This presentation has been reported for its great application in Organic synthesis and effects of various substituents. It will be useful in understanding of many organic reaction mechanisms.

1. Dr. Arun Sharma
M.Sc. (Gold medalist), Ph.D. (DRDE)
Assistant Professor
Arenes and Aromaticity

2. Aromatic hydrocarbons are called “Arenes”.
Benzene was the first isolated compound. It was isolated by Michael
Faraday in 1825 who determined C:H ratio to be 1:1.
Aromatic compounds are cyclic compounds having ring structure.
These are highly stable compounds. Generally, these compounds are
insoluble in water.
Aromatic hydrocarbons follow the Huckle’s Rule:
(When the number of π-electrons = 4n+2)
Where, n= 0, 1, 2, 3,…..are called “Aromatic compounds”.
These are cyclic, planar and completely conjugated.
Introduction

3. Resonance structure of Benzene & MO-picture
Kekulé structure of Benzene
Kekule benzene: Two forms are in rapid equilibrium
Resonance
hybrid
6 -electron delocalized
over 6 carbon atoms
Localized π-electrons Delocalized π-electrons
Resonance
Delocalization of π-electron cloud in Benzene
Each sp2 hybridized C in the ring has an
unhybridized p orbital perpendicular to the
ring which overlaps around the ring.

4. Aromaticity
Aromatic compounds are cyclic, planar, stable, completely conjugated and follow the Huckle’s
Rule i.e. contains (4n+2) π electrons.
Anti-aromatic compounds are cyclic, planar, completely conjugated and follow the 4n π
electrons. These compounds are generally unstable. e.g. cyclobutadiene.
Non-aromatic compounds are the compounds that lack one or more of the four requirements
to be aromatic or anti-aromatic.
Buckminsterfullerene—Is it Aromatic?
C60 is completely conjugated, but it is not aromatic since it is not planar.

5. Aromatic Electrophilic Substitution
Electron deficient species are called “Electrophiles”. Represented by E+
Electrophiles loves to bind with the electron rich molecules.
“Nucleophiles” are electron rich species. Represented by Nu-
Nucleophiles loves to bind with the electron deficient molecules.
Benzene is an electron rich compound. Therefore, it can easily gives the
electrophilic substitution reaction.
Mechanism: Step 1. Attack of electrophile E+
Slow
Resonance stabilizationStep 2. Removal of proton

6. Examples of Aromatic Electrophilic Substitution
(Nitration)
(Nitrobenzene)
(Sulfonation)
(Benzene sulfonic acid)
(Bromination)
(Bromo benzene)
HNO3
H2SO4
H2SO4
SO3
(Ethyl benzene)
(Alkylation)
CH3CH2Cl
AlCl3

7. Mechanism of Nitration
Formation of electrophile:
Nitronium
ion
Nitric acid
Step 1. Attack of electrophile i.e. Nitronium ion
Slow
Intermediate
cation
Step 2. Removal of proton
Nitrobenzene

8. Mechanism of Sulfonation
Formation of electrophile:
Sulfur trioxide is formed (or already present). It is a powerful electrophile, i.e. electron pair
acceptor because of the effect of the three very electronegative oxygen atoms bonded to the
central sulfur atom.
Step 1. Attack of electrophile i.e. SO3
Slow
Resonance stabilization
Step 2. Proton transfer to give the Benzene sulfonic acid.

9. Mechanism of Bromination
This complex is more electrophilic
than Br2 alone
Formation of electrophile:
Step 1. Attack of electrophile i.e. Br2-FeBr3 complex
Resonance stabilization
Step 2. Removal of proton

10. Mechanism of Alkylation
Step 2. Removal of proton
Step 1. Attack of electrophile i.e. CH3CH2
+
Formation of electrophile: CH3CH2Cl + AlCl3 CH3CH2
+ + AlCl4
-

11. Activating and Deactivating groups
Activating or Ortho-Para (o/p) Directing Groups
Rxn of an electrophile at the
para position of anisole

12. Rxn of an electrophile at the meta position of anisole
The charge on the meta-derived intermediate cannot be delocalized onto the -OCH3 group
Electrophilic Aromatic Substitution Reactions of Substituted Benzenes

13. Deactivating or meta (m)-Directing Groups

15. Activating and Deactivating Effects
• Activating group: A group that causes the substituted
benzene ring to react more rapidly than benzene itself
– All ortho-para directing groups except Halogens
• Deactivating group: A group that causes the substituted
benzene ring to react more slowly than benzene itself
– All meta directing groups
1) Inductive/Polar effect
+I effect -I effect

16. Electron-withdrawing (-I effect)
Electron-releasing (+I effect)
2) Resonance effect: Electron withdrawing by resonance

17. • Electron donation by resonance
• Inductive and resonance effects can work in the
same or opposite directions

18. Summary of Substituent Effects
Activating and Deactivating Effects
Table: Substituent Effects in Aromatic Electrophilic Substitution

19. Use of Aromatic Electrophilic Substitution in Organic Synthesis

21. Thanks………………………