Rohini Narwal semester 1

1. SAMRAT PRITHVIRAJ CHAUHAN GOVERNMENT
COLLEGE
AJMER
Resonance and its applications
Submitted by
Rohini Narwal
M.Sc. Chemistry
Semester 1st
2020-2021
Department Of Chemistry

2. T
ABLE
OF
CONTENT
1 • Introduction
2 • What Is Resonance?
3 • Benzene
4 • Rules Of Resonance
5 • Stabilities Of Resonating Structures
6 • Resonance Energy
7 • Resonance Effect
8 • Applications Of Resonance Effect
9 • References

3. “WHAT IN A NAME” SAID SHAKESPEARE
WHAT WILL BE THE NAME? SAY ORGANIC CHEMISTS
Resonance and mesomerism are terms used for
explaining the delocalization of electrons in a conjugated
system, organic chemists used the two terms
interchangeably.
 The study of delocalization of electron based on quantum
mechanical approach led to introduction of the term
resonance[HEISENBERG 1926,PAULING]
 The study of delocalization of electrons as explained by
overlap of p-orbital of conjugated system led to the
introduction of term mesomerism[INGOLD,1934]

4. WHAT IS RESONANCE?
 The representation of a real structure as weighted
average of two or more canonical forms is called
resonance.
 Two or more structures of the same molecule or an ion 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 structures of the molecules is not represented
by anyone of the resonance structure but is considered to
be hybrid of all these structures is called Resonance
Hybrid.

5. Resonating structure of benzene
Kekule structures
39% contributor
(F.A. Kekule)
Dewar structures
7.3% contributor
(J. Dewar)
Resonance hybrid
Delocalised
6p-orbitals

6. RULES OF RESONANCE
 All the atoms taking part in resonance i.e. the atoms that are a
part of delocalised system must lie in a plane or be nearly
planar.
 Only pi electrons movement are allowed.
 The number of unpaired electrons in each of the structure must
be same.
 All resonance structures do not contribute equally to hybrid.
 All canonical structures must be written according to the Lewis
Cyclopropene cation
Picryl iodide

7. STABILITIES OF RESONANCE STRUCTURES
 Structures with greater number of covalent bonds contribute more
towards the resonance hybrid.
 Resonance structures involving charge separation are less stable
than non-polar structures.
 Structure with a maximum of octets are preferred.
 Structures with isolated unlike charges have less contribution than
those in which they are close to each other.
Major
contributor
Minor
contributor
Major
contribution
Minor
contributor
Major
contribution
Minor
contributor
Major
contributor
Minor
contributor
Phenol
1,3-butadiene
Carboxylic acid
diazomethane

8. RESONANCE ENERGY
 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

9. RESONANCE EFFECT
 The generation of centres of low and high electron density
in conjugated system as a result of resonance or
mesomerism is called resonance effect(R-effect)or
mesomeric effect (M-effect).
TYPES OF R(or M) EFFECT
1.Electron –withdrawing resonance effect(-R effect)
2.Electron-donating resonance effect(+R effect)
Electron Donating
Groups
Electron withdrawing groups

10. APPLICATIONS OF RESONANCE EFFECT
 Low reactivity of vinyl and aryl halides towards nucleophilic substitution
 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 .

11.  Comparison of relative base strength of Ethylamine and
Acetamide
 Difference in C-N bond length for o and p-nitro groups in Picryl
iodide
Ethylamine
strong base
Acetamide
weaker base
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

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

13. REFERENCES
Organic Chemistry by
S.M.MUKHERJI
Organic Reactions and
Their Mechanism by
P.S.KALSI
Organic chemistry by
ANUP PATHAK
ANUPA SAHA
Chemistry by
JERRY MARCH
Organic chemistry by
BHUPINDER MEHTA
MANJU MEHTA
Books Name:

14. For Your Time and Attention
Thank You