One of the non-obvious outcomes of structural identification using spectroscopy (particularly NMR and MS) is reduced experience with distillation among organic synthetic chemists. Because even an inexperienced student researcher can now routinely identify a substance using milligrams of a pure compound, using flash chromatography, and then high-performance liquid chromatography or preparative gas chromatography can replace old-fashioned distillation for making samples big enough for identification of the products from most steps in laboratory synthesis. Corroborating evidence of this trend is the virtual disappearance of boiling point as part of physical characterization in the chemical literature.
Finally, as the
catalogs of suppliers of chemical intermediates become thicker, more of the products of early steps in syntheses can simply be purchased. It is these lower molecular
weight entities that formerly were prepared and distilled in the lab.
Standard
distillation has an inherent problem that became a further reason for the substantial abandonment of distillation from the laboratory. Unless a small scale distillation column receives an input of
heat supplied by vigorously boiling the liquid
mixture in the still pot it cannot achieve liquid-vapor equilibrium. Thus, on
the lab scale, there is a hold-up of distillate that is inevitably lost and this
can be up to about 30%. Compounding this inherent difficulty is the annoyance
that all glass laboratory distillation equipment is expensive and does not
easily accommodate the particular amount of crude that you may have. That is,
the amount of crude distillate must be selected to fit the size of the physical
assembly that you have and not the other way round. Fractional distillation
assemblies are not available in your lab drawer in 100 ml, 200 ml, 500 ml, 1L, 5L, and 15L sizes like round bottom flasks are!
The days when distillation units were patched together with hardened cork or rubber stoppers between pieces of blown glass are long past. Now all glass assemblies are a single piece or pieces joined with ground glass joints.
Because of this, now more than ever,
distillation assemblies for vacuum distillation often use the same equipment as
for simple distillation and lab workers don’t appreciate the special requirements imposed
by the low-pressure condition. The boiling point of
the fluid mixture in the still pot of a distilling assembly depends upon the pressure at the surface of the liquid, not the pressure recorded on a pressure
gauge, which may be, and usually is, closer to the vacuum pump. For pressures from
760 mm down to 15 mm of mercury, a regular distillation flask is satisfactory.
For pressures below this level, and particularly pressures 2 mm or less, the
diameter and location of the vapor port linking the distillation portion of
the apparatus to the condensing portion become very important. This is not
usually understood.
The increment in
vapor pressure at the surface of the boiling liquid, over and above the vacuum
pressure reading taken at the receiver is proportional to the length and
inversely proportional to the fourth power of the diameter in centimeters of
that sidearm plus any other narrow portion of the path between still pot and
condenser.
As Hickman, inventor of a famous low pressure still, pointed out many years ago, an experimenter may go to great lengths to produce a vacuum less than 1 micron, yet fail to benefit properly from his/her efforts because the pressure necessary to drive vapors distilling through neck and sidearm is from 1 to 4 mm. The factor limiting the available vacuum is often the distilling assembly shape, not the quality of the vacuum pump or vacuum pump oil. Take as a simple example a vacuum distillation using a Liebig condenser attached by a ground glass joint to a simple distillation flask. A Liebig condenser has a narrow bore tube running inside a wide-bore tube that serves to supply condenser water to the outside of the narrow bore tube. The vacuum, in this example, would be applied through the length of the condenser down the height of the distilling flask and column to the boiling liquid surface. Because the Liebig condenser tube is both long and narrow, it will necessarily add a large pressure to the reading of the vacuum gauge at the receiver. Low-pressure distillation therein is impossible.
As a consequence of understanding the above, with low pressures where the pressure drop in the apparatus is damaging the effective rate of distillation, tipping the entire apparatus to the side can actually help by reducing the height that the gas must be driven to reach the sidearm! This amounts to a patch when you are stuck with inadequate apparatus for a low-pressure distillation.
Regarding another major concern: bumping from super-heating of the still pot liquid is a great time waster and many solutions have been offered. When distilling at atmospheric pressure boiling chips can be used or a bleed from a glass capillary, but the former fails under vacuum and the latter adds to the pressure and is really co-distillation with the gas being bubbled. A very old but effective solution is to place glass wool into the flask so that it is partly above the liquid surface. Using this method magnetic stirring is not possible and an oil bath is the preferred source of heat to avoid over-heating at the flask wall. When using a flask with ground glass joints, the glass wool must be inserted carefully making sure that no wool strands get trapped on the ground glass joint where they will destroy the vacuum seal.
When magnetic stirring is used anti-bumping devices are usually not needed unless the stir bar gets out-of-sink with the magnet inside the stirring motor.
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