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The use of "equivalents" as used in titrations/chemical analysis is an obsolete concept except in some countries. The original meaning of equivalent weights is the weight of a compound that combines with 16 g O or 1 g H and it was developed during the time when electrons did not "exist". With this definition it is easy to determine the equivalent weights, hence equivalents can be calculated directly or indirectly for most of inorganic simple compounds provided a chemical equation is available.

I still occassionally see equivalents in organic synthesis works. For example, one abstract has

"Deprotonation of pyrazine, pyridazine, pyrimidine, and quinoxaline using an in situ mixture of ZnCl$_2$·TMEDA (0.5 equiv) and LiTMP (1.5 equiv) was studied."

This is another paper title

"Why Do Catalytic Quantities of Lewis Acid Generally Yield More Product than 1.1 Equiv in the Intramolecular Diels−Alder Reaction with a Furan Diene? "

It appears that the organic chemist's equivalent is quite different from old-shool's analytical chemist's equivalent. I see this has been asked before, but the answers are not relevant to organic synthesis and it is hard to find a relevant definition in any chemical dictionary or even textbooks with reference to organic compounds. Is there a solid reference which defines how equivalents are defined/used in organic chemistry?

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  • $\begingroup$ books.google.com/ngrams/… $\endgroup$
    – Karsten
    Aug 13 at 3:31
  • $\begingroup$ Karsten, I think this reaction equivalent in J.Chem.Ed. is quite different from what orthocresol is explaining. The article has a nice solved equation for an inorganic reaction but the term reaction equivalent there is not "molar equivalents" as explained in your first link and this answer. Also it will not fit in the examples I quoted. The book is also related to inorganic compounds. They are apparently using equivalents in a classical way (the analytical chemist's way). $\endgroup$
    – AChem
    Aug 13 at 3:45
  • $\begingroup$ “ The use of "equivalents" as used in titrations/chemical analysis is an obsolete concept except in some countries.” What???We don’t use equivalent weight internationally anymore? $\endgroup$ Aug 17 at 12:03
  • $\begingroup$ @insipidintegrator, Equivalent weights are longer taught in a majority of Western universities. $\endgroup$
    – AChem
    Aug 17 at 15:32
  • $\begingroup$ Wow. @AChem and here, (won’t specify country, but it is to the east of Greece and to the west of New Zealand) we literally have a whole chapter called “Redox Reactions “ in school which focuses on redox titrations and where the main task is to find out the n-factor and equivalent weights of a variety of reactions. $\endgroup$ Aug 17 at 15:42

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Is there a solid reference which defines how equivalents are defined/used in organic chemistry?

I'm not sure about a reference per se; but from my (limited) experience as an organic chemist, the usage of 'equivalents' in this context invariably refers to molar equivalents. It's most certainly not based on the concept of 'equivalent weights' or normality, which as you noted is obsolete.

Typically this is measured with respect to the substrate of the reaction, so in the case you quoted,

Deprotonation of pyrazine, pyridazine, pyrimidine, and quinoxaline using an in situ mixture of ZnCl2·TMEDA (0.5 equiv) and LiTMP (1.5 equiv) was studied

would refer to the addition of $\pu{0.5 mol}$ of $\ce{ZnCl2.TMEDA}$ and $\pu{1.5 mol}$ of $\ce{LiTMP}$ per $\pu{1 mol}$ of pyrazine (for example).

As for the second quote, the authors are probably comparing the Diels–Alder reaction using catalytic Lewis acid versus (super)stoichiometric Lewis acid. In this case, the Lewis acid is also measured in terms of molar equivalent, relative to the substrate.

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    $\begingroup$ Molar equivalents make perfect sense in reaction schemees but organic chemistry books are quiet about calculating or quoting equivalents. I will search this term in Google Books. It is clear now that this equivalent is not the analytical / inorganic chemist's equivalent. $\endgroup$
    – AChem
    Aug 10 at 21:59
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    $\begingroup$ Molar equivalents is how synthetic organic chemists apply the term. If substrate A is either expensive or valuable then one would use 1.1 molar equivalents of inexpensive or readily available substrate B to form bond A-B. $\endgroup$
    – user55119
    Aug 11 at 2:09

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