In the Laboratory
Claisen’s Flask and Its Evolution Bruno Lunelli Chemistry Department “G. Ciamician”, University of Bologna, 2 via F. Selmi, I-40126 Bologna, Italy
The apparatus generally used for reduced-pressure distillation, one of the most common physical operations of chemistry, is substantially that known for more than one century as the Claisen flask (1) (Fig. 1). During this time, the relevant changes have been the reduction of the volume of the most used size by two to three orders of magnitude and the use of new connections. The Claisen flask was designed to reduce the probabilC T ity of having to repeat the distillation, which in a common flask would become necessary V upon bumping (“stossF weissem Sieden” [1]), the sudden, violent vaporization of part of the liquid, which projects most or all of the distilland into regions from where any material ends up in the distillate. Bumping is a normal occurrence in lowpressure distillations (2). Its Figure 1. Solid line, Claisen’s flask; dashed line, partial rem- effects are aggravated at miedy to the contamination of the croscale size by the bubble distillate induced by the centrifu- size’s independence of the gal pseudoforce; dotted line, flask dimensions, by the best position for vapor outlet. slight decay of velocities and accelerations over small distances, and by the rate of heating exceeding the maximum vapor flow rate possible in the apparatus. The distinctive feature of the Claisen flask (Fig. 1) is a bifurcation of the flask’s neck into a neck for the capillary, C, and one for the thermometer, T. Such a geometry makes it impossible for any splash of distilland projected by bumping and following nearly rectilinear trajectories (independent of the vapor flow) to reach the distillate. This applies to big (with a large volume-to-surface ratio) splashes, but not to the droplets of distilland mainly originating from bubble bursts, which are small enough (3) to be entrained by the vapors featuring a high speed due to the low pressure. The situation is worsened by the stream of inert gas from the capillary and by the fact that the vapor outlet V is placed on that side of the thermometer neck which is external with respect to its curvilinear part. Thus the inertia (the centrifugal pseudoforce in the noninertial reference frame rotating with the vapor at the bend of neck T) of any entertained distilland droplet (which is liquid, with a density much higher than that of the vapors), directs such particles towards the vapor exit V, facilitating the contamination of the distillate. A more efficient design would have reduced this effect by placing the exit of the vapors on the interior side of the curved arm, as indicated by the dotted, virtual vapor exit F of Figure 1. This drawback is somewhat reduced also by means of a protrusion of the vapor exit pipe V into T (dashed line in Fig. 1), a modification originally conceived to avoid collecting into the 638
distillate condensates that had leached the rubber or cork stoppers (4). In the Claisen flask the centrifugal field induced by the nonrectilinear part of the vapor path is an undesirable feature, whose effects should be eliminated as far as possible. However, some time ago I showed that it is possible to set up a centrifugal field in the vaporizer (or boiling flask) and exploit it in practice to reduce to zero the contamination of the distillate. A new, very compact and efficient condenser was also designed to take advantage of a second centrifugal field generated in the region where vapors condense. The union of the vaporizer and condenser utilizing centrifugal fields gives a reduced pressure distiller outperforming the previous ones, especially with small samples (5). To get an order-of-magnitude appreciation of the centrifugal field intensity in the vaporizer, consider the distillation of 5.0 mL of a compound with density 1.0 and molecular weight 200 (0.025 mol) at 100 °C under an effective pressure (that generated by the rotary pump plus its increase due to the resistance to the flow of the conduit) of 1 torr (10᎑3 bar under this approximation). Under these conditions the volume of the vapor is 0.75 m3; if heating power is such as to get one drop every six seconds, the distillation takes 600 s. If the radius of the upper chamber of the vaporizer is 1.5 cm = 1.5 × 10᎑2 m and the cross section of the vapor exit pipe is 50 mm2 = 5 × 10 ᎑5 m2, the average vapor speed is (0.75 m 3 )/(5 × 10 ᎑5 2 m × 600 s) = 25 m s᎑1 and the centrifugal acceleration on the entrained liquid droplets is (25 2 m 2 s ᎑2)/ T (1.5 × 10 ᎑2 m) = 41,600 b b ms᎑2, between one thousand and ten thousand times the gravitational acceleration. M After years of quite satisfactory use, the main drawbacks of this vaporizer appeared to be the amount of glassblowing necessary to build it and the significant S condensation of higher boilA ing point substances in the B upper cyclone chamber. The latter inconvenience was a a avoided by including the vaporizer into a cylindrical box of mica sheet. The former was tackled by a new, simpler design (6) Figure 2. New centrifugal vapor(shown in Fig. 2), also exizer. A: axial section through ploiting a centrifugal field, plane a-a, plus front view of tanwhich generally proved satgential, tapered inlet conduit T of isfactory. Heating was genboiloff from S. B: cross section erally carried out by means through plane b-b.
Journal of Chemical Education • Vol. 75 No. 5 May 1998 • JChemEd.chem.wisc.edu
In the Laboratory
of a thermostated heat gun of the low-cost household type used to remove paint. The hot air stream was directed at a suitable angle on the side arm S. Boiling and bumping take place only here, where the temperature is slightly higher than in the main flask body M, but the amount of liquid involved is a fraction (determined by the square of the ratio of the diameters of M and S) of the total. In M only a quiet evaporation occurs. Temperature equalization is very efficient owing to the considerable transfer of both vapor and liquid from S to M, and of liquid only in the reverse direction. The lower part of S is a capillary to facilitate a steady, unidirectional transfer of liquid from M to S. The vertical part of the capillary should be a larger fraction of its overall length when most of the planned distillations will involve thermally sensitive materials (which do not tolerate hot spots); this will require heating the vaporizer by means of a stirred liquid bath. The
head supply to S is maximized by shortening its capillary part, which presents a higher resistance to heat flow. Literature Cited 1. Claisen, L. Liebigs Ann. Chem. 1893, 277, 162–205, p 177. Hansen, C. J. In Die Methode der Organischer Chemie, Vol. 1; Houben, J., Ed.; Thieme: Leipzig 1921; pp 526–594. Bowman, J. R.; Tipson, R. S. In Technique of Organic Chemistry, Vol. IV; Weissberger, A., Ed.; Interscience: New York, 1951; Chapter V, p 471. 2. Lubetkin, S. D. Chem. Soc. Rev. 1995, 24, 243–250. 3. Morrison, P. In Physics, 3rd Ed.; Serway, R. A., Ed.; Saunders College Publishing: Philadelphia, 1992; p 19. 4. Vogel, I. Practical Organic Chemistry, 3rd ed.; Longmans: London, 1957; p 116. 5. Lunelli, B. Rev. Sci. Instr. 1980, 51, 832–835. 6. Lunelli, B. Patent applied for.
JChemEd.chem.wisc.edu • Vol. 75 No. 5 May 1998 • Journal of Chemical Education
639
