A challenging experiment today for the third week of O-chem lab. The goal was to separate an acid from a neutral substance and get both pure. For rookies, this was a tough task as they had never used a sep funnel before.
For pre-lab, they had to read the lab manual and the Mohrig techniques book, watch a video from the MIT lab techniques site, then run through an on-line exercise I did in the course web-site. Most of them did that.
Procedure was partially outlined in the lab manual but they had to come up with the idea of re-precipitating the acid from its conjugate base drawing on their experience in the previous lab, where they confirmed the presence of an organic acid by re-precipitating. Ether solution, extraction with 5% NaOH, re precipitate the acid and heat to boiling to digest it, then evaporate off ether to recover the neutral. ID both by melting point from a list of 8 possibles.
But there are always surprises in O-chem lab.
- Adding the clear NaOH to a clear ether solution makes a precipitate? Well, the sodium salt of one of the acids might not be all that soluble.
- You’re not getting a precipitate? Try adding the HCl to the aqueous base layer instead of the ether.
- You’ve boiled off almost all the ether and you still have no solid? Give it a sec… Wham, the whole liquid layer crystallizes because
- you had it supercooled, or
- because the compound was so amazingly soluble you had to get every last bit of Et2O out of there
- Or maybe because you heated your ether flask over boiling water instead of 50-60 degrees, so that liquid you thought was ether was really melted biphenyl.
- Or maybe it’s water from the bath that you slopped in there.
And then there’s the beautifully educational mistakes.
- A 50 mL Erlenmeyer tips over when heated in a 600 mL beaker’s water bath. Ether boils off instantly, leaving a skin of product on the water. “OK, let’s pretend you’ve spent weeks making that compound, so every molecule is precious. How can you get it back?”
- A flask of boiling water with crystals of naphthalene subliming onto the lip (why was that even in your water layer?)
- A sep funnel with fuzzy white crystals growing all around the leaky stopcock.
Making mistakes isn’t bad. Not noticing you’ve made a mistake, that’s bad. Not learning from your mistake, that’s bad. Making the same mistake over and over, not so good.
If there wasn’t room for mistakes, there wouldn’t be as much room for learning. And man, was there a lot of room for learning today.
But then you get the big payoff: