Explain Different Electron Displacement Effects

Electron Displacement Effects

Contents

• Inductive effect

• Resonance effect

• Electromeric effect

• Examples

Learning
Objectives

At the end of this
lecture, student will be able to

• Explain different electron displacement effects

• Electron Displacement effects

• Inductive effect

• Resonance effect

• Electromeric effect

Factors
effecting reactivity

Steric
effects- concerned with the size and shape of groups within molecules

Electronic
effects- result from the electronegativity differences between atoms affect the
way electrons are distributed in molecules

Can be
divided into inductive and mesomeric effects

Inductive
effects- consequence of way that electronegativity differences leads to
polarization of
σ bonds

Mesomeric
effects- affects the distribution of electrons in
π bonds

Inductive
effect- polarization of
σ bond

Cause-
electronegativity difference between the atoms

Creates
some bond polarity between atoms

Most
electronegative atoms pulls electrons in the bond towards itself which results
in polarization of bond

It is a
permanent effect

It
influences physico-chemical properties

Inductive
effect

Inductive
effect weakens away along the chain and is not significant beyond 3rd
carbon atom

Types of
Inductive effect

Negative
inductive effect (-I)

Positive
inductive effect (+I)

Electron
withdrawing nature of groups or atoms is called negative inductive effect

Electron-withdrawing
groups include halogen, nitro, cyano, carboxy, ester and aryloxy

Positive
inductive effect- refers to electron releasing nature of groups or atoms

Alkyl
group are usually considered as electron donating groups

Why alkyl
groups are showing positive inductive effect?

Though
the C-H bond is practically non-polar covalent bond, there is partial positive
charge on hydrogen atom

And
partial negative charge on carbon atom 

Each
hydrogen acts as electron donating group and turns alkyl moiety into electron
donating group 

Applications of inductive
effect

Stability
of carbocations

Increases
with the increase in number of alkyl groups

Due to
the +I effect

Alkyl
groups releases electrons to carbon bearing positive charge and stabilizes ion

Stability
of carbanions

Decreases
with the increase in number of alkyl groups

Electron
donating groups destabilize the carbanions by increasing the electron density

Acidic
strength of carboxylic acids and phenols

Electron
withdrawing groups decreases the negative charge on carboxylate ion and
stabilizes it

Acidic
strength increases when –I groups are present

p-nitro
phenol is more stronger than phenol

Because
nitro group is –I  group and withdraws
electron density

p-cresol
is weaker acid, because methyl group is +I effect

Basic
strength of amines

Electron
donating groups increases the basic strength of amines

Electron
withdrawing groups decreases the basic strength

Alkyl
amines are stronger than ammonia

Aryl
amines are weaker than ammonia

For
example, CH3NH2,NH3,C6H5NH2

Ans: CH3NH2>NH3>C6H5NH2

Problem-01

• Arrange it in the order of acidic strength

A) CH3COOH, CH2FCOOH, CHF2COOH, CF3COOH

Ans: CH3COOH<CH2FCOOH<CHF2COOH<CF3COOH

B) HCOOH, CH3COOH

Ans: Formic acid is stronger than acetic acid

• -CH3 destabilizes the carboxylate ion

Resonance
Effect /Mesomeric Effect

Also
called as resonance effect

Arises
due to substituents or functional groups in a molecule

Represented
by letter M or R

Polarity
produced in the molecule by the interaction of two
π bonds or between a π bond and lone pair of electrons
present on adjacent atom

Negative
mesomeric effect

Shown by
substituents or groups that withdraw electrons

Denoted
by –M or –R effect

Electron
density on rest of the molecule will be decreased by this

For
example, -NO2, carbonyl group C=O, cyano, -COOH, -SO3H,
etc

Positive
mesomeric effect

Shown by
substituents or groups that donates electrons

Denoted
by +M or +R effect

For
example, -OH, -OR, -SH, -SR, -NH2, -NR2

Applications of mesomeric
effect

Nitro
group in nitrobenzene shows –M effect

Electron
density on benzene ring is decreased particularly on ortho and para positions

Nitro
group deactivates the benzene ring towards electrophilic substitution reaction

In
phenol, -OH group shows +M effect due to delocalization of lone pair of
electrons on oxygen atom towards the ring

Electron
density on benzene ring is particularly increased on ortho and para positions

Hence
phenol is more favoured towards electrophilic substitution

Also more
favoured at ortho and para positions

-NH2
in aniline also exhibits +M effect

Releases
electrons towards benzene ring through delocalization

By this
electron density on benzene ring increases particularly at ortho and para
positions

Aniline
activates towards electrophilic substitution

Causes
less basic than ammonia and alkyl amines

Electromeric
effect

• Electromeric effect is a temporary effect

• The polarization of pie bond by a nucleophile to form
temporary addition compound

• Can be regained if the attacking species is expelled out
from it by adding some strong electrophile

Hyperconjugative Effect

Stabilising
interaction that results from the interaction of the electrons in a σ-bond
(usually C-H or C-C) with an adjacent empty or
partially filled p-orbital or a π-orbital

To give
an extended molecular orbital that increases the stability of the system

For example,
hyperconjugation in carbocation centre

• C-H bond

• Conjugative effect

• Through sigma and pi bonds

In
resonating structures of propene, there is no bond between carbon and hydrogen
atom

Hyperconjugation
also called as no bond resonance

Number of
methyl groups bonded in double bonded carbon atom increases, possibility of
hyperconjugation increases

Results
more stability- more substituted alkenes are more stable than less substituted
alkenes   

Summary

• Inductive effect occurs through sigma bond

• Inductive effect is a permanent effect

• Mesomeric effect is a temporary effect

Hyperconjugative
effect is through sigma and pi bonds

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