In this post, everything about steric inhibition of resonance is explained.
What is steric inhibition of resonance effect?
SIR stands for Steric Inhibition in Resonance. As per its name, steric means size, inhibition means some kind of hindrance and resonance means delocalisation or conjugation.
We know that the planarity is the main condition for resonance that means resonance can occur only when all the atoms which are involved in resonance must lie in same plane or nearly in same plane.
Any change in the structure of molecule which changes its planarity will restrict or inhibit resonance, this phenomenon is known as steric inhibition of resonance.
Where is steric inhibition of resonance applicable?
SIR effect finds immense use in explaining:
- Stability of carbocation
- Acidity of organic compounds
- Basicity of organic compounds
For instance,
Ortho substituted benzoic acid is more acidic than meta or para substituted benzoic acid irrespective of the nature of group present i.e., either electron donating group or electron withdrawing group.
When a group present in the ortho position with respect to carboxylic acid (-COOH) group, it creates steric strain or hindrance resulting in rotation of the carboxylic acid (-COOH) group and shifting it out of plane of the benzene ring. As a result, the carboxylic acid group can no longer participate in resonance with the benzene ring, and thereby the acidity increases as delocalisation of negative charge equally on both oxygen atom of conjugate base (i.e., carboxylate ion, -COO–) of the benzoic acid, giving two equivalent resonating structures, hence carboxylate ion is more stabilised. This is also called as ortho effect.
This is illustrated in the figure below:
N,N,2,6-tetramethylaniline is more basic than N,N-dimethylaniline. In case of N,N-dimethylaniline its dimethyl groups derivative, i.e., NMe2 group is coplanar with benzene ring. So, lone pair of electron present on N-atom and p-orbital remain in same plane. For this the lone pair of electron on N-atom can delocalise via p-orbital in the benzene ring and it results in 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-tertamethylaniline having two bulky methyl groups in ortho position of the benzene ring, the NMe2 group can not remain in same plane. That’s why the lone pair on N-atom cannot delocalize through p-orbital in para position. It is localized at nitrogen atom. Thus, 2,6-dimethylaniline is less basic N,N,2,6-tertamethylaniline.
Who really used steric inhibition of resonance?
SIR effect is only applicable to benzoic acid and aniline.
For further reading: Why ortho effect will make you question acidic character?