One of
the non-obvious outcomes of structural identification using spectroscopy
(particularly NMR and MS) is the decrease in experience with distillation,
among organic synthetic chemists.
Because even an inexperienced student researcher can now routinely
identify a substance using milligrams of pure compound, flash chromatography
high performance liquid chromatography or preparative gas chromatography, these can
replace old-fashioned distillation for making samples for identification in
most steps in a laboratory. Corroborating evidence of this trend is the virtual
disappearance of boiling point as part of physical characterization in the
chemical literature.
Finally,
as the catalogues of suppliers of chemical intermediates become thicker, more
of the early steps in syntheses can simply be purchased. It is these lower
molecular weight entities that were formerly prepared and then distilled in the lab.
Standard
distillation has an inherent problem that became a further reason to abandon this methodology in the laboratory. Unless a distillation column receives an input of
heat, which at small scale is usually 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 wasted 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, 200ml, 500ml, 1 L 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 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 becomes very important. This
is not usually allowed for.
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
centimetres of that side arm 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 vapours from the distilling through neck and side arm 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 for 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. When used in a distillation assemblage the vacuum is applied through the length of the condenser down to the boiling liquid surface. Because the
Liebig condenser tube is both long and narrow, it must add a large pressure drop to
the reading of the vacuum gauge at the receiver. Low pressure distillation is
impossible.
Another problem with distillation is bumping. 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 flasks with ground glass joints the glass wool must be inserted carefully to make sure that no wool
strands get trapped on the ground glass joint where it will destroy the vacuum
seal.
When
magnetic stirring is used anti-bumping devices are usually not needed unless the
stirring fails.
When
performing a fractional distillation in a packed column some people do not
realize the importance of a near-perfect vertical positioning of the column
above the flask. Fractionation is achieved by the equilibration of rising
vapors and the descending liquid film and that equilibration is a function of
the surface area and thickness of that film. If the column is tilted the
returning liquid is not spread evenly over all the walls and packing and where it does
run it is in a thicker, less effective layer. In a tilted fractionating column
the height equivalent of a theoretical plate is longer so there is less rectification.
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