Submitted by Arthur W. Weston and C. M. Suter.
Checked by C. F. H. Allen and John W. Gates, Jr..
1. Procedure
To
200 g. (1.64 moles) of benzoic acid in a
1-l. Kjeldahl flask, in the top of which is inserted a loosely fitting cold finger (Note
1), is added
500 ml. of fuming sulfuric acid (Note
2). The mixture is heated in an
oil bath for 5 hours at 240–250° (bath temperature).
After standing overnight the syrupy liquid is poured slowly with stirring into 3 kg. of ice in a
3-gal. crock (Note
3). The solution is then neutralized by adding
barium carbonate in 100-g. portions, stirring occasionally until the gas evolution slackens before each addition: 2.4–2.5 kg. is required. The pasty mass is filtered by suction on a
30-cm. Büchner funnel, and the
barium sulfate is washed with five 300-ml. portions of water. The combined filtrates are evaporated nearly to dryness on a
steam bath and finally dried in an
oven at 125–140° (Note
4). The yield of crude
barium salt is
640–800 g.
The operator should wear goggles and long-sleeved gloves during the next two operations. The dried, pulverized
barium salt is introduced in 200-g. portions into a melt of
600 g. each of sodium and
potassium hydroxides contained in a 14 by 20 cm.
copper beaker. The mixture is stirred with a
copper stirrer and the temperature determined with a
thermometer in a
copper well. (Note
5). Each portion is well stirred in before the next is added. The temperature is then slowly raised to 250–260°, at which point a vigorous reaction occurs with copious evolution of gas. After this has slackened (about 30 minutes) the temperature is raised to 280–310° and maintained there for 1 hour, then allowed to drop to 200°. The melt is ladled into 6 l. of water (Note
6) and (Note
7). The
barium sulfite is filtered by suction, and the filtrate is acidified with concentrated
hydrochloric acid (about 2.5 l. is required).
The resulting solution (about 9 l.) is divided into two portions and each is extracted three times,
600 ml. of ether being used each time (Note
8). The combined extracts are concentrated to about 1 l. and dried overnight over
150 g. of anhydrous sodium sulfate. After filtration and evaporation of the
ether, there remains
137–160 g. (
58–65%) of a slightly colored product, melting with decomposition at
227–229°. It is sufficiently pure for most purposes (Note
9) and (Note
10).
2. Notes
1. An ordinary
glass funnel suspended in the neck serves less satisfactorily, whereas with a water condenser solidification of the
sulfur trioxide causes difficulty.
3. The reaction product may solidify at first, but it dissolves later.
4. The evaporation of the solution is most conveniently done overnight. The final drying is best done in flat metal trays; the length of time required depends upon the temperature. The checkers left trays on a steam coil over a week end.
5. An
18-cm. piece of 10- to 12-mm. copper tubing, pounded together at the lower end, is used. The stirrer can be made of the same tubing.
6. The addition of the hot melt causes some spattering.
7. Alternatively, the melt may be poured into shallow metal trays and allowed to cool and solidify. It is then pulverized and dissolved. Grinding is difficult and unpleasant. The melt should not be allowed to solidify in the reaction vessel.
8. A large automatic extraction apparatus may be used if available.
9. The color can be removed by recrystallization from hot
acetic acid, using a
decolorizing carbon. A solution of 16 g. of the crude acid in
85 ml. of hot acetic acid deposits
13.4 g. of white needles after filtration through a 2-mm. layer of Darco in a hot funnel and cooling. These are anhydrous after drying at 100° at 35 mm. for 2 hours; m.p.
234–235° with decomposition (cor.). The melting point varies with the rate of heating.
10. This material is partially hydrated.
1 The melting point is unchanged after recrystallization from water.
3. Discussion
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