Purification 12/4/21

Thoughts by Richard Bleil

Last night’s dream was interesting to say the least.  Recently I wrote a post about a former student who was interested in the cosmetics industry.  Writing this must have brought her to the forefront of my mind, as in my dream she was working on her capstone project with me.  This might seem a bit sad, but I don’t remember what her actual capstone was, but in the dream, she was doing an organic synthesis.  The backstory to the dream was that I was not her organic chemistry professor (which is possible, but unlikely as the university hired one more chemistry professor for one year and he was an organic professor), so I was kind of quizzing her to see what she really knew of organic laboratory work.  Specifically, I was asking about product purification.

No matter how clean or careful a synthesis is, it is always necessary to purify the final product to get rid of unreacted reagents, side products and just simple impurities.  If it’s a liquid, the most common purification technique is some form of distillation.  By vaporizing and recondensing the product, most (but never all) of the impurities are left behind (or come out ahead of the product).  If it’s a solid, the most common form of purification is recrystallization.  This means dissolving and then causing the crystals to reform, cleaner (but never perfect) each time.  Because neither of these techniques (nor any technique known) is completely effective, it’s not uncommon to perform these recrystallization steps multiple times (typically at least three times), which brings us to the second part of the line of questioning.

See, with each purification step, the product is cleaner, but you also lose some of the product.  It is impossible to purify a product without losing at least a little bit regardless of how careful you are being.  One of the major points of pride in chemists is percent yield, that is, how much product do you actually get compared to what theoretical calculations (called “stoichiometric calculations”) claim you should get.  Remember balancing all of those equations in chemistry class?  These balanced equations give rise to “theoretical yield”, meaning that if you are absolutely perfect and your technique flawless, this is the amount of product you should get.  Of course, perfection is difficult to come by (except for my ex-wife who insisted that she was perfect), so nobody ever reaches 100% yield.  In general chemistry, it’s not difficult to get over 95% (even over 100%, which, if you get it, usually means your product is impure, probably due to incomplete drying), but in organic, the reactions are too delicate and side reactions too plentiful.  A percent yield of even 70% is usually far more than can be expected.  But now we have a problem.

The chemist is faced with the difficult question of how pure the product should be, versus how high the theoretical yield?  It’s all too easy to purify your product into oblivion, but stopping the purification too soon leaves a dirty product (with poor usefulness and characteristics thanks to colligative properties).  How much should the product be purified?  That’s a subjective question.  Some chemists insist on the absolutely cleanest possible product, losing product at each step, and others clean to an “acceptable” level to save as much product as possible.  My students would go crazy when I explained this, and never gave them a straight answer as to how many purification steps they needed to carry out because it truly is subjective. 

It seems to me that this is an allegory.  I’ve had to purify my life more than once, including recently.  My marriage was excessively toxic, with emotional abuse, lack of support and too many one-sided demands.  In essence, my wife was an impurity in my life, dragging me down and keeping me from reaching my true potential.  Truth be told, she is the one who asked me for the divorce, but as soon as she did I recognized the benefits and happily agreed.  Maybe she was hoping I would “fight for her”, but she didn’t have any qualities worth fighting for.  This might sound harsh, but it’s also true, and I wasn’t necessarily very good for her either.  I had become an enabler for her alcoholism, and it was destroying her, me and the marriage.  When I divorced her, I lost a part of myself that she still carries with her to this day whether or not she is aware of it.  But, I’m in a better place, better able to focus on myself, grow, and live the life I want to lead.

It’s often difficult to purify ourselves of people.  Walking away from a toxic relationship that you had known for a very long time is never easy.  If you’ve been trying to help somebody out (for example, of an addiction) it’s painful when you reach the point that you realize that this person just doesn’t want to be better.  We can only give so much of ourselves, and when they try to drag you down to their level it just takes too much effort.  Purification always results in some loss, but what you end up with is so much better.

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