,18 - annulenes are available.
 According to Huckel’s rule, Annulenes containing (4n+2) πe- and
having a coplanar ring should be aromatic in nature.
AROMATICITY
IN
ANNULENE
COMPOUNDS

16. 24) (4 - annulene)
Cyclobutadiene
25) (6 - annulene)
Cyclohexatriene
i.e. Benzene
26) (8 - annulene)
Cyclo-octatetrene
● Planer
● Cyclic
● Planer
● Cyclic
● Cyclo-octatetrene looks
Planer but in actual structure
it is non-planer
● Cyclic
● Fully conjugated
system
● Fully conjugated
system
● Fully conjugated system
● Obey (4n) rule
● n=1, 4n= 4πe-
● Obey (4n+2) rule
● n=1, 4n+2= 6πe-
● Obey (4n) rule
● n=2, 4n= 8πe-
● Anti-aromatic ● Aromatic ● Anti-aromatic
● But Non-aromatic

17. 27) (10 - annulene) 28) (12 - annulene) 29) (14 – annulene)
● Planer
● Cyclic
● Planer
● Cyclic
● Planer
● Cyclic
● Fully conjugated
system
● Fully conjugated
system
● Fully conjugated
system
● Obey (4n+2) rule
● n=2, 4n+2= 10πe-
● Obey (4n) rule
● n=3, 4n= 12πe-
● Obey (4n+2) rule
● n=3, 4n+2= 14πe-
● Aromatic ● Anti-aromatic ● Aromatic

18. 30) (16 - annulene) 31) (18 - annulene)
● Planer
● Cyclic
● Planer
● Cyclic
● Fully conjugated system ● Fully conjugated system
● Obey (4n) rule
● n=4, 4n= 16πe-
● Obey (4n+2) rule
● n=4, 4n+2= 18πe-
● Anti-aromatic ● Aromatic
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16

19. AROMATICITY
IN
FUSED RING COMPOUNDS
i.e. AZULENES

20. 32) Azulenes –
It is a combination of 5
memb. & 7 membered ring
compounds like
cyclopentadiene &
cycloheptatriene.
33) Pentalene –
It is a combination of
two 5 membered ring
compounds like
cyclopentadiene.
34) Heptalene-
It is a
combination of two 7
membered ring
compounds like
cycloheptatriene.
● Planer
● Cyclic
● Planer
● Cyclic
● Planer
● Cyclic
● Fully conjugated
system
● Fully conjugated
system
● Fully conjugated
system
● Obey (4n+2) rule
● n=2, 4n+2= 10πe-
● Obey (4n) rule
● n=2, 4n= 8πe-
● Obey (4n) rule
● n=3, 4n= 12πe-
● Aromatic ● Anti-aromatic ● Anti-aromatic