Simran Bundel Semester 1

1. SAMRAT PRITHVIRAJ CHAUHAN GOVERNMENT
COLLEGE AJMER
2021-2022
DEPARTMENT OF CHEMISTRY
RESONANCE AND ITS APPLICATIONS
Submitted by:-
Simran Bundel
M.Sc. Chemistry
Semester 1st

2. TABLE OF CONTENT
 Introduction
 What is resonance ?
 Structure of Benzene
 Rules of Resonance
 Stabilities and contribution of resonating
structure
 Resonance energy
 Resonance effect
 Applications of resonating effect
 References

3. INTRODUCTION
 Resonance and mesomerism are the two
concepts that explains the delocalization
electrons in a conjugated system, chemists use
the terms interchangeably.
 The study of delocalization of electrons ,based
on the quantum mechanical approach to the
introduction of resonance. [HEISENBERG
PAULING 1926]

4. WHAT IS RESONANCE ?
• Resonance is a hypothetical state of a chmeical species in whuch its
electronic structure can be represented hypothetically by two or more
resonating structures . This representation of real structures as a
weighted average of two or more canonical forms is termed as
resonance.
• Two or more structures of the same molecules or ions with same
geometry with the same number of paired electrons but differing in the
pairing arrangement of these electrons are termed as Resonance
structures.
• The actual structure of all the different possible structures that can
be drawn for a molecule or ion without changing the relative position
of atoms is termed as Resonance hybrid.
• The resonance hybrid of chemical species is the real structure and
the most stable structure then any of the resonating structure.
Example:
Resonance in phenol

5. STRUCTURE OF BENZENE
Kekule structures (Most
stable)
39% contributor (F.A. Kekule)
Dewar structures (less
stable)
7.3% contributor (J. Dewar)
Resonance hybrid
6 p-orbital Delocalized

6. RULES OF RESONANCE
a). The position of atom in a chemical species must
remain unchanged in all the resonating structure.
Carbonate ion
b). Only π bonds can be converted into lone pair of
electrons and vice-versa.

7. c). All the atoms taking part in the resonance i.e. the
atoms that are a part of delocalized system must lie in
same plane or be nearly planar. The reason for
planarity is maximum overlap of the p orbitals .
d). Most stable resonating structure contributes the
most and vice-versa.
e). The number of unpaired electrons must be same
in each canonical form. Thus, the structure below of
1, 3- butadiene is not a valid canonical form.
f). All the resonating do not contributes equally in the
resonance hybrid.
g). The resonance energy of the actual molecule
(hybrid) is lower than that of any form, therefore
delocalization is stabilizing phenomenon.

8. STABILITIES OF RESONANCE
STRUCTURES
The more covalent bonds a structure has the
more stable it is.
Structures of a maximum octets are preferred.
Major
contributor
Minor
contributo
r
1,3-butadiene
Major
contributor
Minor
contributor
Diazomethane

9. Enolate ion
Structure with a negative charge on the most
electronegative atom and positive charge on least
electronegative atom contributes the most.
Major
contributor
Minor
contributor
Major
contributor
Minor
contributor
Vinyl amine
Uncharged structures contributes the most if different
number of covalent bonds is present in different
resonating structures.

10. Major
contributor
Minor
contributor
Carboxyllic acid
Structures with isolated unlike charges have less
contribution than those in which they are close to each
other.
Major
contributor
Minor
contributor
Phenol
Structures with separated opposite charge will
be less stable.

11. RESONANCE ENERGY
o Resonance energy is the difference between the
actual energy of a molecule(i.e. Resonance hybrid) and
the energy calculated for the most stable of the
resonating structure i.e. having minimum energy among
the resonating structures.
Resonance energy of
benzene

12. RESONANCE EFFECT
1.) (-R ) or (+M) effect :-when an electronegative group
or atom donates its lone pair of electrons in a
conjugated system then it is said to possess electron
donating effect or (-R ) effect.
oThe lowering and decreasing of electron densities in a
conjugated system when results in resonance and
mesomerism , some effects are produces which are
termed as Resonance effect (R-effect) or Mesomeric
effect (M-effect).
oTypes of (R) or (M) effect:-
Aniline

13. 2.) (+R ) effect :- when a more electronegative group or
atom in a conjugated system , withdraws the π –
electrons from the system , then it is said to possess
electron withdrawing effect or (+R effect).
Nitrobenzene

14. APPLICATIONS OF RESONANCE
EFFECT
1. Low reactivity of vinyl and aryl halides towards nucleophili
substitution.
2. High reactivity of allyl and benzyl halides.
Due to the resonance the
carbon chlorine bond
acquires some double bond
character, thus binding
halogen to carbon atom
cannot be easily displaced in
a substitution reaction.
Both allyl and
benzyl halides
undergo
ionization to give
resonance
stabilized allyl
and benzyl
cations .

15. Resonance may increase or decrease the magnitude of this
resultant dipole moment depending upon the nature of the polar
bonds present in the molecule.
a.) Lowering of dipole moment
»Dipole moment of
vinyl chloride
is much lower than
ethyl chloride
b.) Increasing the dipole moment
» Dipole moment
of p-nitro
aniline
is higher than that
of
nitrobenzene
3. Effect of dipole moment

16. Ethylamine
strong
base
Acetamide
weaker base
5.) Difference in C-N bond length for o and p-nitro
groups
The large iodide atom forces o-nitro
groups
out of the plane of benzene ring and
has no
effect on p-nitro group which remains in
the
plane of the ring. Therefore ,resonance
operates in benzene ring involving p-
nitro
4.) Comparison of relative base strength

17. REFRENCES
 Organic reactions and their mechanism by
P.S. Kalsi
 Advanced organic chemistry (pragati prakashan)
 March’s organic chemistry by jerry march
 Reaction mechanism in organic chemistry by S.P .
Singh and S.M. Mukherjee
 Organic chemistry by Anup pathak and Anupa
saha
 Organic chemistry by Manju mehta and Bhupinder
mehta

18. THANK YOU.......