Guidelines for care & Use of Dry Solvent Stills [Example]

The oxidation potential of sodium is such that it can reduce water and liberate hydrogen gas
concomitantly forming sodium hydroxide. In the presence of alcohols the corresponding sodium alkoxide is
formed. In the absence of water and alcohols (the most likely contaminants) sodium metal can donate an
electron to the carbonyl group of benzophenone to form the resonance stabilized ketyl radical.
Ketyl radical is a very sensitive indicator for traces of moisture and oxygen(a diradical in the
ground state) and the blue color will occasionally be lost during the normal usage of the still due to
consumption of benzophenone ketyl by the above contaminants. The blue color can be returned in one of
the following ways:
i) addition of further benzophenone and continued refluxing until dry.
ii) crushing the sodium with a glass rod to expose a clean, reactive surface.
iii) starting the still up from scratch. This will periodically occur when the amount of reduction
products in the distillation flask build up beyond a certain level or if some idiot pours ether in the
still.
To start the still up from scratch
First destroy any remaining sodium in the flask by slow addition of ethanol to the residue
remaining in the flask. This is best carried out in a fume hood as hydrogen will evolve, the flask will grow
warm and ether may be boiled off. the reaction may be slowed by dilution with hexane. Once rapid
evolution of hydrogen has ceased cautiously add water to make certain that all the sodium has been
destroyed. Clean out and dry the flask. Refill the flask with pre-dried ether and then add sodium ribbon.
Reflux the ether for several hours to make sure it is thoroughly dry before adding benzophenone.
THF
Instructions for the use of the THF are still the same as for the ether still except that the pre-dried
THF bottle (kept in the solvent cupboard under the oven) may be filled with regular grade solvent from the
blue Fisher can in the solvent room.
Hexane
As with the ether and THF, hexane is dried over and distilled from sodium using benzophenone as
its indicator. Because of the low solubility of benzophenone in this solvent it is necessary to add
approximately ten milliliters of dry triglyme (a high boiling point poly-ether) as a co-solvent so that the
strong color reaction is observed. The blue color of the ketyl radical may at times be lost when the still is
left to stand at room temperature because of this solubility problem but the color should return upon
refluxing for a short while.
Hexane does not absorb a lot of water and the regular grade of hexane found in the blue cans in the
solvent room can be used to top up the pre-dried solvent bottle.
Dichloromethane
Halogenated solvents should never be dried over sodium. This could cause the formation of
carbenes.
Dichloromethane is dried by distillation from calcium hydride (kept in a desiccator above the
balances.) There is no need to pre-dry dichloromethane for use in the still, so the still may be topped up
with solvent directly from the can. Never use sodium metal as a drying reagent for Halogenated solvent
(Carbenes are formed).
Sodium Ribbon
As noted before, sodium ribbon is used to dry ether, THF, and hexane because of its large surfaceto-volume
ratio and the high reactivity of the fresh sodium face which is exposed. Sodium ribbon is formed
by use of a sodium press (kept in the bottom corner cupboard under the stills.)
Secure the body of the press assembly to the edge of a work top with two clamps provided. The
movable barrel of the die screws on to the notched vertical bar in the press assembly and is moved up and
down by means of the lever at the right hand side.
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