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I have been told that the inductive effect is only present in sigma bonds by my teacher, and the book I've referenced also says the same thing.

However, I can't think of why this is. Answers to this question seem to agree with me. But Wikipedia also says that inductive effect only operates through sigma bonds.

So which way is it and why?

Paula

(From Paula Bruice, page 47)

Amadeus
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  • The presence of double bonds implies 1)a double faced electronegativity for the C atom 2) resonance (mesomeric effect) eventually at work. This might render I effect propagating through pi bonds not so important to be a subject in introductory chapters. Look at https://chemistry.stackexchange.com/questions/4904/why-inductive-effect-operates-only-through-single-bonds where at least one user think as you (and me). – Alchimista Nov 08 '21 at 13:47
  • Also, a couple of considerations I do: 1 statements you mentioned are about sigma and pi, not really about single & double. 2 a double bond is anyway a reservoir, it logically damps the I effect propagation through bonds. Like you need water or dump water in a tank placed between two pipe lines. – Alchimista Nov 08 '21 at 13:56
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    It's just terminology. Electron donation or withdrawal through the sigma system is termed "inductive". Electron donation or withdrawal through the p-orbitals of a pi system is termed "resonance". – ron Nov 08 '21 at 14:02
  • @ron not really / not the point. In fact my damping example refer only to the inductive part,normally negligible. The answer is given by user Zhu in the thread I've linked just above. – Alchimista Nov 08 '21 at 14:32
  • Sorry, I see that you were aware of the other question. If it helps, I like the answer by Zhu. – Alchimista Nov 08 '21 at 15:10
  • @ Alchimista I'm not sure I buy Zhu's answer. Fluorobenzene undergoes EAS faster than chlorobenzene due to resonance effects. See this earlier answer. – ron Nov 08 '21 at 16:10
  • @ron but inductive effect is about partial charges and formation of dipoles, while resonance is supposed to be a complete dislocation? – Amadeus Nov 09 '21 at 02:45
  • @Amadeus resonance can lead to partial charges on atoms. Look at the allyl carbocation for example. – ron Nov 09 '21 at 04:25
  • @ron oh yeah, good point. I'm most satisfied with your answer then; if you convert your comment into an answer I could accept it if you want – Amadeus Nov 09 '21 at 05:41
  • @ron. Resonance effect delocalise electrons, inductive ones "pin" them to an atom. A double bond includes a sigma one. This last line makes the question even superfluous for a yes or no answer. It remains to see how big the effect can be, if it is worth consideration, if something useful can be concluded. And the EAS discussion seems totally different to me. It is about the prevalent effect exerted by the halogen. I will read that later. – Alchimista Nov 09 '21 at 08:24
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    @Alchimista I don't think there is any reason to doubt that inductive effect would work through the sigma bond in a double bond as the physics involved is the same, so I've edited the question (and Zhu's answer also gives an example though ron has contested that, but I don't believe he disagrees either with the claim that inductive effect works through sigma bonds of double bond, just that the given example is wrong). Actually I had misinterpreted Zhu's answer earlier, now I understand that he was also only talking about inductive effect through the sigma bond in a double/multiple bond. – Amadeus Nov 09 '21 at 09:03
  • @Amadeus that is the point I am considering. I also think that, there is no physical reason for not having inductive effect through bonds. However, if you think carefully, the Double bond is effectively a "damper", especially because one side is already electron poor and one electron reach. – Alchimista Nov 09 '21 at 11:43
  • @Alchimista that's a very interesting point! I also thought some more about what Ron said and i no longer think it makes sense, resonance only acts in conjugated pi systems (or in presence of lone pairs or ± charges). What about induction in something like Cyclohexanone? – Amadeus Nov 10 '21 at 04:33
  • For me the final answer then would be that induction, which is just shifting electron densities due to electrostatic attraction, happens just fine for both sigma and double bonds since the physics involved is the same and there's nothing special about the pi bonds position, but as total electron shifting has to be pretty much the same the induction gets dampened, distributed between the two bonds. Thus in effect it doesn't matter and you can form a general rule (which is wrong but always gives the right result) that all induction only happens through sigma bonds – Amadeus Nov 10 '21 at 04:38
  • Creates more confusion than help but oh well – Amadeus Nov 10 '21 at 04:39
  • @Amadeus I think your second last comment although could be more elegantly formulated resume the situation. And now that you have changed double into pi in the title, even more. In a way one side of the C involved tends to "repel" electrons, while the other tend to "attract" them. This makes the unit involved a kind of stopper. But the physics is just buried, still at work. – Alchimista Nov 10 '21 at 07:02

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