                               Mandelic Acid
    
This a-hydroxy acid is prepared from benzaldehyde through the cyanohydrin,
which is produced by interaction of the benzaldehyde bisulfite addition
compound with potassium cyanide. This interchange reaction eliminates the
hazard of working with volatile, toxic hydrogen cyanide, but even potassium
cyanide is dangerous if safety measures are not observed. The interchange
reaction is reversible and excess cyanide is used to shift the equilibrium
in favor of the cyanohydrin. Mandelonitrile, a liquid, undergoes changes on
standing and hence should be processed further without delay; it is
extracted with ether, the solution is carefully washed free of cyanide ion,
and the nitrile is then hydrolyzed with hydrochloric acid. An intermediate
ketimine hydrochloride, C6H5CH(OH)C=NH*HCl soon gives way to mandelic acid
and ammonium chloride, both of which are soluble in the aqueous acid. The
solution is cooled and extracted with ether, and the ether is then
displaced by benzene for crystallization, since mandelic acid is much less
soluble in this solvent than in ether.

                                 EXPERIMENT

1. Caution

Potassium cyanide is extremely poisonous if it gets into the blood through
a cut, if transferred from the hand to the mouth, or if it comes into
contact with acid and liberates hydrogen cyanide. Never touch the solid
with the fingers; be very careful not to spill it and if you do clean it up
at once. Take care not to spill a solution or reaction mixture containing
the substance, and discard the waste solution directly into the drain and
wash down the sink with excess water.

2. Procedure

In a 125 ml Erlenmeyer flask dissolve 11 g of sodium bisulfite in 50 ml of
water by brief swirling, add 10 ml. of benzaldehyde and swirl vigorously
and stir until the oily aldehyde is all converted into the crystalline
bisulfite addition compound. Cool to room temperature but not below, add 14
g. of potassium cyanide (caution) and 25 ml. of water (rinse down the
walls), and take the mixture to a hood. Swirl and stir for about 10 min.
until all but a trace of solid has dissolved (break up lumps with the rod).
Mandelonitrile separates as a thick oil. Pour the mixture into a separatory
funnel, rinse the flask with small amounts of ether and water (and at once
wash the flask free of cyanide), and then shake the mixture vigorously for
a full minute to insure complete reaction. Add 50 ml. of ether, shake, and
run the aqueous solution down the drain. Wash the ether extract with 25 ml.
of water and then with 25 ml. of saturated salt solution (note and account
for the difference in appearance of the ether layer). Then run the solution
into a 125 ml distilling flask containing 15 ml. each of coned.
hydrochloric acid and water. Add a boiling stone, stopper the flask, and
mount it on a steam bath for distillation through a condenser into an
ice-cooled receiver. Distill off the ether and then disconnect the
condenser, note the time, and continue heating on the steam bath with
frequent swirling to mix the layers and promote hydrolysis. Note that the
initially lighter-than-water layer of mandelonitrile gradually changes to
an oil of density greater than that of the aqueous acid. In a little less
than 1 hour the oil dissolves to a clear solution; however if the solution
is cooled at this point it will become cloudy from separation of
unhydrolyzed oil still present. Hence continue heating for one half hour
more to complete the reaction and then cool to room temperature. Measure
100 ml. of benzene into a 250 ml. distilling flask and make a mark at the
level of the liquid. Transfer the acid solution to a separatory funnel and
rinse the flask with a little ether (5 ml). Then add to ml. of ether, shake
well, let the layers separate, and draw off the aqueous layer into a second
separatory funnel or a dry flask, and wait a few minutes for separation of
a little more aqueous solution. Run the ethereal extract into the flask
containing benzene, and reextract the aqueous solution with two further 20
ml portions of ether, and add the extracts to the flask containing benzene.
Add a boiling stone, connect the flask to a condenser and an ice-cooled
receiver, put a thermometer in place, and distil the solvent, cautiously at
first since the flask is very full. Water is eliminated gradually by
azeotropic distillation and the boiling point rises as ether and water are
eliminated. Continue the distillation until the solution has cleared, water
is no longer evident in the neck of the distilling flask, and the volume of
the solution has been reduced to the 100 ml. mark. Disconnect the flask and
inspect the bottom to see if a trace of ammonium chloride has separated, as
either a solid or a gum, make sure that no flames are near, and decant the
hot solution into a  250 ml. Erlenmeyer flask. Crystallization usually
starts soon; yield 10.0 gram, mp 118-119 deg C. In case no crystals appear,
pour the solution into a 25 ml. distilling flask, evaporate the solvent,
scratch to induce crystallization, and digest the solid with benzene.

Note. When mandelic acid that has been crystallized from benzene is allowed
to stand in contact with the mother liquor for several days, the needles
gradually change into granular, sugarlike crystals of a molecular compound
containing one molecule each of mandelic acid and benzene. These crystals
are stable only in contact with benzene and at temperatures below 32.6 deg
C on exposure to the air the benzene of crystallization evaporates and the
mandelic acid is left as a white powder.
