Three-score and beyond

The postbaccalaureate students for whom I am the TA are currently working on a particularly grueling organic chemistry lab. The resolution of enantiomers lasts three long lab sessions and is fairly unforgiving in terms of technique. I see the hollowness in their weary eyes and remember what it was like, almost exactly one year ago, as I slogged through the same drudgery.

It was, in some ways, the last straw for my already tottering sanity - the Amadis de Gaul to my Alonso Quixano - and it drove me to the excesses with which you, my faithful readers, are perhaps already acquainted. Unlike a Lovecraftian unfortunate, my madness did not mark the end of the tale but its commencement, and it was this brain-sick humour that pervaded the remainder of my postbaccalaureate endeavors.

The first fruit of my strange affliction still savored of the soil from which the mad tree had sprung, and so was directly concerned with the resolution of enantiomers. It only loses luster the more I describe it, and with so little merit at the outset (Einhard, Einhard, Einhard...), it cannot afford much tarnish.


The following appeared in my laboratory notebook, under the heading "VII. Resolution," without explanation:


Ένάντιος means “opposite” in old Hellenic speech
So ’twixt two twinned enantiomers, they’re mirrored, each in each
But do not be so hasty as to hastily suppose
That these two twinned enantiomers you can superimpose
The truth is made quite tangible by basic polarimetry
Whereby the matched-up molecules reveal their true asymmetry
When two enantiomers are present in the same proportion
The light that’s been plane-polarized will suffer no distortion
But when the brew’s scalemic, and there’s excess of one strain,
The light that’s been plane-polarized is twisted ’twixt the twain
To disconnect the duo, purge from each its pal’s pollution,
The chymist breaks the brace and pares the pair by resolution.
It is a daunting prospect, this most subtle separation
The chymist must use all his cunning in the distillation
And ne’er be shy to use his whole experimental coterie:
Extraction, crystallizing, and evaporation rotary.
Into an Erlenmeyer weigh the acid of the Khan
(Six grams in honor of the Russian princes feasted on)
Then methanol is added, eighty cubic c’s complete,
And to dissolve the acid, we apply judicious heat
Five mils methylbenzylamine, but of the alpha kind,
Are added once the brew is hot, and swirling they’re combined.
A scattering of prism seeds are subsequently sown
Then for a week you’d better leave them bloody well alone
For in the flask the seeds will grow, and if you don’t harass it,
The dragon’s teeth will spawn a host with orthorhombic facets
Wait half a fortnight’s passing, and recommence no quicker,
But once a week’s gone by you may decant the mother liquor
The crystals are collected by a plain vacuum filtration,
Then dried and weighed and subject to percent-yield calculation
Take half the mother liquor and decant it, without spilling,
Into a rounded flask, and simply start simply distilling.
Once thirty milliliters have been carefully collected
Pour all the liquor left into the flask that you selected
Watch carefully the level of the distillate distilled
For once you’ve sixty mils, the quota has been quite fulfilled.
Distilling takes a while but stay busy while you’re waitin’
There still is much to do and idle hands are tools of Satan!
While distillate is dripping you’ll begin another task
Put all the crystals in a fifty milliliter flask
We add three-molar natrium hydroxide to the mix
To form an amine with C4H12N2O6
The free amine sits in a less-dense layer up above
And in a sep the aqueous may then be drained thereof
Anhydrous sulfate sodium add to the ether extract
For it will bind to any excess water it can contact
By now your distillation should be totally complete
But if the flask cools down the crystals may become discrete
So quickly pour the last remains of liquor most maternal
While yet the liquid temperature’s still gen’rally infernal
And when this Erlenmeyer becomes cool enough to hold
Immerse it in an icy bath and crystals will unfold!
These needles of ammonium tartarate (double plus)
Should form along the bottom of the flask without a fuss
But this takes time as well so while your crystals are complying
Go back and find that flask where your amine solution’s drying
Decant the ether off the salt where it was left to bask
Into a dry, weighed, fifty cubic c round-bottom flask
Then clip it to the Rotovap, release the vacuum vent,
And spin that little bugger until all the solvent’s spent
With all the liquid gone and only amine still remaining
The new weight of the flask is what you need to be obtaining
Now all that’s left is just to find the optical rotation
A measurement of passing light’s compuls’ry aberration
Inside the polarimeter, light waves are polarized
And when you see a dark spot your rotation’s realized
You’re not done yet, though sun may set, and certain it’s quite late
Go back to your old crystals of ammonium (plus, plus) tartarate
In your Buchner place the rocks and drain the methanol off
And store them in a place that’s dry so liquid levels fall off
One week to wait, but what’s another week? You’ve waited three
Then find their optical rotation, right down to a degree
And now you’re done, and you have seen events that were quite nice occur:
Successfully you’ve isolated isomer from isomer!

© D.S.E. 2008