First Organic Lab of the Year

Maybe it's because it was the first full moon of the year yesterday. Maybe it's because it's my first organic lab of the year. Maybe it's because there was 5-cent cotton candy on sale on campus today and I was high on sugar. Anyway, today's lab is just plain strange.

The reaction itself is very simple: reflux an alcohol and an acid anhydride together to get an ester, then purify it by column chromatography. The point of the reaction itself is twofold: one, to serve as a gentle reminder to students how to do organic labs, and two, to teach students a few new techniques (i.e. TLC, column chromatography, running the IR).

Anyway, so the reaction was supposed to take one hour, with a TLC plate run every 15 minutes. The first weird part? Apparently my reaction was complete within that first 15 minutes. Seriously, no reactant spot in the TLC place at the "reaction mixture" at all.

The second step was purification by chromatography. Oddly enough, the eluent for column chromatography (DCM*) differs from the eluent for TLC (hexanes/EtOAc). I should state now that my packing skills sucked and there were bubbles (ick), but since the product differ quite greatly in polarity from the reactant... Anyway, I ran about 30 mL** solvent through, and got four fractions out of it. According to TLC, only the third one (and vaguely, possibly the second one) held anything. Fair enough; I combined and rotavapped the two fractions. Result: an empty round-bottom flask.

I then went off to cry on a TA's shoulder. (Well, ok. More like: "TLC says I have product, so where's my product?!" He said: "Elute more solvent through the column directly into the RBF".)

So I did; ran 20 mL more solvent through the column. Then I ran yet another TLC on the newly eluted solution, all my non-evaporated fractions, along with a reactant spot; this time around, what TLC said actually held up in the rotavap, and I ended up with my product (a clear and colourless liquid, which is kind of hard to visually tell apart from the equally clear and colourless solvent except for their difference in boiling point).

In the end, it took about twice as much solvent as the manual predicted to elute the product off. Sigh. My yield isn't that great either - about 60%, probably because I haven't eluted everything off yet (being an idiot, I trusted my TA and went with the eluent volume he suggested. Should've collected more fractions and done a TLC).

The strange part is the result from my TLC plates. They are lying, I tell you! (Or, something got contaminated/was inhomogeneous.)

Incidentally, it seems that a good reagent:solvent ratio for TLC is 1 (generous) drop of reagent per mL of solvent or so. I had very nice spots that way.

Also incidentally: the lab had awesome TLC spotters! They are much finer than capillary tubes, slightly flexible, and long enough to reach through the (microscale) condenser into the reaction mixture. However, since the spotter is flexible, one must "aim" very carefully on the TLC plate, otherwise the spot will end up in unintended places; in addition, the temporary "wet spot" is very difficult to see (for those who are used to seeing them). Too bad we didn't have enough; I only got to run a single TLC with one of those beauties. (All the TLC spots made with capillary tubes were ungainly, in comparison.)

(One last weird part: one of my lab TAs is related to one of my previous math professors. Small world.)

*Dichloromethane feels somewhat burn-y on the skin - a few drops of the stuff went merrily through my gloves, and goodness, it was obvious.
**As this is a microscale lab - the total mass of my reactants was about 0.8 g (about 2 mL or so) - 30 mL of solvent ought to have been plenty.