(a) Inductive Effect -The polarization of a σ-bond due to electron withdrawing or electron donating
effect of adjacent groups or atoms is called inductive effect. It is permanent effect. It is represented by I.
It is of two types.
(i) -I effect - The electron withdrawing nature of groups or atoms is known as —I effect.
e.g., —NO2, —CN, —SO3H, —F, —Cl etc.
(ii) +I effect -The electron donating nature of groups or atoms is known as +I effect.
e.g., (CH3)3 C—, (CH3)2 CH – (CH3 )3 —,
CH3CH2 —, (CH3)3 —CH3
(b) Electromeric Effect - It is defined as the complete transfer of a shared pair of π-electrons to one of the atoms joined by a multiple bond on the demand of an attacking reagent. It is temporary effect. It is represented by E and the shifting of the electrons is
shown by a curved arrow
It is of two types :
(i) +E effect- When the transfer of electrons takes place towards the attacking reagent, the effect is called +E effect.
e.g., Addition of acid to alkene.
(ii) -E effect : When the transfer of electrons takes place away from the attacking reagent, the effect is called -E effect.
e.g., Addition of cyanide ion to carbonyl compound.
(c) Mesomeric Effect- The electron withdrawing or releasing effect attributed to a substituent through delocalization of π-electrons which can be visualized by drawing various canonical forms, is called mesomeric effect. It is represented by M.
(i) -M effect - It is shown by the substituents that withdraw electrons by delocalisation mechanism from rest of the molecule and are denoted by -M effect.
(ii) +M effect- It is shown by the substituents that release electrons by delocalisation mechanism to rest of the molecule and are denoted by +M effect.
e.g., -OH, -SH, -NH2, -NR2 etc.
(d) Hyperconjucation- The delocalization of σ-electrons or lone pair of electrons into adjacent π-orbital or p-orbital is called hyperconjugation. It occurs due to overlapping of σ-bonding orbital or the orbital containing a lone pair with adjacent π-orbital or p-orbital. It is also known as “no bond resonance” or “Baker-Nathan effect”.
Conditions for Hyperconjugation: There must be an a-CH group or a lone pair on atom adjacent to sp2 hybrid carbon or other atoms like nitrogen, Oxygen etc.
Example : Alkenes, alkyl carbocations, alkyl free radicals, nitro compounds with α-hydrogen etc.
