Wednesday, December 12, 2012

Process Wednesday: byproduct versus sideproduct

From the sticklers at Scientific Update, a comment in Organic Process Research and Development on a grammatical issue in chemistry by Will Watson:
Following my colleague John Knight’s editorial about the misuse of various terms, particularly the distinction between crystallisation and precipitation, my beef is about the use of “side products” and “byproducts”. 
These two terms are bandied about indiscriminately and concern materials other than the desired products that result from the reactions. However, there are two distinct types of materials that are formed during a chemical reaction, and if the terms are used correctly, these can be classified as either byproducts or side products... 
Byproducts are materials that are produced as a direct result of the desired reaction, and so they will appear as part of the fully balanced chemical equation. Side products, on the other hand, are the result of side reactions. Let me explain a little further. If we carry out a decarboxylation reaction, we will expect a stoichiometric amount of CO2 to be produced; if we carry out a Suzuki coupling, we will similarly produce a stoichiometric amount of borate byproducts and HX, with HX usually being neutralised by the base present in the reaction mixture. 
Side products are impurities which appear during the reaction as a result of (1) side reactions that can be alternative reaction pathways or (2) further reaction/degradation of the desired product after it has formed. Once we isolate our product, it is likely to contain some impurities, and these can be either byproducts or side products.
...So please, let us use the terms byproduct and side product correctly. “What about impurities that are carried through from a previous step?” you ask? Does it matter if they are byproducts or side products from the previous step? Are there any suggestions? Should they be called preproducts, pre-impurities, or...?
I confess that I've never really thought very hard about this issue (the world for me being divided into "desired" and "undesired")... but it's an interesting question. I suppose that for his last question, I would have to go with "side products." 

4 comments:

  1. I'd ask why you're carrying impurities through from previous steps...

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  2. Impurities from previous steps aren't byproducts or side products - they aren't products at all if they didn't form during the current reaction.

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  3. @CanChem

    Do you really want to stop after every chemical step to rigorously purge all impurities? While the ideal process generates no impurities at all, the next best thing would be a process in which all steps could be telescoped and any impurities purged by one final isolation at the API. Understanding the fate of impurities generated in upstream steps as they come downstream is an often overlooked and underappreciated part of process chemistry.

    As for the original post, I routinely use the word "byproduct" as intended by its definition, but when it comes the word "side product", I always opt for use of "impurity" instead. The "product" in "side product" to me has some connotation of value, whereas in practice they're mostly just a thorn in your side.

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