Weygand/Hilgetag Preparative Organic Chemistry

478 Formation of carbon-nitrogen bonds
Alkyl azides are usually prepared by warming alkyl iodides or bromides with
an excess of sodium azide in an organic solvent. 661 Glycol ethers are recommended
as solvents for the simple alkyl azides (C3-C10). 662
Pentyl azide: 662 A mixture of activated sodium azide 663 (27 g), diethylene glycol monoethyl
ether ("Carbitol") (450 ml), and water (75 ml) is placed in a 1-1 three-necked flask
fitted with a reflux condenser, thermometer, and stirrer; the mixture is stirred while pure
pentyl iodide (60 g) is added in one portion. The solid dissolves within a few minutes. Whilst
being continuously stirred, the mixture is heated during 3 h to 95° and then kept at that
temperature for 20 h. It is then cooled and poured in two portions into separate portions of
ice-water (each 800 ml). The aqueous phases are separated and extracted twice with ether
(200-ml batches). The ether solutions are united with the previous organic phases, and the
ether is evaporated. The residue (42 g) is fractionated in a vacuum, giving the azide, b.p.
77-78°/112 mm, nD 20 1.4266 (27.8 g, 81.4%).
Further examples, where other solvents are used, are to be found in the literature. 664 " 666
Carbonyl azides are usually obtained from the hydrazides, although in
principle they can also be prepared directly from the carbonyl chlorides and
sodium azide. They are frequently used as intermediates for synthesis of isocyanates,
urethanes, ureas, amines, etc., and are often used in such work
without isolation.
They can be prepared either in an inert organic solvent or, particularly when
from an aromatic acid chloride, in an aqueous-organic medium. Cooling is
necessary to avoid decomposition of the azide to the isocyanate.
fw-Nitrobenzoyl azide: 667 An ice-cold, concentrated aqueous solution of sodium azide
(20 g) is stirred into a cooled acetone solution of /w-nitrobenzoyl chloride (50 g). The azide
crystallizes when the mixture is kept in the ice-bath. Purified by dissolution in acetone and
precipitation by water, it melts at 68° (yield 53 g).
Ethyl azidoformate: 668 A solution of ethyl chloroformate (50 g) in pure ether (50 ml) is
dropped, with stilting, into a solution, cooled in ice, of sodium azide (35 g) in distilled
water (190 ml). Stirring is continued for a further 2.5 h with continued cooling, then the
ether phase is separated, the aqueous phase is extracted with ether, and the combined ether
solutions are dried over sodium sulfate. The ether is distilled off at atmospheric pressure and
the residue is distilled in a vacuum through a short Vigreux column (87.4%; b.p. 39-41°/
30 mm). Before use the product should be distilled again in a vacuum; it then has nD 25 1.4180.
It is best kept in a refrigerator, since it decomposes readily at room temperature, particularly
in light. (Caution: poisonous, explosive!)
Owing to the insolubility of sodium azide in inert organic solvents its rate
of reaction therein becomes reasonable only when the mixture is warmed to
about 60-80°. To facilitate reaction it is advisable to activate the sodium
azide in advance, 663 although the azides then often decompose to isocyanates
with loss of nitrogen; methyl, hexyl, and phenyl isocyanate, inter alia, have
indeed been prepared in this way. 669 Organic Syntheses contains a detailed
661 J. H. Boyer and F. C. Canter, Chem. Rev., 54, 1 (1954).
662 E. Lieber, T. S. Chao, and C. N. R. Rao, /. Org. Chem., 22, 238 (1957).
663 P. A. S. Smith, Org. Reactions, 3, 382 (1946); cf. J. Nelles, Ber. Deut. Chem. Ges.,
65, 1345 (1932).
664 J.H. Boyer and J. Hamer, /. Amer. Chem. Soc, 11, 951 (1955); J. H. Boyer, F. C.
Canter, J. Hamer, and R. K. Putney, /. Amer. Chem. Soc, 78, 325 (1956).
665 T. Wieland and H. J. Hennig, Chem. Ber., 93, 1236 (1960).
666 L.Horner and A.GroQ,Ann. Chem., 591,117 (1955).
667 C. Naegeli and A. Tyabji, Helv. Chim. Ada, 16, 349 (1933).
668 W. Lwowski and T. W. Mattingly, /. Amer. Chem. Soc, 87, 1947 (1965).
669 G. Schroeter, Ber. Deut. Chem. Ges., 42, 3356 (1909).