CHE101 (B.Tech. Chemistry Course)

Lecture - 23

Basic of Electron Displacement Effect

Edited By

Mr. Harminder Singh

Department of Chemistry Lovely Professional University, Phagwara, Punjab, India.

E-mail: harminder.singh@lpu.co.in

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The inductive effect arises in the organic molecules when some atom or group of different electronegativity is attached to the carbon atom chain.

As a result, the electron pairs of the chain get displaced either towards it if it is more electronegative then carbon or away from it in case it is less electronegative.

• the electron displacement will lead to polarity( i.e. partial + and –ve charge)

The permanent displacement of the electrons along the chain of carbon atoms due to the presence of an atom or group of different electronegativity at the end of the chain is called Inductive effect.

X is more electronegative

X-

CY is less electronegative

Y-

C

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Inductive effect

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TYPES OF INDUCTIVE EFFECTSTYPES OF INDUCTIVE EFFECTSELECTRONWITHDRAWINGGROUPS (-I effect)

ELECTRONDONATINGGROUPS (+I effect)

Cl-

C CH3

-

C

F, Cl, Br, N, O R, CH3, B, Sielectronegative elements take electron densityfrom cabon

alkyl groups and elements less electronegative than carbon donate electron density to carbon

These electron withdrawing and donating groups work throughthe sigma bond system, unlike the similarly named resonance

groups that work through the system.3

-I effect- NO2 > -SO3H > -CN > -COOH > -F > -Cl > -Br > -I > -OCH3 > -C6H5 > -H

+I effect

(CH3)3C- > (CH3)2CH- > CH3- CH2- CH2- > CH3- CH2- > CH3- > -H

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Applications of inductive effect:

•Relative strength of organic acids•Relative basic strength of ammonia and primary amines

Inductive effect

Cl C C C-

- + - +

O

O

INDUCTIVE EFFECTS INDUCTIVE EFFECTS HALOACIDSHALOACIDS

The effect diminishes with distance - it carries for about 3 bonds.

Cl C

O-

Chlorine helps

to stabilize -CO2-

by withdrawingelectrons

O

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Inductive effect

Inductive EffectsElectronic effects that are transmitted through

space and through the bonds of a moleculeThe effect gets weaker with increasing distance

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Inductive effect

The inductive effectthe electron-deficient carbon bearing the

positive charge polarizes electrons of the adjacent sigma bonds toward it

the positive charge on the cation is not localized on the trivalent carbon, but delocalized over nearby atoms

the larger the volume over which the positive charge is delocalized, the greater the stability of the cation

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Inductive effect

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Inductive effect

Factors that Determine Acid Strength—Inductive Effects

• When electron density is pulled away from the negative charge through bonds by very electronegative atoms, it is referred to as an electron withdrawing inductive effect.

• More electronegative atoms stabilize regions of high electron density by an electron withdrawing inductive effect.

• The more electronegative the atom and the closer it is to the site of the negative charge, the greater the effect.

• The acidity of H—A increases with the presence of electron withdrawing groups in A.

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Electromeric effect

Electromeric effect

This is a temporary effect and takes place between two atoms joined by a multiple bond, i.e., a double or triple bond. It occurs at the requirements of the attacking reagent, and involves instantaneous transfer of a shared pair of electrons of the multiple bond to one of the linked atoms. 

It is temporary in nature because the molecule acquires its original electronic condition upon removal of the attacking reagent.

For example, consider the carbonyl group, >C=O, present in aldehydes and ketones. When a negatively charged reagent say approaches the molecule seeking positive site, it causes instantaneous shift of electron pair of carbonyl group to oxygen (more electronegative than carbon). The carbon thus becomes deprived of its share in this transferred-pair of electrons and acquires positive charge. In the meanwhile oxygen takes complete control of the electron pair and becomes negatively charged. Therefore, in the presence of attacking reagent, one bond is lost and this negatively charged attacking reagent links to the carbon having positive charge.

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Electromeric effect

This phenomenon of movement of electrons from one atom to another at the demand of attacking reagent in multi-bonded atoms is called electromeric effect, denoted as E effect. The electromeric shift of electrons takes place only at the moment of reaction. Like the inductive effect, the electromeric effect is also classified as +E and E:

 +E- effect

When the transfer of electrons takes place towards the attacking reagent, it is called + E (positive electromeric) effect. For example,

 

 

+E- effectWhen the transfer of electrons takes place away from the attacking reagent, it is called, -E (negative electromeric) effect. For example, 

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Electromeric effectEx.of application of electromeric effect:

Additions to Alkenes

Generally the reaction is exothermic because one and one bond are converted to two bonds

The electrons of the double bond are loosely held and are a source of electron density, i.e. they are nucleophilicAlkenes react with electrophiles such as H+ from a hydrogen halide to form a carbocation

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