Submitted by C. S. Marvel
1. Procedure
The
ether solution is filtered through a dry
filter paper into a
5-l. round-bottomed flask which is fitted with an
efficient mechanical stirrer and immersed in an
ice-salt bath. When the temperature of the solution has fallen to 0° (Note
2), a rapid stream of dry
ammonia, from a cylinder of the compressed gas, is passed into the solution.
After about fifteen minutes, the theoretical amount of freshly distilled
n-butyl nitrite (95 g. for every
100 g. of phenylhydroxylamine) is added slowly through a
dropping funnel (Note
3) and (Note
4). The addition of
butyl nitrite usually requires about one hour (Note
5), during which time the stream of
ammonia gas is continued in order that
ammonia may always be in excess. If this precaution is not observed, a colored product will result. The temperature of the reaction mixture should be maintained below 10° and this may be done best by controlling the rate at which the
butyl nitrite is added (Note
6). An appreciable rise in temperature will cause the volatilization of considerable quantities of
ether and of
ammonia (Note
7).
After the
butyl nitrite has been added, the reaction mixture is stirred for about ten minutes longer in order to insure completion of the reaction, after which the
cupferron is filtered off and washed several times with small portions of fresh
ether. The product is spread on sheets of paper until all traces of
ether have been lost, and is then stored in bottles where it is exposed to the vapors of
ammonium carbonate. This may be done by protecting each
cork with a double sheet of filter paper and placing a lump of
ammonium carbonate between the cork and the filter paper (Note
8).
2. Notes
1. The solvent
ether may be replaced by
benzene, but this modification offers no advantages for the preparation of
cupferron on a laboratory scale.
2. The temperature must be kept low. If it is not, the material is generally colored brown and the reaction does not run smoothly. Probably the most important factor in securing successful results is always to have an excess of
ammonia present.
3. The
butyl nitrite is freshly distilled as a general precaution, because a product which has stood for some time is often partially decomposed.
4. It is suggested that gaseous
ethyl nitrite be passed in until the precipitate ceases to increase (W. W. Hartman, private communication). However, it is probable that
butyl nitrite is more convenient for a laboratory method. A method for the preparation of
ethyl nitrite is described in Org. Syn.
10, 22.
5. In the preparation of
cupferron, it has been recommended that the
butyl nitrite be added all at one time. This procedure is satisfactory only when the amount of
phenylhydroxylamine used is less than 200 g.; otherwise the reaction becomes extremely vigorous and an excessive proportion of
ether is lost. The directions given above, on the other hand, are adaptable for the preparation of large quantities of
cupferron. For the rapid preparation of small quantities of material, the
butyl nitrite may be added all at one time, provided sufficient excess of
ammonia is present.
6. If
ammonia is kept in a definite excess at all times, a rise of temperature to the boiling point of
ether appears to do no harm. The lower temperature was used because it makes possible a more rapid addition of the
nitrite.
7. It is found that
75 per cent of the ether and
95 per cent of the butyl alcohol used may be recovered, and that one person, working six to seven hours, is able to prepare
800 g. of
cupferron. The recovered ether may be used over again, provided the following procedure is observed: The
phenylhydroxylamine is dissolved in a little fresh ether, the solution cooled and treated with
ammonia, then the recovered ether added. This precaution is necessary owing to the presence of some
butyl nitrite in the recovered ether.
Cupferron has been kept in
dark bottles sealed with viscose caps for a year without decomposition.
3. Discussion
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