Explain Different Electron Displacement Effects

Electron Displacement Effects


• 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


• Arrange it in the order of acidic strength




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   


• 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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