Alexander Shumski
Fractional Distillation
Definition Type: Process Definition
Sentence Definition
Fractional distillation is a process in which a solution containing two or more chemically distinct
compounds with boiling points separated by a difference of less 25°C are separated by heat
induced vaporization followed by condensation.
Extended Definition
Fractional distillation is used to separate organic solutions with boiling points that are too close
to allow for a simple distillation. In practice, this will mean that the constituents of the solution
have boiling points within 25°C of each other. The tendency of these solutions to boil rapidly in
sequence means that the individual “fractions” of pure liquid are collected during the brief
periods when only one of the components is boiling.
Restrictions
There are several conditions in which fractional distillation is rarely or never used, due to a
combination of infeasibility or safety concerns. For instance, solutions that may contain
peroxides are only distilled under highly controlled conditions due to the threat of explosive
reactivity when purified. Certain solutions may also form azeotropes; mixtures that vaporize
together at a specific temperature, forming a vapor with roughly the same composition as the
original solution. If an azeotrope is produced any collected fractions will be comprised of a
mixture very close to the intensive (not reliant on total quantity) azeotropic proportion of the two
components in solution. As the fractional distillation process relies on obtaining separate, pure
fractions, full separation does not occur under for these solutions.
Another condition preventing fractional distillation is the potential reactive behavior of the
mixture. For instance, carboxylic acids and alcohols will undergo Fischer esterification if
sufficiently heated, producing esters. Many organics are heat reactive in some way, with the
most extreme cases resulting in flash ignition or explosive gas evolution. As a result, the
composition is always the primary restriction of what can and cannot be separated safely.
Fractional distillation is almost always performed to purify miscible organic materials due to the
low boiling points and common occurrence of experimental mixtures. It is a fast alternative to
the more thorough separation provided by column chromatography when the components are
known and the mixture is composed of a small number of miscible liquids. If the two solutions
differ sufficiently in polarity or acid-base behavior, it is usually easier to perform an extraction,
adding another limitation to the usefulness of fractional distillation.
Process
The process by which a solution is separated begins with an apparatus constructed with common
organic laboratory glassware. The integral components of a fractional distillation column are a
round bottom flask or glass bulb, a fractionating column, a condenser, and a heat source. While
the layout may vary, all of these components are required for the distillation to occur. A diagram
of a fairly typical burner based column can be seen below.
The specific type of heat source is less important
than the method of controlling the rate at which
heating occurs. In the example, the heat source is a
Bunsen burner; when the distillation is performed
on more delicate solutions, the heat source is
usually a controlled oil or sand bath. No matter
what the heat source, the device must be capable of
safely reaching a temperature higher than that of
the second highest boiling point. At the bare
minimum, the heat source needs to vaporize all
other substances before the solution is a pure liquid;
the final component does not need to be vaporized
as it will be pure after the removal of all other
fractions.
The solution heats up to a controlled boiling point
and generates a mixture of vapors due to a
combination of the vaporization caused by boiling
and the increasing vapor pressure of the other
components. The mixture of vapor rises up the fractionating column, and the compounds with
higher boiling points condense more quickly as the gas cools. As a result, only the compound
with the lowest boiling point will reach the condenser. Once the boiling hot vapor hits the
condenser, it is cooled by water running through the external compartment of the tube and the
pure liquid is collected and seperated as the first fraction.
The solution gets boiled away complete and the process repeats for the next component of the
solution. As the goal of the process is to separate the mixture, it goes without saying that each
liquid is captured in a separate container. If everything was done correctly, these fractions are
pure and ready to be analyzed or used in other reactions. Sometimes the distillation is insufficient
or the procedure was performed incorrectly, requiring a secondary purification method. In these
cases, the distillation may be repeated or an alternative method of purification used to remove
minor impurities.
Microscale Version
Fractional distillations can also be performed in microscale, using a greatly simplified version of
the above set-up. The glass bulb is exchanged for a microscale test-tube, and a pipet becomes
both the fractionating column and the condenser by systematically sucking up vapor and
depositing liquid as fractions. This method is harder to control and yields only a small quantity
of usable material, making it superior for purely analytical processes.
Conclusions
While it appears to be a fairly complex process, fractional distillation boils down to several
relatively simple steps that are repeated for multiple components in solution. First, the solution is
raised to the boiling point of the most easily vaporized substance. That substance is then boiled
off, and separated from excess vapor through the fractionating column. The hot vapor is then
cooled in the condenser by running water, and the fraction collected. Once the solution stops
boiling, the fraction container is changed out for a new one, the solution heated to the next
boiling point, and the process repeated until only pure substances are obtained.
Recognize that fractional distillation is only one of many tools available to you when attempting
to separate a homogenous solution and that all possible options should be considered before any
separations are attempted. Due to the risk of lost product, distillations of this kind are almost
never done on an entire sample, and a small to moderate amount of the crude solution saved as a
backup. Much in the same way that samples are kept safe, the safety of the people performing
these experiments is always top priority. Before and after every successful fractional distillation,
the components are handled carefully and any unwanted results removed via proper laboratory
waste disposal systems.