What is Steric Inhibition in Resonance of SIR effect ?

SIR stand for Steric Inhibition in Resonance, means as per name steric means size, inhibition means some kind of hindrance, little hurdle, and resonance means delocalisation of conjugate points.

We know that planarity is the main condition for resonance that means resonance can occur only when all the atom involved in resonance lie in the same plane or nearly in the same plane. Any change in structure which destroys planarity of molecule will restrict or inhibit resonance, this phenomenon is known as steric inhibition of resonance.

SIR effect finds immense use in explaining Stability of carbocation, acidity and basicity of organic compounds:

For example(1): Ortho substituted Benzoic acid is more acidic then meta or para substituted benzoic acid irrespective of nature of group (electron donating group or electron withdrawing group) 

When a group present in the ortho position with respect to carboxylic group creates steric strain resulting in rotation of the carboxylic group and shifting it out of plane of the benzene ring as a result the carboxylic group can no longer participate in ring resonance and thereby the acidity increases as delocalization of negative charge equally on the both oxygen atom of  conjugate base ( carboxylate ion) of the benzoic acid , hence carboxylate ion more stablised. This is  also called Ortho effect.

Note:  However for groups like -NH₂ or -OH does not experience SIR effect due to small size and –CN is linear group so SIR effect not applicable.

For example (2):  N,N,2,6-Tetramethyl aniline is more basic than  N,N-Dimethyl aniline. In cas of N,N-Dimethyl aniline its dialkyl derivatives the NMe₂ group is coplanar with benzene ring.  So p-electron (lone pair) on N atom and pi orbitals are remain in the same plane. For this the p-electron on N-atom can delocalized via pi orbital in the benzene ring and its result the electron availability in para position is high and less available at nitrogen atom and hence less basic.

On the other hand , in case of N,N,2,6 tetramethyl aniline having two bulky methyl group in ortho position of the benzene ring , the NMe₂ group can not remain in the same plane. That is why the p-electron on N-atom cannot delocalized through pi orbital in para position. (due to steric inhibition of resonance)  It is localized at nitrogen atom Thus 2,6-Dimethyl aniline  derivative is more basic.