Meaning of equivalents in organic synthesis

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?





> 
> 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 It's most certainly *not* based on the concept of 'equivalent weights' or normality, which as you noted is obsolete.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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