Due to conjugation, it is possible for 1,4-addition, or conjugate addition, to occur. I was thinking, since conjugation and resonance effects do no diminish with distance, unlike inductive effects, does that mean that we can indefinitely conjugate and even have 1,6-addition? And even 1,8-addition and so on? My chemistry teacher mentions that 1,6-additions are possible but is there really no limit as to the effects of conjugations? Perhaps, there is increasing "resistance" as the carbon chain lengthens, like how a longer wire would have a larger resistance? I wonder if that would be an appropriate analogy.

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    $\begingroup$ Roughly speaking, there is no resistance. $\endgroup$ – Ivan Neretin Jul 5 '18 at 9:58

1,6-additions are indeed possible (1) but since you have competing 1,4-addition and 1,2-addition it becomes more difficult. To push it to the 1,6-addition bulky organometallic catalysts are used. Reaction pathways are also determined by the electrostatic interactions, and these diminish the further away you get from the carbonyl.

(1) https://onlinelibrary.wiley.com/doi/epdf/10.1002/adsc.201601342, https://www.organic-chemistry.org/abstracts/lit1/353.shtm

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