Roger Giese Massachusetts Institute of Technology
I
LOW Temperature Recrystallization Tube
Recrystallization is one of the foremost purification techniques. It is employed whenever an organic compound is found to he crystalline a t room temperature. But it is not as commonly used for lower melting compounds due to the difficulties introduced by low temperature recrystallization. For one thing, to prevent the recrystallized compound from melting on the filter pad, one needs a cooledfiltration apparatus. There are two general types available. The first involves a system which is completely immersed in a cold bath and then inverted to cause filtration across a scintered glass disc in the middle of the tube. This system is simple in theory but awkward in practice. The second, more sophisticated and popular, either of home or commercial design, involves maintaining the solution to be recrystallized on a scintered glass disc in a tube by means of nitrogen pressure from below, followed by vacuum suction of the mother liquors after crystallization is complete. The only disadvantages are the cost of purchase or construction of such an apparatus and the upward nitrogen pressure, which may lead to unwanted agitation or foaming of the solution t o be recrystallized and does not always prevent leakage of this solution through the scintered disc. One can always plunge a flask of the solution to be recrystallized into a cold bath and pour off or syringewithdraw mother liquor after crystals form. This easy procedure can he effective hut sometimes leaves excessive quantities of entrapped mother liquors, withdrawable only by suction filtration, so as to beinefficient. The apparatus pictured here overcomes all of the above difficulties. It consists of a straight glass tube B with a scintered glass disc C and stem G through rubber stoppers E and F and mounted in a cut-off, polyethylene bottle I. Step by step, one first stoppers A , inverts the apparatus, fills compartment D with solvent, and then stoppers G. No-air
610
/
Journal of Chemicol Edumtian
stoppers are especially suitable. The apparatus is turned back over, the stopper at A is removed, a small quantity of solvent is deposited in compartment B on top of the scintered glass disc C, and a cooling mixture (e.g., dry ice and isopropyl alcohol) is poured into If approximately to the level of the disc C to equilibrate that part of the system. Then the excess solvent in B that was not contracted into compartment D by the cooling is withdrawn, the solution to be recrystallized is poured into B, cooling mixture is poured into H,and opening A is stoppered to keep out moist air. By this procedure the solution in B will not flow into compartment D. After the crystals baveformed, the stopper at G is removed, the apparatus is secured on a filter flask at stopper F, and suction is applied. Prior to filtration, the solvent in compartment D may be removed by tapping. I t may be preferable to leave stopper A intact so as not to melt the crystals by pulling warm air through them. One can attach a flask at A and invert the apparatus, after emptying H, to recover the filtered crystals. The apparatus as dimensioned hmdles 5-25 g quantities and may be readily scaled up or down for other size work. One can exclude air with the presence of an inert, dry gas throughout the procedure with no undue difficulty.
18crn
f I"
Recry+dliration apparatur.
),
Sometimes one's efforts to find a suitable solvent system for recrystallization of a compouud whichis crystalline at room temperature can be relieved by a low temperature recrystallization in hexane or pentane-the usual low temperature recrystallization solvents-where the laree tem.* perahre change available diminishes t,he importance of the nat,ure of the solvent ~ystem.
