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Our teacher told us that the inductive effect or the electron displacement effect can only affect the sigma electrons. I did not understand why this should be.

I reasoned it can not affect the pi electrons, because they are "delocalized".

Also, for measuring the displacement, the C-H bond is taken as a reference. The C-H bond does not have a pi bond, and so we can not consider pi electrons as being affected

Is it true that the inductive effect or the electron displacement effect affects only sigma electrons? Why should it be so?

I am not asking if it operates through single bonds only, as asked in this question.

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    $\begingroup$ "I did not understand why this should be". Well, perhaps because it's not like that? $\endgroup$
    – Mithoron
    Jun 23, 2020 at 17:44
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    $\begingroup$ I guess you are too fixated on defining things as distinct "effects", while it may actually be more sensible to look at it from a perspective of unequal distribution of electron densities across bonds. In both the cases, sigma, and pi bonds, due to variations in electronegativity, electrostatic forces are set up due to formation of charges, and so the electron density tries to shift towards the negative end in both cases. The only real difference is that as the pi electrons are pretty "loosely" bonded as compared to sigma electrons, they can actually shift themselves in their entirety... $\endgroup$
    – Yusuf Hasan
    Jun 24, 2020 at 0:04
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    $\begingroup$ ..so as to give rise to the so-called "mesomeric" effect, where the pi electrons delocalize over the system.Sigma electrons can't really move completely from one atom to another,as their bond strengths are relatively higher,and also,their geometry somewhat prevents them from delocalizing.So they have to make do with just shifting towards one end of the bond,which is called the "inductive" effect.So it is like a spectrum of ways of distribution of electron densities,and just because pi electrons manage to move completely themselves,doesn't mean that they can shift within bonds like the I effect $\endgroup$
    – Yusuf Hasan
    Jun 24, 2020 at 0:12
  • $\begingroup$ @YusufHasan Thank you,now I understand.Can you convert your comment into an answer? $\endgroup$
    – Micelle
    Jun 24, 2020 at 3:36
  • $\begingroup$ @YusufHasan you said pi electrons completely shift themselves to more electronegative atom which would lead to development of complete charges rather than fractional charges but that doesn't happens $\endgroup$
    – Lalit
    Jun 25, 2021 at 6:58

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