Saturday, October 23, 2010

How to isolate a drug

Every year a Huron High school would be selected to shadow me when I was working as a chemist. What does an organic chemist do anyway?   We have nothing to do with food grown without pesticides. I made compounds to be tested as potential pharmaceutically active agents. We did not like to use the word 'drugs' as it is a pejorative term. Running reactions did not take up much of my time. What did was trying to isolate my desired compound from a mixture.(later writing reports justifying my existance started to eclipse this but I digress comme d'habitude)To give my young charge some idea how I did that, I had them isolate caffeine from tea. Caffeine is the most widely consumed drug there is with more than 90% of the world ingesting it in the form of coffee or tea or soft drinks. It is a central nervous system (CNS) stimulant.I am addicted to it but I have weaned myself off of it in the past so it is not a strong addiction.  Caffeine is fairly soluble in water especially hot water but it is more soluble in organic solvents such as chloroform or ethyl acetate. One tea bag infused in hot water provides about 90 mg of caffeine. To ensure we had enough to look at, we used two tea bags.  Now tea is not pure caffeine. There are plenty of other compounds such as tannins, polyphenols, etc that we would need to separate the caffeine from. Chloroform would be my first choice to extract but mindful of liability, I opted for the 'natural' solvent ethyl acetate as I see it sometimes referred too as it is found in small quantities in fruit. For some biology project when I was a child, I was given a 'killing' jar for insects that had cotton balls saturated with ethyl acetate on the bottom. I remembered the smell. Esters (which ethyl acetate is one formed by reacting ethanol with acetic acid-this situation arises all the time in nature) generally are supposed to smell good. Ethyl acetate does not.
To extract, we took the cooled tea (cooled so we would not have a bomb situation) and shook it up in a separatory funnel, carefully venting it, with ethyl acetate. Separatory funnels are wide at the top and thin at the bottom where there is a stopcock. We let the layers settle. Ethyl acetate is less dense than water so it is the top layer. If we had used the very dense chloroform, it would be the bottom layer and because of that alone, it would have been a better choice but I am trying to not expose anyone to its toxicity. Even so, all work is done in a hood which sucks any fumes away from us. We have gloves and safety glasses on too. We extract the aqueous layer (the water) twice. With a pH strip of paper, we make sure the aqueous layer is not acidic. If it were, the caffeine would form a water soluble salt and we would not be able to extract it until we made it basic. Caffeine is colorless but it does absorb uv light. I have my student place a small drop of it onto an absorbant glass plate and look at it under uv light. Yes, something is definitely in our 'organic' layer. The aqueous layer remains brown but the organic layer is a bit colored also. We let the water layer go through the stopcock into one flask and let the ethyl acetate layer go into another. We add powdered magnesium sulfate to our ethyl acetate flask. This will form crystals of magnesium sulfate (Epsom's Salts) when it combines with the little bit of water that dissolved in the ethyl acetate.  We filter the solids away on a sintered glass funnel (huge improvement over a pleated paper funnel used in college) that is fitted over a thick walled glass flask fitted with an adaptor that we can attach a vacuum to. Gravity takes too long. In our hood, we have valves that can access 2 levels of vacuum. One level is about 25 torrs and the other about 10 (normal pressure is about 760 torrs). If we want to have a stronger vacuum, we have pumps good for about 1 torr or so. Some days, if the big pumps somewhere in the sky have not been maintained, we hardly get less than  400 torrs. In this case (often) we say the vacuum sucks. This is an example of a chemist's oxymoron. The vacuum sucks because it doesn't suck. Now we need to get rid of the ethyl acetate. It boils at 78 deg C vs water boiling at 100 deg C. In general, we don't like to heat our compounds any more than necessary as they can decompose, although caffeine is a very stable compound surviving roasting and fermentations. To get rid of
 the ethyl acetate, we use a device called a rotary evaporater. You heat your rotating flask in a water bath under a vacuum converting your solvent into a gas. Cold water in coils converts it into a liquid again. Eventually all the ethyl acetate is evaporated away leaving what was dissolved, in our case, crude caffeine. Even using the 'low' vac, ethyl acetate will now boil away at 30 deg. We trap the fumes with a dry ice condensor. Dry ice keeps the condensor at minus 78 deg so no fumes destroy the pump in the sky and our fair city is safe from breathing its fumes.

Of all the materials I use in  the lab, kids love dry ice the most. It is solid carbon dioxide. At atmospheric  pressure, it goes directly into a gas as it warms (it is liquid only under great pressure). When visiting classrooms, I will  put a small piece into a latex glove, tie off the wrist part and quickly the glove expands into an enormous udder. Kids ask if it will pop and I just shrug. Well we just have to see.
Meanwhile ice crystals form on the outside of the glove particularly if the room is humid. I remember a particular teacher alarmed by its appearance. She thought it must be some toxic substance. I in turn was alarmed that a teacher would have no understanding of what constitutes air. Young kids also enjoy another demonstration with dry ice. I half fill a graduated cylinder with water (distilled gives the best results, who knows what is in tap water)and add a drop of base and universal indicator. The fluid is now dark purple. I add pieces of dry ice. Huge bubbles form and lots of fog. The purple turns blue then green then yellow then red until I add more base and the rainbow starts over. Magic! the kids ooh and ahh. Nothing is magic, I say if you know what is going on. Not all of the carbon dioxide is converted into fog; some of it dissolved in the water forming an acid  causing the pH to drop gradually causing the indicators to change colors
Carbonic acid: would you dare drink that?

 No!! They resound..
That sounds toxic!!
I explain the production of soda pop and why it goes flat...yadda.yadda.yadda.
We want to see more magic!

Back to caffeine. In our flask, we have a brownish solid. To purify it, we have some options. Chromatography (time and solvent consuming), recrystallization (poor recovery and this stuff dissolves in almost anything..maybe heptane would be a good solvent choice) or short path distillation with the aid of some device called a Kugelrohr made from a coffee percolator and an automobile window wiper mechanism.To turn a substance that melts at 218 deg into a gas, we'll need a high vac meaning, we'll need a vacuum pump meaning more set-up of traps (I hate setting up traps). Our impure compound is in a flask within
the coffee percolator body. I heat  it to about 100 deg. The flask is turned by the wiper mechanism: pshoo, pshoo, pshoo. It is under vacuum at about 2 torr or so. The receiver is on the outside of the perc body. Soon white crystals appear in it. Success. We shut everything down. Only a black tar remains in the original flask.
I have the student put a small amount into a small glass capillary tube to see what temperature that it melts in the melting point apparatus. Taking melting points became much less important in the later days of my career but I had the student take it for historical purposes. It melts where it should. But lots of things melt around 218. How do we prove it is caffeine? We take a Nuclear Magnetic Resonance spectrum ( medicine they use the same technology to create MRIs..somehow they don't use the word nuclear ). We dissolve about a milligram up in a milliliter of deuterated cholorform (the lone hydrogen in chloroform is replace by its isotope deuterium so it does not interfere with our results) and put the solution into a special thin tube. Our compound is exposed to a changing magnetic field. At certain frequencies, the protons or hydrogens of our compound will flip in a predictable matter. The magnetic field is produced by a supercooled magnet that renders heart pacemakers, credit cards and some watches, useless if you get too close. Running a NMR used to be a major ordeal fine tuning it, shimming it etc but now, all is done by computers and I just type how many scans I want and what solvent I am using. All spectra are permanently stored and dated. This can be important to the company if it is necessary to prove that they made a certain compound first. Caffeine has  a simple structure with the formula C8H10N4O2. Looking at the structure above, there are 4 types of hydrogens. We will have 4 peaks in our spectra: 3 of the same height and one  only one third the height of the other three. The smaller peak is on the left side of the spectra. Two of the methyl groups are very similar but the third methyl is further downfield. Our spectra is consistat with the structure. We could have a combustion analysis burning the compound and seeing what percent C, N, and H there are but that would take a day and my student would be gone.
That's how to isolate a drug.
Making a drug, another story.

Meanwhile my life is full of gurgling and sometimes crying babies. I will write more later.


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