Notes On Organic Compounds: Electronic Displacements - CBSE Class 11 Chemistry
The factors which influence the electron density in the substrate are:
  • Inductive effect
  • Resonance effect
  • Electromeric effect and
  • Hyperconjugation

Inductive effect:
The polarisation of a sigma bond caused by the polarisation of an adjacent sigma bond is called the inductive effect.



The inductive effect is related to the ability of a group to either withdraw (or) donate electron density to the attached Carbon atom of the alkyl group.

If any atom (or) group attracts electrons less strongly than hydrogen does then it is classified as electron donating group and is said to have +inductive effect.
          + Inductive Effect Groups
     Name   Alkyl Groups
  Methyl   -CH3
  Ethyl   -C2H5


Ex:


If any atom or group attracts electrons more strongly than hydrogen, it is classified as electron withdrawing group and  is said to have a -inductive effect.
                  - Inductive Effect Groups
          Name    Group
      Halogens   -F,-Cl,-Br,-I
      Nitro  -NO2
      Cyano  -CN
      Carboxy  -COOH
      Ester  -COOR


Ex:



Resonance effect:
Resonance refers to the phenomenon where two (or) more structures can be written for a molecule. The polarity produced in a molecule by the interaction of two pi-bonds (or) between a pi-bond and lone pair of electrons present on an adjacent atom is known as the resonance (or) mesomeric effect.
Ex: Interaction of pi-bond –lonpair in acetate ion.


In the positive resonance effect, the transfer of electrons is away from an atom or substituent group attached to the conjugated system.

            + Resonance Effect Groups
         Name    Alkyl Groups
      Halogens    -F,-Cl,-Br,-I
     Amino Group    -NH2
     Hydroxyl Group    -OH
     Methoxy group    -OCH3


When the electron shift is towards the atom (or) substituent group attached to the conjugated system it is called the negative resonance effect.

         - Resonance Effect Groups
   Name    Alkyl Groups
  Nitro group  - NO2
  Cyano group  - CN
   Carbonyl group   ≻C=O


Electromeric effect:
Electromeric effect is a temporary effect and observed only in organic compounds with multiple bonds in the presence of an attacking reagent.


      
Electromeric effect may be defined as the complete transfer of  shared pair of pi electrons of multiple bonds  to one of the atoms in presence of an attacking reagent.

In the positive electromeric effect the pi electrons of the multiple bond are transferred to that atom to which the attacking reagent is bonded.

When the pi electrons of the multiple bond are transferred to the atom to which the attacking reagent does not get attached  is called the negative electromeric effect.

Summary

The factors which influence the electron density in the substrate are:
  • Inductive effect
  • Resonance effect
  • Electromeric effect and
  • Hyperconjugation

Inductive effect:
The polarisation of a sigma bond caused by the polarisation of an adjacent sigma bond is called the inductive effect.



The inductive effect is related to the ability of a group to either withdraw (or) donate electron density to the attached Carbon atom of the alkyl group.

If any atom (or) group attracts electrons less strongly than hydrogen does then it is classified as electron donating group and is said to have +inductive effect.
          + Inductive Effect Groups
     Name   Alkyl Groups
  Methyl   -CH3
  Ethyl   -C2H5


Ex:


If any atom or group attracts electrons more strongly than hydrogen, it is classified as electron withdrawing group and  is said to have a -inductive effect.
                  - Inductive Effect Groups
          Name    Group
      Halogens   -F,-Cl,-Br,-I
      Nitro  -NO2
      Cyano  -CN
      Carboxy  -COOH
      Ester  -COOR


Ex:



Resonance effect:
Resonance refers to the phenomenon where two (or) more structures can be written for a molecule. The polarity produced in a molecule by the interaction of two pi-bonds (or) between a pi-bond and lone pair of electrons present on an adjacent atom is known as the resonance (or) mesomeric effect.
Ex: Interaction of pi-bond –lonpair in acetate ion.


In the positive resonance effect, the transfer of electrons is away from an atom or substituent group attached to the conjugated system.

            + Resonance Effect Groups
         Name    Alkyl Groups
      Halogens    -F,-Cl,-Br,-I
     Amino Group    -NH2
     Hydroxyl Group    -OH
     Methoxy group    -OCH3


When the electron shift is towards the atom (or) substituent group attached to the conjugated system it is called the negative resonance effect.

         - Resonance Effect Groups
   Name    Alkyl Groups
  Nitro group  - NO2
  Cyano group  - CN
   Carbonyl group   ≻C=O


Electromeric effect:
Electromeric effect is a temporary effect and observed only in organic compounds with multiple bonds in the presence of an attacking reagent.


      
Electromeric effect may be defined as the complete transfer of  shared pair of pi electrons of multiple bonds  to one of the atoms in presence of an attacking reagent.

In the positive electromeric effect the pi electrons of the multiple bond are transferred to that atom to which the attacking reagent is bonded.

When the pi electrons of the multiple bond are transferred to the atom to which the attacking reagent does not get attached  is called the negative electromeric effect.

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