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?
Great explanation good job👍
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