Sakshi Arora semester 1

1. SAMRAT PRITHVIRAJ CHAUHAN
GOVERNMENT COLLEGE, AJMER
2020-2021
Department Of Chemistry
Aromaticity and Anti-Aromaticity
Submitted By:
Sakshi Arora
M.Sc.(Chemistry) Semester I

2. Contents
• Aromaticity
• NMR Spectroscopy for Aromatic Compounds
• Aromatic Compounds
• Anti-Aromaticity
• Non Aromatic Compounds
• Reference

3. • Aromaticity is the ability of a compound to sustain induced diamagnetic ring
current i.e. they are diatropic.
• Planar, monocyclic, completely conjugated systems with (4n + 2) delocalisable π
electrons all in bonding orbitals and are exceptionally stable. Such systems are said
to be aromatic.
• They undergo electrophilic substitution rather than addition reaction.
• The rings of 2,6,10,14…. π electrons will be aromatic.
• Delocalization energy of cyclic conjugated polyene system is greater than that of
its acyclic analogue

4. Criteria for Aromaticity
1. The molecule must be cyclic
To be aromatic, each p orbital must overlap with p
orbitals
on adjacent atoms.
Benzene 1,3,5 Hexatriene
Aromatic Not Aromatic
2. The molecule must be Conjugated

5. 3. The molecule must be planar
All adjacent p orbitals must be aligned so that the π electron density can be delocalized.
Benzene

6. 4. The molecule must follow Hückel's Rule i.e. it must have
(4n+2)π electrons

7. • According to Hückel's Molecular Orbital Theory, a compound is particularly
stable if all of its bonding molecular orbitals are filled with paired electrons.
• With aromatic compounds, 2 electrons fill the lowest energy molecular orbital,
and 4 electrons fill each subsequent energy level, leaving all bonding orbitals
filled and no anti-bonding orbitals occupied. This gives a total of 4n+2π
electrons.
• Benzene has 6π electrons. Its first 2π electrons fill the lowest energy orbital,
and it has 4π electrons remaining. These 4 fill in the orbitals of the succeeding
energy level.
Why 4n+2 π Electrons?

9. Why do aromatic compounds burns with sooty flame?
• Aromatic compound burn with sooty flame because they have a
ring structure of carbon atom. This causes incomplete combustion
of the carbon chain.
• They have relatively high percentage of carbon.
• Thus they give sooty flame.

10. NMR Spectroscopy Of Aromatic Compounds
• The protons above or within the aromatic ring are shielded and will be shifted upfield(lower δ
ppm)
• The protons outside the aromatic ring are deshielded and appear at a downfield position(higher
δ ppm)
• Ordinary alkene hydrogens are found at ~5-6 δ, while the hydrogens of aromatic ring are
located at ~7-8 δ.

11. BENZENOID AROMATIC COMPOUNDS
Naphthalene Anthracene Phenanthrene
10π electron system 14π electron system 14π electron system
NON BENZENOID AROMATIC COMPOUNDS
Cyclopropenyl cation
2π electron system

12. Cyclopentadienyl anion Ferrocene
6π electron System Has two cyclopentadienyl rings
Aromatic Stable
Pyridine
Lone pair is not part of the
aromatic system

13. Cyclotetradecaheptaene
14π Electron System
Not Stable
Azulene Dipolar Structure
10π electron system

14. • These are planar, cyclic and conjugated compounds with 4n π electrons.
• They sustain paramagnetic ring current hence are called paratropic.
• They are highly unstable and highly reactive.
• Delocalization energy of cyclic conjugated polyene system is lower than that of the
corresponding acyclic analogues.
• The rings of 4,8,12…π electrons will be non aromatic.
ANTI-AROMATICITY

15. Cyclobutadiene Cyclopentadienyl cation
4π electron system 4π electron system
Only stable below -100°
Pentalene Cyclooctadiendiyne
8π electron system 8π electron system
(four extra triple bond electrons
do not participate)
Anti-Aromatic Compounds

16. • Cyclic, non-planar, completely conjugated polyene with localized π electrons are said to be
non-aromatic.
• Delocalisation energy of cyclic conjugated polyene is equal to that of the corresponding
acyclic analogue.
Cyclooctatetraene Cyclooctatetraene Tub Shaped
Non-Planar sp2
angles are most stable at 120°
Bond angle=135°
Cyclodecapentaene
Non-Planar Cyclododecahexaene
Non-Planar
Non-Aromatic Compounds

17. REFERENCE
• Advanced Organic Chemistry (Sixth Edition) by Jerry March and Michael
B.Smith
• Organic Reaction and Their Mechanisms (Third Revised Edition) by P.S.
Kalsi
• Organic Chemistry(Second Edition) by Jonathan Clayden, Nick Greeves
and Stuart Warren

18. Thank You