Tags
Alumina, column chromatography, crystallization, fieser, Flash chromatography, functional group test, HPLC, IR, Morin, MS, recrystallization, x-ray
The past is a foreign country: they do things differently there.
-L. P. Hartley
Imagine a world without flash chromatography, HPLC, high resolution MS or high field NMR, though if you ask nice you may be able to get time on the department’s new 60 MHz NMR (1D proton spectroscopy only, of course).
In this environment, how did chemists ever determine what lay in the bottom of their flasks? IR is an excellent method for determining which functional groups are present, but like elemental analysis it says very little about how the bonds are put together. Closely related compounds and isomers would be nearly impossible to distinguish by these methods, and so changes in physical properties became key.
While presently chief paperweight [1], tracking physical properties was what role that the CRC handbook was made for. Older versions of the book had even less of the spectroscopic properties we now rely on for compound characterization, instead focusing on tests we may no longer even routinely perform. Chief among these is of course melting point, but density and optical rotation also receive special mention [2].
Necessity drove innovation in purification techniques as well. Distillation at reduced pressure can do wonders with low molecular weight oils, and with enough time almost any solid can be recrystallized [3]. Recrystallization solves many of the above mentioned structural assignment problems as well, but unfortunately could not be applied to every case. Oils are nearly impossible to recrystallize and analyze (some do solidify at low temperatures), and many solids have enough disorder to highlight the fine crystallographic distinction between “theoretically possible” and “incredibly difficult.” To combat these beasts several classes of recrystallization aids were devised.
Sodium bisulfite is likely the best known crystallization agent, and the most generally applicable. Aldehydes combine with bisulfite to form stable sulfonic acid/alcohol adducts that are at best sparingly soluble in water. Filtration of the resulting precipitate removes water soluble impurities, while alcohol/ether washes draw off the more organic soluble contaminants. A final exposure to weak acid or base regenerates the original aldehyde, which is then collected via simple liquid/liquid extraction.
Somewhat less amenable were reagents like p-phenylazobenzenesulfonyl chlroide and p-phenylazobenzoyl chloride, which react with free amines and alcohols respectively to form brightly coloured azo dyes. These compounds could be readily isolated by paper or silica chromatography–no stains required [4]–and recrystalized to allow for X-ray analysis, but only through destruction of the original material. Carboxylic acids and amine salts which could do the same were highly prized, and there are far too many to list here. One interesting example is dehydroabieyylamine, which selectively recrystallizes various penicillins. Recrystallization aids fell out of favour in the research lab with the advent of flash chromatogrpahy, but these agents are still very useful for resolving mixtures of chiral compounds.
[1] The CRC handbooks were giants, because in the pre-computer age they had to be to contain all relevant compounds. The 1963 edition was 3600 pages long.
[2] Augmenting IR and providing more detailed structural information was of course a multitude of functional group tests. In the 1960’s though their sun was setting, so I’ll cover them another time.
[3] Cfi. The entire field of protein crystallography.
[4] Few of the TLC stains we take for granted seem to be common at this point. Instead compounds could be tracked on column, provided one worked with specially prepared morin-stained alumina.
