Submitted by F. C. Whitmore and Marion G. Whitmore.
Checked by H. T. Clarke and J. H. Bishop.
1. Procedure
To a mixture of
500 g. (5.3 moles) of chloroacetic acid and 500 g. of cracked ice, is added, with stirring, just sufficient cold
40 per cent sodium hydroxide solution to make the solution faintly alkaline to
phenolphthalein. About 360 cc. is required; the temperature should not rise above 20° (Note
1). The solution is then mixed with
365 g. (5.3 moles) of sodium nitrite dissolved in 500 cc. of water and heated in a
3-l. round-bottomed flask fitted with a
two-holed stopper containing a
bent tube of large diameter connecting with an
efficient condenser (set downward for distillation) and a
thermometer dipping into the liquid. The
receiver should be so arranged that it can be cooled, if necessary, by a stream of water.
The solution is heated slowly until the first appearance of bubbles of
carbon dioxide, which occurs when the temperature has reached about 80°. The flame is then removed and the reaction allowed to proceed by itself (Note
2). If no rise in temperature occurs, heat is very cautiously applied until the temperature rises to 85°, when the flame is again removed (Note
3). At this temperature the exothermic decomposition of the
sodium nitroacetate becomes so rapid that the temperature rises almost to 100° without further application of external heat. If heat is applied after the temperature of the liquid reaches 85°, violent frothing will occur, with serious loss of
nitromethane. If the reaction becomes too vigorous it may be checked by applying a wet towel to the flask.
Nitromethane starts to distil over at about 90°. During the spontaneous heating, about
120 cc. of nitromethane distils over, accompanied by about 170 cc. of water (Note
4). This water is saved for redistillation.
When the mixture ceases to maintain its temperature spontaneously at 95–100°, heat is applied cautiously until the temperature reaches 110°. About
13 cc. of nitromethane and 200 cc. of water distil over. Further distillation gives water which contains too little
nitromethane to pay for recovery. At this point enough
sodium chloride and
sodium carbonate have separated to cause serious bumping.
The water separated from the
nitromethane distillates is mixed with one-fourth of its weight of salt, and redistilled, yielding
10–12 cc. of nitromethane and about 100 cc. of water. This water on distillation gives
3–4 cc. of nitromethane and about 45 cc. of water (Note
5).
The total yield of moist product is
125–135 g.; on drying with a little
calcium chloride and distilling,
115–125 g. of pure
nitromethane, boiling at
98–101°, is obtained (
35–38 per cent of the theoretical amount). A small forerun distils over below 98°, and a little dark brown residue remains (Note
6).
2. Notes
2. The thermometer dipping
in the liquid is absolutely necessary, as the success of the preparation depends on proper temperature control. The vital point of the whole preparation is to remove all external heat as soon as the reaction is well started. This is between 80 and 85°. No frothing ever occurs when this precaution is rigidly adhered to.
3. Several of these distilling sets can be set up and run by one operator, if each lot is started only when the preceding one has reached 85° and external heat has been removed. The corresponding water layers from different sets can be united and distilled at once.
4. It has been recommended
1 that use be made of a
gravity separator connected with the
adapter in order to separate the
nitromethane as fast as it condenses. It was found inadvisable to use such a separator, as the
nitromethane is so little heavier than water that a rapid separation is not possible; drops of
nitromethane remain suspended and pass over with the water. It is best to allow the distillates to stand in
cylinders
for at least one-half hour before separating the
nitromethane by means of a
separatory funnel. During this standing, as much as
2 per cent more of
nitromethane sometimes settles out.
5. If the time consumed is an important consideration, it is well to note that the spontaneous heating from 85° to 100° gives three-fourths of all the
nitromethane obtained in the preparation. This takes less than one hour. The further heating above 100° and the distillations of the water layers take over two hours, and give only one-fourth of the total yield.
6. It is suggested that improved yields are obtainable when
374 g. of crystalline boric acid is added subsequent to the
sodium nitrite and prior to the heating. In this way,
carbon dioxide together with some oxides of nitrogen are evolved at about 45°. The reason offered for the higher yields is the destruction of
sodium bicarbonate by the
boric acid, which thereby prevents alkaline hydrolysis of the
sodium chloroacetate (C. L. Tseng, private communication;
2).
The yield is somewhat improved by using
625 g. of chloroacetic acid and about
450 cc. of 40 per cent sodium hydroxide solution instead of the 500 g. and 360 cc., respectively, now specified (H. Adkins, private communication;
2).
3. Discussion
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