Fundamental Concepts of Inorganic Reaction Mechanisms III

Resonance Effect:

• Resonance effect is defined as the ‘polarity produced in the molecule by the interaction of two π bonds or between a π bond and lone pair of electrons present on an adjacent atom’.
• It is designated as R or M effect. Now, a resonance effect can be either positive or negative. Look below positive and negative resonance effects one by one.

Positive Resonance Effect (+R Effect):

Transfer of electrons is away from an atom or substituent group attached to the conjugated system.

+R effect showing groups: -halogen, -OH, -OR, -OCOR, - NH2, - NHCOR

Negative Resonance Effect (-R Effect):

Transfer of electrons is towards the atom or substituent group attached to the conjugated system

-R effect showing groups: -COOH, -CHO, >C = O, - CN, -NO2

Conditions needed for a compound to show resonance are as follows:

• The molecule must have conjugation.
• Part of the molecule having conjugation should be exactly or nearly planar. The orbitals participating in resonance must be of almost equal sizes and should also be smaller in size. Bigger orbital do not overlap effectively. For example,
  CH₂ = CH - OH will show greater resonance than CH₂= CH - Cl as the size of O and C (belongs to the second period, while Cl belongs to the third period).

Hyper conjugation:

• Hyper conjugation is the stabilizing 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.
• This effect takes place only when a saturated carbon with a H-atom is linked to an unsaturated carbon. This involves the conjugation between σ-electrons of a single carbon - hydrogen bond and the π-electron of the adjacent multiple bond. 
• Hyper conjugation is also possible in alkenes and alkyl arenas. For example, for propene, it can be shown as follows:
  
  
• The hyper conjugation effect in toluene (methyl benzene) can be shown as follows: 
• This helps us in predicting the o/p directing influence of CH₃ towards electrophilic substitution. 
• The hyper conjugation in alkyl groups is classified as +H, while trichloromethyl is classified as −H effect. This explains the m-directing nature of −CCl₃ group.