gdflp - 13-10-2015 at 16:36
In chemistry, especially organic chemistry, solvents are necessary for most reactions. In simple inorganic chemistry, water is a suitable solvent for
most reactions and distilled water is available inexpensively in most parts of the world. For synthetic organic chemistry however, water is quite
often an unsuitable solvent for a variety of reasons, and thus other solvents must be used.
Having a wide array of pure solvents at hand greatly increases the number of reactions which can be performed and is necessary for anyone seriously
interested in organic chemistry. Unfortunately, organic solvents tend to be quite expensive when ordered from lab suppliers, especially when a
certain quality of solvent is required, such as truly anhydrous solvents, or solvents lacking a certain common contaminant. In addition, laboratory
suppliers which carry these grades of solvents may be unwilling to sell such reagents to members of the public, and thus they must be purified by
oneself.
In most parts of the world, hardware stores carry a wide array of technical grade organic solvents for uses such as cleaning, paint removal, and fuel.
Unmixed technical grade solvents that are available are often usable in some reactions, but for more sensitive applications and syntheses, they
should be purified before use. This prevents the introduction of unnecessary impurities which could interfere with various reactions. Thus, I felt
that a thread which compiled such purification procedures would be useful. Some reactions also require solvents to be specially purified to
rigorously remove certain impurities, such as water, carbon dioxide, etc.; these purifications belong here as well.
Unfortunately, many solvents are not sold in pure forms, rather they are premixed to give the resulting combination of solvents a superior performance
in their intended application. These mixtures are often not suitable for use directly in reactions. The different properties of the solvents can be
used to separate them however, providing an economical source of a variety of organic solvents. Procedures for separation are scattered around the
board and difficult to find, thus I thought that a single thread dedicated to such procedures would greatly increase ease of finding such procedures,
and reduce the number of questions asked desiring help in separating common mixtures which have been previously discussed in other threads.
As mentioned previously, this thread is for procedures for purifying technical grade solvents, isolating solvents from OTC mixtures, and purifying lab
grade solvents to the purity needed for sensitive reactions. Some background on the solvent being purified is desirable, as well as physical
properties necessary to the separation such as boiling point, melting point, density, solubility, etc. The procedure for purification should be
repeatable, thus provide exact amounts, details on equipment and glassware used, etc. Finally, any tests of the final product which confirm its
identity, confirm the presence/absence of a certain compound, or any other aspect of the product would be nice. Pictures are unnecessary unless they
are showing a certain color change, specialized setup of equipment, etc., as we have all seen pictures of colorless liquid distilling.
Here are several to get the thread started :
Toluene
Introduction
Toluene, or methylbenzene, is a common non-polar solvent that finds many uses in organic chemistry. It is an aromatic hydrocarbon that, while
relatively inert to many things, is also a great building block for a variety of aromatic compounds. Common impurities in technical toluene are
Fe<sup>3+</sup> ions, which is especially intrusive as they catalyzes many EAS reactions, and water. Commercially, toluene is produced by
fractional distillation of various petroleum sources, such as crude oil. Thus, commercial grades of toluene often contain methylthiophene, a sulfur
containing aromatic compound which can char in many reactions and prevent isolation of the desired product. Both toluene and methylthiophene are
readily sulfonated by concentrated sulfuric acid, but methylthiophene reacts faster. This difference can be exploited to separate the two.
Procedure
450ml of technical grade toluene was placed in a flask and cooled in an ice bath to 0°C. 100ml of concentrated sulfuric acid were separately
measured out and cooled to 0°C as well. After both were chilled, the toluene was placed in a separatory funnel, and shaken with one half of the
sulfuric acid. The lower acid layer was discarded, and the toluene was washed with the second portion of the acid. This should be done as quickly as
possible to prevent the temperature of the toluene rising above 30°C. The toluene can be chilled between washings if necessary. At this point, the
toluene should be tested for the presence of thiophenes by adding 3ml of it to 10mg of isatin in 10ml of concentrated sulfuric acid. If a blue-green
coloration is produced after about a minute, the main batch should be chilled and washed with cold sulfuric acid again until this test is negative.
Unfortunately, I didn't take a picture of this test, but I will if I do this again. The toluene was then washed successively with 50ml of cold
distilled water, 50ml of sat. sodium bicarbonate solution, then finally with 50ml of water. The toluene was dried over anhydrous magnesium sulfate,
then fractionally distilled using a 300mm Vigreux column. The first 45ml of distillate are rejected, then the middle fraction was collected as
product. Finally, approximately 45ml are left in the pot. The collected toluene was placed in an amber glass bottle and stored over activated 4A
molecular sieves. This toluene is quite dry and can be used for most reactions. To dry it further, it can be refluxed over sodium metal or
phosphorus pentoxide, then distilled.
Pure toluene is a clear, colorless liquid; Boiling point 110.5°C Specific gravity 0.886
Isopropanol
Introduction
Isopropanol is the simplest 2° alcohol and is a good solvent for recrystallizing many organic compounds, as well as it's use as a precursor for
several reagents. It is sold as a disinfectant at pharmacies around the world, and is typically available as a 70%, 91%, or 99% aqueous solution.
Typical impurities present are lower alcohols, acetone, aldehydes, and obviously water.
Caution! Isopropanol should not be heated unless it has been tested for peroxides immediately prior.
Purification
1L of 70% isopropanol was placed into a flask and, with strong stirring, solid sodium chloride was added until no more would dissolve.(This requires
about 110g) A lower brine layer should form, this was removed with a separatory funnel. The isopropanol was returned to the same flask and chilled
in an ice bath.(Alternatively, start from this point with commercial 91% isopropanol) 100g of sodium hydroxide was then added slowly with stirring.
Again, a lower layer should form and this was removed with a separatory funnel. The isopropanol was returned to the flask and another 100g of sodium
hydroxide were added, chilling is unnecessary this time.(Alternatively, start from this point with commercial 99% isopropanol) The mixture was left
to stir for 10 minutes, preferably refluxing, then the isopropanol was decanted from excess sodium hydroxide and the aqueous layer, if present, was
removed.(This removes excess water, acetone, aldehydes, and carboxylic acids) The isopropanol was then fractionally distilled using a 300mm Vigreux
column and rejecting the first and last 5% of distillate. The middle fraction can be regarded as absolute isopropanol and should be stored in the
dark to minimize peroxide formation. To further dry the alcohol, 50ml of absolute alcohol were placed in a large, dried flask along with 5g of
magnesium and 0.5g of dry iodine, or several drops of CCl4, and the mixture was refluxed until the magnesium dissolves. 900ml of absolute
alcohol were then added and the mixture was refluxed for 30 minutes, then distilled through a dried distillation apparatus, under a dry atmosphere,
directly into its storage container.
Pure isopropanol is a clear, colorless liquid; Boiling point 82.6°C Specific gravity 0.786
Dichloromethane
Introduction
Dichloromethane is a useful laboratory solvent, particularly for extractions as it is easily removed due to its volatility, yet its not flammable. It
is not widely available in a technical grade from hardware stores, however it is quite often available in mixtures as paint thinner. This is
typically a mixture of dichloromethane, methanol, and "petroleum distillates" to thicken the mixture to a gooey consistency, and is a good source of
the solvent as it is present in quite a high concentration.
Isolation
A 1 liter flask was charged with 750ml of paint thinner(see MSDS below) and several grams of pumice were added. The mixture was distilled until the
stillhead temperature rose to 50°C, then the apparatus was allowed to cool. Without cleaning out the stillpot, it was refilled to the 750ml mark
with fresh paint thinner and the mixture was distilled again until the stillhead temperature rose to 50°C. This was repeated until the flask
contained 500ml of goo, approximately 3 times in total. The dichloromethane collected, about 1 liter, still contains methanol, so it was split into
two portions and each portion was washed twice with 250ml of distilled water. The dichloromethane was recombined and approximately 800ml of crude
dichloromethane remained.
Purification
The crude dichloromethane from above was dried over 60g of anhydrous calcium chloride, then again split into two portions. Each was washed with 50ml
of concentrated sulfuric acid, then with 50ml of water, sat. sodium bicarbonate solution, then finally with water. Finally, the dichloromethane was
dried over an excess of anhydrous calcium chloride and fractionally distilled using a 300mm Vigreux column, rejecting the first and last 5% and, if
necessary, collecting the portion boiling between 40°C-41°C.
Pure dichloromethane is clear, colorless liquid; Boiling point 40.0°C Specific gravity 1.325
References
1. Armarego, Wilfred L. F.; Chai, Christina L. L. Purification of Laboratory Chemicals 6<sup>th</sup> Edition pg. 132,
152
2. Vogel, Arthur I. Practical Organic Chemistry 3<sup>rd</sup> Edition pg. 163-178
Attachment: Dichloromethane MSDS.pdf (36kB)
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Magpie - 14-10-2015 at 08:02
When on vacation in Boston I wandered into the organic chemistry section of the Mallinckrodt laboratory at Harvard University. Against one wall I saw
about 5 large distillation columns in operation. A technician was purifying various solvents for reuse. I had never seen anything like this before.
Purifying/stabilizing recycled chloroform can be important to the home chemist. After making PCl5 I treated the chloroform solvent as follows: (1)
washed with sat aqueous NaHCO3, (2) dried with CaCl2, (3) distilled, and (4) added 1% ethanol as stabilizer.
I think the treatment used is somewhat dependent on how the solvent was used.
[Edited on 14-10-2015 by Magpie]