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These two compounds are given in my workbook, and I am supposed to compare their net dipole:

enter image description here

The solution given in the workbook is that:

  1. Molecule A (N,N-dimethyl-4-nitroaniline) has a "net shifting of electrons" in the downward direction - due to $+M$ effect of $\ce{NMe2}$ and $-M$ effect of $\ce{-NO2}$;
  2. Molecule B (N,N,2,3,5,6-hexamethyl-4-nitroaniline) - due to steric hindrance - does not undergo resonance to an appreciable extent, hence, there is negligible "net shifting of electrons" in any direction. The lone pair of $\ce{NMe2}$ almost remains in its place.

Hence, molecule A has a higher dipole than molecule B. I am content with this explanation.

However, my question is:

Is this explanation actually correct, or is it just a theoretical mask to explain an experimental observation (i.e. they just made up a random theory on their own to explain an observation)?

Are such types of explanations common, universally accepted, etc?

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  • $\begingroup$ What is "actually correct"? We can know for sure what are the dipole moments, but not why. To me, the explanation sounds good. $\endgroup$ – Ivan Neretin Oct 23 '17 at 15:45
  • $\begingroup$ @IvanNeretin i wonder if this explanation is a "standard" one, universally accepted for such compounds? $\endgroup$ – Gaurang Tandon Oct 23 '17 at 16:12
  • $\begingroup$ Yes, it is a fairly common explanation, but only if the groups are large enough to actually cause steric repulsions (in this case, two methyl groups are just fine, because of the flatness of the molecule). $\endgroup$ – orthocresol♦ Oct 23 '17 at 16:46
  • $\begingroup$ Try googling “steric inhibition of resonance” $\endgroup$ – orthocresol♦ Oct 23 '17 at 16:49
  • $\begingroup$ @orthocresol I know (and have solved several workbook questions based on) that topic, unless there was a specific link which you wanted to point me to? $\endgroup$ – Gaurang Tandon Oct 23 '17 at 17:01
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The workbook's explanation could have been more specific. If you build a 3D model, you may see that the orbital interaction between the lone pair on the $\ce{NMe_2}$ and the $\pi$-system on the ring is somewhat hindered by bad overlap because the steric interaction of the methyl groups (on the ring and the $\ce{N}$) prevents perfect alignment. Same for the nitro substituent. A higher interaction would lead to more charge shift.

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