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