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 3^rd
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