Distillation for Compound Identification: Boiling Point Determination
The organic teaching labs employ distillation routinely, both for the identification
and the purification of organic compounds. The boiling point of a compound –
determined by distillation – is well-defined and thus is one of the physical properties of a
compound by which it can be identified. Distillation is used to purify a compound by
separating it from a non-volatile or less-volatile material. Because different compounds
often have different boiling points, the components often separate from a mixture when
the mixture is distilled.
The boiling point is the temperature at which the vapor pressure of the liquid
phase of a compound equals the external pressure acting on the surface of the liquid. The
external pressure is usually the atmospheric pressure. For instance, consider a liquid
heated in an open flask. The vapor pressure of the liquid will increase as the temperature
of the liquid increases, and when the vapor pressure equals the atmospheric pressure, the
liquid will boil. Different compounds boil at different temperatures because each has a
different, characteristic vapor pressure: compounds with higher vapor pressures will boil
at lower temperatures.
Boiling points are usually measured by recording the boiling point (or range) on a
thermometer while performing a distillation. This method is used whenever there is
enough of the compound to perform a distillation. The distillation method of boiling point
determination measures the temperature of the vapors above the liquid. Since these
vapors are in equilibrium with the boiling liquid, they are the same temperature as the
boiling liquid. The vapor temperature rather than the pot temperature is measured
because if you put a thermometer actually in the boiling liquid mixture, the temperature
reading would likely be higher than that of the vapors. This is because the liquid can be
superheated or contaminated with other substances, and therefore its temperature is not
an accurate measurement of the boiling temperature.
If you are using the boiling point to identify a compound which you have isolated in
the lab, you will need to compare its boiling point with that of the true compound.
Boiling points are listed in various sources of scientific data.
If you look up the boiling point of a compound in more than one source, you may find
that the values reported differ slightly. The literature boiling point depends on the method
and ability of the technician taking the boiling point, and also on the purity of the
compound. While theoretically all boiling points should be constant from source to
source, in reality the reported boiling points sometimes vary. Therefore:
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Always reference the source of the physical data which you write in your lab
report.
Distillation for Compound Purification
Simple distillations are used frequently in the organic chemistry teaching labs. They are
useful in the following circumstances:
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the liquid is relatively pure to begin with (e.g., no more than 10% liquid
contaminants)
the liquid has a non-volatile component, for example, a solid contaminant
the liquid is contaminated by a liquid with a boiling point that differs by at least
70°C
“Simple” distillation may be a miss-leading term to the beginning organic chemistry
student, since it takes a lot of practice in simple distillation to become proficient in this
technique. It is especially important to do a perfect simple distillation when determining a
boiling point for identification purposes.
How to do a Simple Distillation
1) Set up the distillation apparatus as demonstrated in the video. Make sure that the
thermometer is at the correct height.
Distillation: placement of thermometer in the Y­adaptor The "bulb" part of the thermometer is positioned near the side arm of the still head so that
it monitors the temperature of the vapors. See the illustrations below.
Too High
Too Low
Just Right
2) Place a sample of a volatile organic substance in the distillation flask.
Distillation: filling the flask At the beginning of the distillation, the distillation pot should be between one-half and
two-thirds full. If the pot is too full, the surface area is too small for rapid evaporation
and the distillation proceeds very slowly. If the pot is not full enough, there will be a
large holdup volume and loss of sample. Holdup volume is the amount of vapor in the
flask and head along with the liquid required to wet the inner walls of the apparatus. A
typical holdup volume is one or two milliliters, which can lead to significant product loss,
especially in a small scale experiment.
If you fill a flask and find it either too full or not full enough, choose another size flask
from your lab drawer. The following photos show improper and proper levels of liquid in
a distilling flask.
Too Low
Too High
Just Right
3) Heat the liquid slowly so that only small amounts of liquid flows over into the
collection vessel. If at some point there is a lot of liquid flowing over to the collection
vessel, the boiling point reading will be too high and the liquids will not be as pure.
4) As drops begin to fall from the thermometer and a small but steady amount of
liquid begins to flow into the collection flask, read the temperature. If it is well below the
boiling point you expected, switch to a fresh collection flask when the head temperature
is closer to the correct boiling point (within 3 degrees).
5) Watch the temperature as the distillation progresses. A steady temperature means
that you have heated properly, and the substance is as pure as it can be for a simple
distillation. If the head temperature falls drastically and then increases to above the
previous high temperature, a new substance is starting to com off the column. You need
to change the collection vessel again.
Fractional and Vacuum Distillations
Mixtures of liquids whose boiling points are similar (separated by less than 70°C) cannot
be separated by a single simple distillation. In these situations, a fractional distillation is
used.
Vacuum distillation is distillation at a reduced pressure. Since the boiling point of a
compound is lower at a lower external pressure, the compound will not have to be heated
to as high a temperature in order for it to boil. Vacuum distillation is used to distill
compounds that have a high boiling point or any compound which might undergo
decomposition on heating at atmospheric pressure. The vacuum is provided either by a
water aspirator or by a mechanical pump.
Always check for star cracks in the flasks before beginning a vacuum distillation.
Photos of star cracks in round bottom glassware:
Quick link to photos of fractional distillation set-up:
Fractional Distillation Set­up The photo below is of a fractional distillation set-up. The only difference between this
set-up and that of a simple distillation set-up is the inclusion of a fractionating column
(see close-up below) between the round bottom flask and the Y-adaptor. In the
illustration, the fractionating column is a regular condenser filled with glass beads. Note
that you do not hook the tubing connectors on this fractionating column/condenser to
tubing and water.
The photo below is of two different styles of fractionating column. The upper one is an
ordinary condenser filled with small glass beads. This is the style you will use in the
organic chemistry teaching labs. The lower one is a Vigreux column.