Saturday, February 9, 2019

Steric inhibition of resonance and its influence of on acidity and basicity.



What is steric inhibition of resonance?


Resonance can occur only when all the atoms involved in resonance lie in the same plane or nearly in the same plane. 

Any change in structure, which preventsplanarity, will diminish or inhibits resonance, this phenomenon is known as steric inhibition of resonance.

Few example of steric inhibition of resonance with explanation are as follows,



Why N,N dimethyl aniline couple with diazonium salt but N,N,2,6 tetramethyl does not?


N, N-dimethyl aniline and its 2, 6-dialkyl derivatives the NMe2 group in N,N- dimethyl aniline  and the ring is co-planar. So p-electron 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. So N, N dimethyl aniline couple with diazonium cation PhN2 + to form diazonium salts.

On the other hand , in case of  N, N, 2, 6 tetra methyl aniline having two bulky methyl group in ortho position of the benzene ring , the NMe2  group can’t remain in the same plane. 

That is why the p-electron on N-atom can’t delocalize through pi orbital in para position. Thus  2, 6 dimethyl derivative does not couple with diazonium cation under these condition that is due to steric inhibition of resonance.




An interesting example of steric inhibition of resonance is activate   nucleophilic  substitution of 1, 3 dichloro 2,5 dinitro benzene. 

When this is warmed with methanolic sodium methoxide, only one product 1,3 dichloro-5 –methoxy-2-nitro benzene  is formed . 

The two nitro group can’t stabilize the carbanion leading to (I), since it is prevented from entering into resonance with the benzene ring because of the steric effect of the o-chloro atoms.

On the other hand this steric inhibition of resonance is absent in the carbanion


leading to (II). Hence the carbanion leading to (II) has a lower energy of activation energy than that leading to (I) and so only (II) is produced.




How does steric inhibition of resonance affect the acidity and basicity of a organic compounds?

Steric inhibition of resonance has a great effect on acidity and basicity of organic compounds as like steric hindrance.

The influence of steric inhibition of resonance on acidity and basicity are discussed below.


Why cyclohexane-1,3dione is much more acidic than bicyclo[2,2,2] oct-2,6-dione ?


 Cyclohexane1,3-dione is much more acidic than bicycle [2,2,2] oct-2,6-dione , because the enolate ion of the cyclohexadione stabilizes through resonance , whereas bicycle[2,2,2] oct-2,6-dione does not due to the steric inhibition of resonance. 

Besides, the filled p-orbital of the enolate ion of the later compound being out of plane of the C = O groups, it can’t overlap with the pi electrons of the carbonyl group.



Why 3,5 dimethyl 4-nitro aniline  is a stronger base  than 2,6 dimethyl 4-nitro aniloine ?


3, 5 dimethyl 4-nitro aniline is a stronger base than 2, 6 dimethyl 4-nitro aniline.
Because in 3, 5 dimethyl 4-nitro aniline, amino group can’t enter into resonance with the nitro group due to the steric inhibition of resonance, whereas it can 2, 6-dimethyl isomer. 

Thus in the former, the lone pair on the N-atom is more available for   protonation and consequently, the former is more basic than the later.


Why N-ethyl amine  is stronger base than N-methyl aniline ?


Since the ethyl group is larger than the methyl group. So the steric effect is greater for the former and hence there is greater steric inhibition of resonance in the former.


 Thus, in the former the lone pair on N-atom is more available for protonation and consequently the basicity of N-ethyl amine is greater than that of N-methyl aniline.




Summary:



What is steric inhibition of resonance
Why N, N dimethyl aniline couple with diazonium salt but N, N, 2, 6 tetra methyl does not?
Why 3,5 dimethyl 4-nitro aniline  is a stronger base  than 2,6 dimethyl 4-nitro aniline?

Why N-ethyl amine  is stronger base than N-methyl aniline?

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