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I recently studied Zaitsev's and Hoffman's rules for deciding which product is formed via elimination and noticed that the rules for formulated much before the discovery of electrons and any conceivable atomic theory. So how did they work out such rules without even knowing how these compounds bond and behave at microscopic levels.

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    $\begingroup$ As Wikipedia notes, Zaitsev's rule is 'empirical' and 'subject to many exceptions'. They did a lot of chemistry and noticed trends. It didn't matter exactly how it worked, they pieced together a phenomenological theory that worked some of the time. $\endgroup$ – Jon Custer Apr 29 '19 at 17:18
  • $\begingroup$ Yeah I read that but how did they even talk about there being double bonds after elimination if there wasn't any theory describing their existence. Also can you please elaborate on what does empirically exactly mean in this context and can I find somewhere what trends did they notice or how they concluded? $\endgroup$ – user78585 Apr 29 '19 at 17:20
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    $\begingroup$ The term "double bond" stems from their time, afaik. Since the ~1930ies we know that the two bonds are not a pair of equals. They knew about the molecular geometry (or rather connectivity), and about addition and elimination. $\endgroup$ – Karl Apr 29 '19 at 18:13
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    $\begingroup$ History of chemistry can be very confusing. So hard to keep track who knew what, and when, when the old journals are hard to get, and textbooks even harder, and the nomenclature wasn't as uniform yet. Good opportunity to practice your school French and German, though. ;-) $\endgroup$ – Karl Apr 29 '19 at 18:20
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    $\begingroup$ This might be a starting point for the history of chemistry: web.lemoyne.edu/~giunta/papers2.html#organic $\endgroup$ – Karsten Theis Apr 30 '19 at 0:52
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This excerpt is from an article in J Chem Ed (J. Chem. Educ., 1961, 38 (6), p 297 DOI: 10.1021/ed038p297) describing the contents of Markovnikov's 1870 paper in Liebig’s Annalen (translated into English, I guess):

enter image description here

So this was before the discovery of the electron, but as you can see from the diagrams, the concept of chemical structure, atoms and valence was already in place.

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    $\begingroup$ Right what I wanted to add. Chemistry is first of all an experimental discipline, I always like to mention that structures like phtalocyanines or something like that were deduced solely by building/dismantling them on the bench! And just using melting points and the likes! $\endgroup$ – Alchimista Apr 30 '19 at 7:34

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