Starch Nitrate

STARCH NITRATE BY GEORGEV. CAESAR* Starch Consultant, Harbor Beach, Michigan

The nitrate ester is the oldest known derivative of starch. Since the beginning of this century, it has been, industrially, by far the most important, high-substituted starch derivative. The origin of starch nitrate,’ frequently called “nitrostarch” and actually listed under that heading in Chemical Abstracts, is nearly as old as dextrin, which was first described2 in 1804 and named3 in 1833-the same year in which Braconnot4 aroused the interest of the small chemical world of that time by his discovery of the clear gummy product of nitric acid and potato starch which he called “xyloidin.” It seems a rather curious coincidence that the three important nitrate esters-those of starch, cellulose, and glycerol-should have appeared within a span of thirteen years (starch nitrate in 1833, and cellulose and glycerol nitrates in 1846) and a t a time when there was little or no understanding of their structural kinship. But it was an eager era of science, and the advent of something like “xyloidin” made a far greater impression than it would probably have made today. It is curious, too, that the first chemist to look critically a t “xyloidin” seems t o have had a more accurate grasp of its constitution than very many who followed him. Pelouze6 published his findings on “xyloidin” from 183846. His empirical formula of 1838 was CeHa04NOa, correct for the mononitrate [CeHs06NOz],which was probably what was formed by the prescribed technique of “dissolving starch in a sufficient quantity of concentrated nitric acid” and coagulating it by the addition of water. * The author is greatly indebted to Dr. Walter Snelling, retired “dean of highexplosive experts in the U. s. A.,” for his friendly advice and his helpful criticism of the manuscript; also to Drs. M. L. Wolfrom and R. S.Tipson. (1) See J. Honeyman and J. W.W. Morgan, Advances i n Carbohydrate Chem., 12, 117, 134 (1957); R . L. Whistler, ibid., 1, 279 (1945). (2) J. L. Roard, Ann. chim. et phys., [ l ] 60, 220 (1804). (3) J . B. Biot and J. Persoe, Ann. chim. et phys., [2] 62,72 (1833). (4) H . Braconnot, Ann., 7. 242 (1833); Ann. chim. et phys., [2] 62, 290 (1833). (5) J . Pelouze, Compt. rend., 7, 713 (1838); 23, 892 (1846); Ann., 29, 38 (1839); J . prakt. Chem., 16, 168 (1839). 331

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The inferred ratio of acid to starch seems to have been too low to produce appreciably more than about 7 % of nitrogen, although Pelouze, by using a “large excess” of nitric acid, made flammable products, some of which he detonated. He also found that nitrated starch is very unstable, a property that has hampered its industrial development throughout its history. He concluded that “xyloidin” and the new and fascinating cellulose derivative-“pyroxylin”-were different substances. Pelouze’s studies, and particularly his 1838 empirical formula, were quite remarkable. Up until the late 1920’s, the literature of starch was little better than a mass of more or less contradictory d i t a , made still more confusing by a grandiloquent lexicon of indefinable terms, which served well, however, as always, to conceal extensive ignorance of the subject. Pringsheime strenuously opposed the view that starch’ could be anything but an association or “mosaic” of comparatively small molecules, such as hexosans; and he denied that maltose is a constituent of the starch molecule. This was as late as 1928, when Haworth and his coworkers7 published their classical paper on the methylation of potato starch, the results of which proved that C Y - D - ( ~-+ 4) linkages are present in starch and are not artifacts as Pringsheim had held. The structure of maltose had been proved two years earlier? The pyranose ring-structure of D-glucose residues was but vaguely perceived until the middle ’20’s. And so we find such terms as penta-, hexa-, and octa-nitrostarch being used, with confusion as to whether true nitro compounds or nitrate esters were actually present. Table I shows some formulas advanced during these “dark ages” of starch chemistry, together with the modern unit structure and the empirical formulas. The modern formula is stated to contain a degree of substitution (DS) of 3 and should contain 14.14%of nitrogen. Possibly the first chemist to experiment with the making of starch nitrate by the modern practice of using mixed nitric and sulfuric acids was Hugo Reinsch in 1849. He used equal parts of nitric acid and sulfuric acid and added 5-9 parts of this mixture to starch,’ forming a slurry instead of a colloid; whereas Buis-Ballot? a few years earlier, was still using Braconnot’s technique to make a translucent gel by grinding starch in a (6) H . Pringsheim, i n “A Comprehensive Survey of Starch Chemistry,” R. P . Walton, ed., The Chemical Catalog Co., New York, 1928, Vol. I, p. 35. (7) W. N . Haworth, E. L. Hirst and J . I. Webb, J . Chem. Sac., 2681 (1928); see D. French, Advances in Carbohydrate Chem., 12, 198 (1957); D. J. Manners, ibid., 12, 269 (1957). (8) W. N. Haworth and S. Peat, J . Chem. Soc., 3094 (1926); J. C. Irvine and I. M. A. Black, ibid., 862 (1926). (9) H . Reinsch, RBpert. pharm., 3, 6 (1849). (10) B . Ballot, Rapport ann. prog. chim., 4, 222 (1842-1843).

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S‘l’AkCH NITRATE

mortar with cohcentrated nitric acid. No casualties seem to have come from these early fumblings. The primitive esters could seldom have contained more than about 11 % of nitrogen, and they were so crudely stabilized that they decomposed rapidly. From 1854-62, BBchampll published numerous papers on starch nitrate. He held some definite views, to wit: “Dinitrodextrin . . . . . . . . . . . . C12H1808N06” “Braconnot’s ‘Xyloidin’ . . . . . Cl2HgO9NO6”-attributed by him to Pelouze, but possibly a misquotation “Dinitrostarch . . . . . . . . . . . . .C1zHsOs(NO6)2.” TABLE I Starch Nitrate Formulas Name

“Xyloidin” “ ‘I

“Dinitrostarch”

Formula

CizHgOeN06 CiaHizOizNOi CsHoOrNOa (1 DS) CizHsO,(N06)z CssHtaOi3(Noa)n

H

Starch trinitrate

Investigator

Year

BBcharnp’l Ballot’o PelouseK B6champ1l SaposchnikoffZ4

1860 1S42 1838 1860 1903

Haworth-Irvine

1926

ONO,

[C,H,O,(NO,),],

Some of his products were made by precipitation with sulfuric acid from a solution in nitric acid, a method which soon became the favorite technique for laboratory and commercial preparation, especially in Europe, but which is chemically inefficient. Another practical disadvantage which arises from employing the solution of the starch is the formation of the ester in an amorphous powder even more dangerous to handle when it is dry than it is in the natural, granular form. BBchamp’s formulas, of course, mean nothing; and how they, and others like them, were derived is now obscure. I n many cases, the nitrogen determinations were probably quite inaccurate. The nitrometer did not appear until 1873, and it still needs expert, experienced handling to afford correct results. (11) A . BBchamp, C o m p t . rend., 39, 653 (1854); 61, 255 (1860); Chem. Zentr., 53, 865 (1862); Phil. Mag., 24,526 (1862); Ann. chim. el phys., [3] 64,311 (1862); BUZZ.~ O C . chim. (France), 4. 358 (1862).

334

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v.

CAESAR

One of the earliest attempts to make commercial use of starch nitrate was advanced by Davey,12 who, in 1863, obtained a British patent for “improvement,s in the manufacture of gunpowder and explosive compounds.” His use of mixed acids in the proportion of 1 of nitric acid to 3 of sulfuric acid should have been fairly satisfactory, but he “boiled” the slurry! Whatever resulted could hardly have contributed to the propellants of the day. As late as 1876, Braconnot’s “xyloidin” was still being investigated. Berthelot, in his famous studies on caloric values,13stated that 12.4 calories per mole were liberated in its formation. Perhaps the first of the numerous patents for mixtures of starch nitrate with oxidants, and the like, was assigned to W. SchUckherl4in 1889 for the production of a “smokeless gunpowder.” His mix included ammonium, barium, potassium, sodium and starch nitrates, together with potassium chlorate, picric acid salts, nitronaphthalene, and carbon. The starch nitrate used was probably defective in stability. In 1892, Petit148 described a method of treating starch in dihcte nitric acid, to afford a gummy product. No evaluation of it is possible, however, because of a lack of experimental data; but the product could not have been much other than a low-nitrogen form of degraded starch nitrate. I n 1896, von R ~ m o c k i made ’ ~ ~ an approach to modern nitration practice by dissolving pre-dried potato starch in nitric acid, precipitating it in mixed nitric-sulfuric acid, and then using aniline in stabilizing the precipitate. Another chemist of about this time, Pvliihlhauser16was outstanding (after Pelouze) in his chemical studies of starch nitration. He seems to have been the first laboratory experimenter to make starch nitrate on modern lines so that it possessed a high nitrogen content. We learn through him that the celebrated firm of Actiengesellschaft Dynamit Nobel was producing a potato-starch nitrate,16 probably as a constituent of dynamite, by drying the starch and dissolving it in 10 parts by weight of nitric acid of s.g. 1.5; the starch nitrate was then precipitated by pouring it into spent mixed acids from the manufacture of nitroglycerine. The stabiliza(12) T. Davey, British Pat. 2,072 (1863). (13) M. Berthelot, Ann. chim. et p h y s . , [5] 9, 316 (1876). (14) W. Schiickher, German Patents 51,755 (1889); 54,434 (1890); Chem. Zentr., 61(II), 416 (1890). (14a) P. Petit, Compt. rend., 114, 1375 (1892). (14b) S. J . von Romocki, “Geshichte der Explosivstoffe,” Robert Oppenheim, Berlin, 1896, Vol. II,.p. 89. (15) 0. Miihlhauser, Dinglers Polytech. J . , 284, 137 (1892); Chem. Zentr., 63(I). b 982 (1892). (16) A. Nobel, German Pat. 57,711 (1891), Jahresber. Leistung. Chem. Technol., 37, 431 (1891).

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tion was carried out by washing with water and with soda ash and aniline solutions. No method as efficient as this had yet been described in the literature, and it set the stage for the roles of the scientific investigator and the industrial chemist; with few exceptions, industry was far ahead of academic chemistry in the theory and practice of the nitration of starch. Muhlhauser seems to have been aware of this. He studied the Nobel methods, using small quantities, and obtained a nitrate containing up to 13.2% of nitrogen, a figure approximating the content of the modern, commercial explosive. His formula for this product was C12H1404(ON02)6 having a theoretical content of 14.14% of nitrogen, the true value for the trinitrate of the empirical composition [CsH,Os(NO!Ja],. Muhlhauser stated that, although starch nitrate prepared according to the Nobel company’s method “could not be worked up into a military powder as easily as guncotton, it dissolved very easily in nitroglycerine” and could also be mixed with nitrocellulose. He was, however, concerned with the problem of the stability of a starch nitrate and he suggested that the trouble lay in the “sulfonating (sulfating) action” of sulfuric acid, a guess which was remarkably intuitive. Will and Lenze’s studies,I7 published in 1898, reflect an increasing appreciation of this vital problem of adequate stabilization of the nitrate ester. They fractionated starch nitrates (produced by the inferior technique of initial dissolution in nitric acid) by means of acetone-alcohol, and the fraction insoluble in alcohol was purified by several boilings in alcohol. A maximum iiitrogen content of 14.04% was reported. This figure is possible but, taking into account the analytical accuracy of the time, was probably an iiiaccurately high value. In 1899, Brown and Millarls published two papers on the nitration of potato starch; they dissolved it in nitric acid and precipitated the product with sulfuric acid. An ether-chloroform technique was used for so-called stabilization. The results were of little theoretical or practical intercst. Furthermore, a ‘Lsmokelesspropellant” was claimed’s in a 1901 patent of Schmidt and Buttner in which a wet “dough of nitrocellulose and starch” was “slimed” in nitric acid and precipitated with water or sulfuric acid. But, from about this time, near the turn of the century, the industry-if not the chemistry-of starch nitrate took a great step forward. It seems to have begun, particularly in the United States, with the work of Arthur (17) W . Will and F. Lenze, Ber., 31, 68 (1898). (18) H. T. Brown and J . H. Millar, J. Chem. Soc., 76,308 (1899); J . SOC.Chem. Znd. (London), 18, 159 (1899). (19) Schmidt and Buttner, German Pat. 130,523 (1901) ; Jahresber. Leistung. Chem. Technol.,48,3G3 (1902).

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Hough,20 a shrewd English chemist who had become interested in high explosives in the late ’80s. He built an experimental plant near Asbury Park, New Jersey, about 1900, and, in 1903, a small plant near Wharton, New Jersey, to make from starch nitrate a non-freezing “dynamite” called “Hough’s Powder.” W. 0. Snelling, then the young assistant to the celebrated Charles E. Munroe of the United States Bureau of Mines, examined a sample of “Hough’s Powder” in 1903 and found it21to be “practically pure starch nitrate . . . very low in density and highly flammable.” On these accounts, Munroe reported on it unfavorably; and an explosion destroyed the plant and killed Hough’s brother. The double disaster put poor Hough out of the starch nitrate business. But his U. S. patent20fs)of 1904, and two foreign patents,2O(b) (c) were unique in specifying a more nearly anhydrous process of nitration than had hitherto been tried. He used mixed nitric and sulfuric acids of high concentrations, reinforced by sulfur trioxide, throughout the treatment of thoroughly dried starch. The product was stabilized by a drastic treatment in hot ammonium hydroxide, which probably reduced its nitrogen content. Snelling, however, found it to contain 13.3 % of nitrogen, and a subsequent check of Hough’s method of preparation yieldedz1a starch nitrate having 13.55 % of nitrogen, about the maximum obtainable with mixed acids. Hough described a product containing 16.5% of nitrogen, referred to as “octanitrate,” both of them impossible claims; for the one he was, probably, led astray by inaccurate analytical methods, and for the other, by the ignorance of his day about starch structure. According to Snelling,2l “starch nitrate was first made in the United States around 1888 as a constituent of Dr. Volney’s powder.” The du Ponts experimented with starch nitrate as early as 1899, but it was not until 1905 that a plant was erected for its manufacture and use in the nonfreezing explosive called “Nyalite.” The manufacture (by Du Pont) of “Arctic” starch nitrate explosives was started about 1907, and this explosive was made and sold extensively for a period of more than ten years. Starch nitrate explosives were also made in Canada. The Trojan Powder Co. was manufacturing starch nitrate for commercial blasting in this pre-World War I era. I n the construction of the Panama Canal, “large amounts of starch nitrate based explosives were used . . . and produced by the Trojan Powder Co. a t its Eastern and West Coast plants.’’22This company is a t present the principal, if not the sole, source of commercial starch nitrate. 3

(20) A. Hough, (a) U. S. Pat. 751,076 (1904); (b) German Pat. 172,549 (1903), Chem. Zentr., 77(II), 938 (1906); (c) British Pat. 12627 (1904); (d) 2. ges. 8chiess.-u. Sprengstofw., 2, 295 (1907). (21) W. 0. Snelling, Private communication. (22) Trojan Powder Co., Private communication.

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Another (and the best) patent of this period, covering the production of starch nitrate, was to F. B. Holmes of Woodbury, New Jersey, and assigned t o the Eastern Dynamite Corporation of Wilmington, Delaware. He treated starch with mixed acids (in a preferred proportion of nitric acid, 32.5 %:sulfuric acid, 64.5 %:water, 3 %), using 25 parts of dried starch to 100 parts of the mixed acids. He stabilized the product by boiling it for 75-100 hours in water containing calcium carbonate. Later patents covered a wide variety of ammonium compounds as stabilizers. Millions of pounds of starch nitrate were made by Holmes’ patents. At about the same time, Saposchnikoff24 attempted a molecular-weight determination on starch nitrate produced by dissolution in nitric acid, and arrived a t the figure of 1845--“agreeing best” with the formula “C36H43013. (NO&, .” This formula, wherein the DP is 6, is not unlikely, as the conditions used probably effected degradation. I n Britain, Arnold and his associatesz6 were granted patents for the nitration of potato starch in mixed acids. The product was probably worthless, owing to the naive assumption that it was ‘(freed of acids by currents of fresh water.” The all-important problem of stability was receiving further attention in the United States of America during this pre-World War I period. J. B. Bronstein26z27 took out a number of U. S. patents covering the stabilization of starch nitrate with borates, soda ash, lime, and the like. His contributions t o the U. S, patent literature, for the purpose of improved stabilization treatments and for the increased efficiency of explosive compositions of starch nitrate, extended from 1907-26; and J. B. Bronstein, Jr.,28was granted a patent in 1939 for improving colloidal compositions of starch nitrate. C. E. Waller29 was another creative member of the unusual Trojan Powder group of the post-World War I era. His patents covered the improvement of starch nitrate compositions. (23) F. B. Holmes, U. S. Patents 779,421 (1905); 779,422 (1905); 875,913 (1907); 875,928 (1907) ; Cheni. Abstracts, 2, 1499 (1908). (24) A Saposchnikoff, Z h u r . Russ. Fiz.-Khim. Obshchestva, 36, 126 (1903). (25) G. E. Arnold, A. C. Scott and H. E. U. Roberts, British Pat. 3,449; Chem. Abstracts, 1, 1191 (1907); G. E. Arnold, A. S. Fox, A. C. Scott and H. E. U. Roberta, British Pat. 3,450; Chem. Abstracts, 1. 1487 (1907). (26) J . B. Braunstein, U. S. Patents 868,636 (1907); 868,637 (1907); 868,638 (1907); 868,837 (1907); Chem. Abstracts, 2, 912 (1908); 869,051 (1907); Chem. Abstracts, 2, 913 (1908); J. B. Bronstein, U. S. Patents 1,398,931 (1922); Chem. Abstracts, 16,1014 (1922); 1,573,673 (1926); Chem. Abstracts, 20, 1525 (1926). (27) J . B. Bronstein and C. E. Waller, U. S. Patents 1,188,244 (1916); 1,188,245 (1916); 1,188,246 (1916); Chem. Abstracts, 10, 2150 (1916). (28) J . B. Bronstein, Jr., U. S. P at. 2,170,629 (1939); Chem. Abstracts, 34,266 (1940). (29) C . E. Waller, U. S. Patents 1,305,845 (1919); 1,305,846 (1919); Chem. Abstracts, 13, 2130 (1919) ; 1,386,478 (1921) ; Chem. Abstracts, 16, 4052 (1921) ; 1,462,093 (1923) ; Chem. Abstracts, 17, 3101 (1923).

338

0.

V. CAESAR

Hough’s method of nitration in the presence of sulfur trioxide was checked in 1910 by Berl and Butler.30 They made intercomparisons of various starch nitrates and showed the very great difference in viscosities between a starch nitrate and a cellulose nitrate. An interesting departure in the techniques for stabilizing starch nitrate was indicated by Nathan, Rintoul, and Baker31 of the Nobel Explosives Company in 1912; British patents31 were issued covering the use of urea derivatives as stabilizers. A U. S. patent to S. S. Ss~dtler3~‘a) in 1917 was an interesting disclosure. He went t o considerable trouble to purify starch prior to nitration, and nitrated the purified material with a mixed acid (composed of nitric acid, 25%:sulfuric acid, 65%:water, 10%) used in a weight “25 to 30 times the weight of the starch taken.” We may note the higher ratio of sulfuric acid t o nitric acid, the much higher proportion of water, and the very much higher proportion of mixed acids to starch as compared to Holmes’ pr0portions.2~I n a subsequent paper, Sadtler32(b)was unusually specific regarding the stability of his product, citing a potassium iodide-starch test of 65 minutes at 70°C. His product must have suffered some denitration from a drastic treatment with soda ash. In the same year in which Sadtler’s patent appeared, W. 0. Snelling became Director of Research a t the Trojan Powder Company, i n d he remained in that capacity for 37 years, until his retirement in 1954. The use of starch nitrate as an exceptionally adaptable, military, high explosive, in two World Wars, is intimately associated with the work of Dr. Snelling, as is also its development for civil use through improvements in its stability and in its explosive compositions. His U. S. patent literature is extensive.33 (30) E. Bed and R. Biitler, J. SOC.Chem. Znd. (London), 29, 373 (1910); 2. ges. Schiess. -u. Sprengstofw., 6 , 82 (1910); Chem. Zentr., 81(I), 2074 (1910). (31) F. L. Nathan, W. Rintoul and F. Baker, British Patents 12,742 (1912) ; 12,743 (1912); 12,745 (1912); 12,746 (1912) ; Chem. Abstracts, 7,3842 (1913). (32) S. S. Sadtler (a) U. S. Pat. 1,211,761 (1917); Chem. Abstracts, 11, 1042 (1917); (b) Met. Chem. Eng., 16, 361 (1917). (33) W. 0. Snelling, U. S. Patents 1,274,343 (1918), Chem. Abstracts, 12,2055 (1918); 1,305,946 (1919), Chem. Abstracts, 13, 2130 (1919) ; 1,308,453 (1919), Chem. Abstracts, 13, 2251 (1919); 1,310,969 (1919), Chem. Abstracts, 13, 2450 (1919); 1,316,396, Chem. Abstracts, 13, 3013 (1919); (and W. R. Lams) 1,329,211 (1920), 1,329,212 (1920), Chem. Abstracts, 14, 1045 (1920) ; 1,343,063 (1920), Chem. Abstracts, 14, 2420 (1920) ; 1,382,563 (1921), Chem. Abstracts, 16, 3751 (1921) ; 1,386,437 (1921), 1,386,438 (1921), 1,386,439 (1921), 1,386,440 (1921), Chem. Abstracts, 16. 4052 (1921); 1,395,775 (1922), 1,395,776 (1922), Chem. Abstracts, 16, 648 (1922); 1,410,037 (1922), Chem. Abstracts, 16, 1829 (1922); (and W. R. Lams) 1,441,130 (1923), Chem. Abstracts, 17, 1029 (1923); 1,456,341 (1923), Chem. Abstracts, 17, 2506 (1923); 1,462,074 (1923), Chem. Abstracts, 17, 3101 (1923); 1,462,075 (1923), Chem. Abstracts, 17, 3101 (1923); 1,464,667 (1923), Chem. Abstracts, 17, 3255 (1923); 1,472,691 (1924), Chern. Abstracts, 18, 472 (1924); 1,473,257 (1924), Chem. Abstracts, 18. 471 (1924); 1,497,600 (1924), Chem. Abstracts, 18, 2405 (1924), 1,504,986 (1924), Chem. Abstracts, 18, 3274 (1924); 1,510,348 (1924), Chem. A b -

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That the age-old problem of better stabilization was still present as late as 1920 was shown by the issuance of a U. S. patent to B. F l ~ r s c h e i mfor ~~ stabilizing starch nitrate by means of cyanamid or calcium cyanamid; on the other hand, G. R. Anchors,35quite evidently unimpressed by the difficulties of obtaining a stable nitrate ester from starch, undertook the nitration of “puffed cereals” in mixed acids, using soap for a stabilizer! A year later, O l ~ e npatented ~~ a blasting composition of starch nitrate and a smokeless propellant. I n the late ’ ~ O S ,O k a d s ~carried ~ ~ out some extensive experimental studies on starch nitration. One of his most important conclusions was that mixed acids give better results than nitric acid alone, a fact already well appreciated by the American industry, but apparently then still obscure to scientific investigators-for, as late as 1929, we find Taylor and W a l t ~ copyn~~ ing the methodsI7of Will and Lenze of 1898 ! They emphasized3*the “marked instability” of starch nitrate, and experimented with pyridine and ammonia as stabilizers a t a time when the commercial product was selling by millions of pounds in a highly competitive market. Its stability could even then be improved; but presumably through failure to read the extant patents, the university chemists seemed almost wholly unaware of the strides that had already been made. In 1930, A. Schrimpff39studied the effect of heating starch nitrate alone and with ammonium nitrate, a compound long used in admixture with starch nitrate; and, in 1933, G. M. Normanm assigned a patent to the Hercules Powder Company for the nitration of starch in a flake form, whereby the nitrate was presumably stabilized with greater ease. The continued stress on stabilization, particularly in the patent literature, indicated that there was still need for it, although the commercial success of starch nitrate suggested, too, that there were some tricks t o the trade which had not yet been disclosed. stracts, 18, 3722 (1924); 1,579,964 (1926), Chem. Abstracts, 20, 1717 (1926); 1,588,277 (1926), Chem. Abstracts, 20, 2751 (1926); 1,617,182 (1927), Chem. Abstracts, 21, 1013 (1927); 1,631,070 (1927), Chem. Abstracts, 21, 2386 (1927); (and G. A . Rupp) 1,652,960 (1928), Chem. Abstracts, 22, 870 (1928); (and C . A . Rupp) 1,659,449 (1928), Chem. Abstracts, 22, 1477 (1928); 1,709,636 (1929), Chem. Abstracts, 23,2827 (1929); (and J. A . Wyler) 1,749,613 (1930), Chem. Abstracts, 24, 2297 (1930); 1,808,613 (1931), Chern. A b stracts, 26, 4405 (1931); (and J . A. Wyler) 1,835,011 (1932), Chem. Abstracts, 26, 1125 (1932); (and G. E. Rees) 2,271,877 (1942), Chem. Abstracts, 36, 3696 (1942); 2,333,275 (1944), Chem. Abstracts, 38. 2492 (1944); 2,371,000 (1945), Chem. Abstracts, 39, 3162 (1945). (34) B. J. Flurscheim, U. S.Pat. 1,343,317 (1920); Chem. Abstracts, 14,2420 (1920). (35) G. R . Anchors, U. S.Pat. 1,329,353 (1920); Chem. Abstracts, 14, 1045 (1920). (36) F. Olsen, U . S. Pat. 1,376,030 (1921); Chem. Abstracts, 16, 2725 (1921). (37) H . Okada, Cellitlose Ind., 3, 3 (1927); Chem. Abstracts, 22, 686 (1928). (38) T. C . Taylor and R . P. Walton, J. A m . Chem. Soc., 61, 3431 (1929). (30) A. Schrimpff, 2. ges. Schiess. -u. Sprengstofw., 26, 273 (1930). (40) G . M. Norman, U. S. P a t . 1,908,857 (1933); Chem. Abstracts, 27, 3824 (1933).

340

G. V. CAESAR

From 1932-35, four papers by Hackel and Urbadski appeared.41Various types of starch were nitrated in various proportions of nitric acid:sulfuric acid: water, for one hour a t 10°C., with average products containing from 11.45 to 13.30 % of nitrogen and with no very clear-cut deductions possible, probably owing to the use of too great an excess of mixed acids in proportion to the starch (50: 1). Many of the investigations of nitration by independent chemists can be criticized in this respect; for example, the ratio 4: 1 was used by H01mes~~ in his U. S. patent 779,421. An x-ray study of potato starch, unnitrated, nitrated, and denitrated, was made by Kolaczkowska and U r b a h ~ k iPerhaps .~~ the most useful finding by Hackel and Urbahski4‘ was that, for any given acid composition, nitration is substantially complete in half an hour. In 1935, E. Berl published another paper, this time with W. C. Kunze,43 which described the use of phosphoric acid in place of sulfuric acid. They claimed higher viscosities in the starch nitrates (through the occurrence of less degradation) and easier stabilization, a not unlikely result. But their experiments showed no appreciable increase in the nitrogen content, and the costs of the mixed acids were increased. They found that potato “amylose and amylopectin” yielded different nitrogen contents. The difficulty was that they did not have either a true amylose (unbranched, “A-fraction”) or a true amylopectin (branched, “B-fraction”), since, a t that time, these two principal components of starch were not thoroughly established.44 A year later, these findings and others were reviewed by who dwelt particularly upon the manufacturing hazards. Dry, organic dust is ever a danger, but dust from starch nitrate is especially so! This hazard seemed particularly great in European factories. On the other hand, the nitration phase seems to have had a satisfactory safety record. According to Snelling,21 nitrating pots have “fumed off,” but, insofar as he knows, have never exploded-quite a different “game” indeed from the nitration of glycerol! ~~ tapioca, arrowroot, and rice In 1936, Staudinger and E i l e r ~nitrated starches in mixed acids at a ratio of 50:l. They obtained products having about 12.5 % of nitrogen and they correctly estimated 14.14 % of nitrogen for the trinitrate. But then they proceeded to study the properties of the nitrates as a function of their molecular weights, employing values which probably were wholly fictitious (and, alas, still are, more or less!). Staud(41) J. Hackel and T. Urbahki, Roczniki Chem., 12, 276 (1932); 13, 221 (1933); 2. ges. Sehiess. -u. Sprengstofw., 28, 350 (1933); SO, 98 (1935). (42) M. Kolacskowska and T. Urbafiski, Roczniki Chem., 16, 339 (1935). (43) E. Berl and W. C. Kunse, Ann., 620, 270 (1935). (44) T. J . Schoch, Advances in Carbohydrate Chem., 1, 247 (1945). (45) W. Matla, Chem. Weekblad, 33, 120 (1936) (46) H. Staudiqer and H . Eilers, Ber., 69, 819 (1936).

STARCH NITRATE

341

inger and H ~ s e m a n i studied i~~ the degradation of various starches during nitration, but here again their results mean little, since they probably had no accurate measure of the molecular weights of the materials with which they were dealing. Hungarian patents issued to Ipartelepek4*in 1938-39 covered the nitration of starch (in mixed acids containing 75-90 % of nitric acid) at temperatures below 5°C. for 30 minutes, but in two stages. Unusual stability of the products was claimed. J. A. Wyler, another member of the Trojan group, was granted patents49 in 1930 and 1938. The first was for a solid fuel consisting of alcohol plus starch nitrate-a development certainly ahead of its time! The 1938 patent covered a nitration of mixed sugars plus starch. His most recent patent,” issued with Boyd in 1943, covered the stabilization of the nitrate in an aqueous solution of bicarbonate and a 0.2 % solution of dicyandiamide. In 1942, U. S. patents6’ were issued to still another of the Trojan group, W. Bowlby, for lacquer compositions of starch nitrate; it has been unsuccessful in this use, however. Walter Snelling’s last U. S. patent on starch nitrate was issued in 194533 and covered an explosive composition of ammonium nitrate partially sensitized by starch nitrate. His first patent33 was issued in 1918 and covered an explosive, fertilizer cartridge for treating soils. He patented starch nitrate as a water-insoluble fe rtili~ e ?in ~ 1922 and 1924. In 1946, Will and Lenze’s method” was revived once again62by Ashford, Cooke, and Hibbert in Canada. Starch nitrates were fractionated in acetone and alcohol, and were examined for their nitrogen content and their molecular weight (lowest for the alcohol-soluble fractions). A nitrate from the “B-fraction” (branched) was reported to be less stable than one from the “A-fraction,” a not unlikely possibility. Before touching upon the literature from 1946 on, a brief discussion of the chemistry of esterijication by mixed acids may be of general interest. As early as 1908, and extending through 1928, HantzschS3concluded, (47) H. Staudinger and E. Husemann, A n n . , 627, 195 (1937); Ber., 71, 1057 (1938). (48) N . Ipartelepek, Hungarian Patents 118,102 (1938), Chem. Abstracts, 32, 6064 (1938); 121,180 (1939), Chem. Abstracts, 33, 8993 (1939). (49) J. A . Wyler, U. S. Patents 1,752,935 (1930), Chem. Abstracts, 24, 2580 (1930); 2,105,389 (1938), Chem. Abstracts, 32, 2357 (1938). (50) J. A. Wyler and R. N. Boyd, U . S. Pat. 2,297,734 (1943), Chem. Abstracts, 37, 1622 (1943). (51) W. D. Bowlby, U. S. Patents 2,261,642 (1941), Chem. Abstracts, 36,1202 (1942); 2,279,438 (1942) ; 2,279,439 (1942), Chem. Abstracts, 36, 5368 (1942). (52) W . R . Auhford, I,. M . Cooke and H. Hibbert, Can. J . Research, 24B,238 (1946). (53) A . Hantauch, 2. physik. Chem. (Leipaig), 66, 41 (1908); Ber., 68, 941 (1925); A . Hantesch and K . Berger, ibid., 61, 1328 (1928).

342

G . V. CAESAR

from analytical, cryoscopic, and ultraviolet absorption data, that nitric acid ionizes in sulfuric acid to “nitracidium” cations, such as HzNOF; and, in 1946, the presence of cationic nitrogen in mixed acids was proved by electrolytic s t ~ d i e s . ~Although 4 it is considered that nitracidium might exist in the presence of an appreciable proportion of water in the mixedacids system,66it was shown by Westheimer and Kharasch,66 and Ingold and co~orkers,~7 that the great increase in the rate of nitration (with decrease in water present) is best explained by the “nitronium” cation N O P as the active nitrating agent. Raman spectra have also confirmed the existence of this ion. I n mixed acids, the presence of four ions is indicated66according to the equation : HNOZ

+ 2 HzSOi + NOz’ + Ha0’ + 2 HSOde.

Nitric acid ionizes as a base, toward the sulfuric acid, and the maximum rate of nitration occurP a t a definite “acidity” defined68by Hammett’s “Ho function,” a logarithmic measure which is equivalent to pH in dilute acids but which acquires a negative value in super-acid solutions. For the nitration of aromatic compounds, on which these rate studiess6were made, the maximum was reached in sulfuric acid of 90% concentration. So the simple reaction ROH

+ HONOz -+ RONOz + HzO

is a little too simple to describe what actually takes place, although it does sum up the result. Nitration in pure nitric acid is considered t o proceed according to the following equations. 3 HONOz $ ONOze ..

..

R-O:eH’

+ NOz@+ HON0z.HzO

+ ONOpe + NO*” + RONOz + HONOz ROH + HONOz + RONOz + HzO

Nitration in mixed acids (nitric and sulfuric) should then proceed as follows.

.. ..

R-OZeH’

HONO2

+ 2HzS04 * 2HSOae + H”(Hz0) + NOz’

+ 2 H S O I ~+ H’(Hz0) + NOz’ ROH

+ HONOz

--+

+

2 HzSO4

RONOz

+ HzO

+ HzO + RON02

(54) G . M. Bennett, J. C. D. Brand and G. Williams, J . Chem. SOC.,869 (1946). (55) T. Moeller, “Inorganic Chemistry,” John Wiley & Sons, Inc., New York, 1950, pp. 608, 610. (56) F. H. Westheimer and M. S. Kharasch, J . A m . Chem. SOC.,68, 1871 (1946). (57) D. R. Goddard, E. D. Hughes and C. K. Ingold, Nature, 168, 480 (1946); C. K. Ingold, D. J. Millen and H . G. Poole, J . Chem. SOC.,2576 (1950). (58) L. P. Hammett and A. J. Deyrup, J . A m . Chem. Soc., 64, 2721 (1932).

STARCH NITRATE

[

343

Nitration with dinitrogen pentoxide may be represented as shown below.

.. ..

R-0ZeH'

+ NOz'ONOze + RON02 + HONOz :o":o-N
..

:0

..

0:

The "acidity" of the mixed acids, not their dehydrating power, has been established as being the rate criterion for the formation of true nitro compounds; that it might be extended t o the case of nitric esters seems a reasonable a s ~ u m p t i o n . ~ ~ During the last war, the author and his coworkersw-mundertook a study of esterification (largely, of starches) by dinitrogen pentoxide under anhydrous conditions, with the principal objective of forming substantially undegraded esters. The literature on the pentoxide' was meager, with only one reference on the nitration of starch.s4 Chloroform is an excellent inert solvent for dinitrogen pentoxide and we decided to use it for our nitrations. We had first, however, to prepare the pentoxide in sufficient quantities; our described methodB6proved satisfactory. The second problem was to remove the nitric acid formed (one mole per nitrate group introduced) which tended to dissolve the starch partially and to impede complete nitration. The use of phosphoric anhydride in a porous container, through which the chloroform solution of dinitrogen pentoxide was circulated, solved the nitric acid problem; and later on, when the phosphoric anhydride was replaced by sodium fluoride (which removed the nitric acid very efficiently through hydrogen bonding to give Na F . * * HON02),we had a rather good, laboratory process by which esterification could be brought to practically complete substitution (3-DS) on starch or cellulose, with the barest minimum of degradation, because of the anhydrous character of the reaction, its low temperature, and its high rate. Degree of substitution, as expressed by the percentage of nitrogen, could be readily controlled by the ratio of nitrogen pentoxide to starch; stabiliza(59) (60) (61) (1947). (62) (63) (1948). (64) (65)

S. Israelashvili, Nature, 166, 686 (1950). G . V. Caesar and M. Goldfrank, J. A m . Chem. Soc., 68,372 (1946). G. V. Caesar, N . S.Gruenhut andM. L. Cushing, J. A m . Chem. SOC.,69, 617 G. V. Caesar, U. S. Pat. 2,400,287 (1946); Chem. Abstracts, 40, 4487 (1946). N . S.Gruenhut, M . L. Cushing and G. V. Caesar, J . A m . Chem. Soc., 70,424

T. Urbafiski and Z. Janissewski, Roczniki Chem., 17, 349 (1937). N . S . Oruenhut, M. Goldfrank, M . L. Cushing and G. V. Caesar, Inorg. Syntheses, 3 , 78 (1950).

344

0. V. CAESAR

tion was relatively simple, owing to the absence of sulfuric esters; and the process was comparatively safe. The procedure was also applied successfully to the formation of some true nitro compounds. It was of particular service in molecular-weight studies because of the probable lack of any appreciable 63 Some results of nitration by degradation effects during the this method are shown in Table 11. The high-nitrogen esters of starch are exceedingly sensitive to shock, and, in this respect, they resemble such primary explosives as lead azide. In 1950, the “mechanism of the nitration of starch” was studied by Israela~hvili~~ who came to the conclusion that conditions of nitration favoring the production of a high proportion of nitronium ion (NOP) produce TABLE I1 Nitrate Esters Prepared by the Procedure of Caesar and GoldfrankOQ Nifrafc of

Corn starch Corn, “A-Fraction” Corn, “B-Fraction” Corn, “Thin-boiling” Corn, dextrins Cellulose Lactose Schardinger a-dextrin Schardinger 8-dextrin a

Nitrogen, %

Pound“

13.92 13.96 13.98 13.95-14.03 13.87-14.01 13.97 15.68 13.61 13.52

Calcd.

14.14 14.14 14.14 14.14 14.14 14.14 15.95 14.14 14.14

Average of 28 experiments.

high nitrogen-contents. Parenthetically, dinitrogen pentoxide might appropriately be called 67 “nitronium nitrate.” In 1951, Vollmerte6 confirmed the assumptiona0,61 that degradation of starch is reduced to the minimum in the process of esterification by dinitrogen pentoxide. One of the most recent papers on starch nitrate is from Poland. Smolehski and Strz0ndala,~7in 1954, investigated the heats of formation of starch nitrates prepared (a) by nitric acid alone and (b) together with sulfuric acid, and found that they were a linear function of the degree of esterification. Commercial starch nitrate has an average nitrogen content of about 13.2%. Its efficiency as an explosive is a direct function of the nitrogen 661

(66) B. Vollmert, Makromol. Chem., 6, 78 (1951). (67) D. Smoledski and J. Strzondala, Zeszyty Nauk. Politech. Wroclaw. N o . 4 , Chem. No. 1, 49 (1954); Chem. Abstracts, 49, 15436 (1955).

STARCH NITRATE

345

content; but the cost of making it also rises with the proportion of nitrogen, and the “happy medium” seems to be about 13.2 %. For civil uses, the properties of starch nitrate as a high explosive are competitive with those of other types employed; and, for military uses, it has been proved to be an exceptionally good substitute for 1 3 5-trinitrotoluene (TNT). There was an acute famine in this all-important T N T when we plunged into World War I . Snelling developed the famous Trojan Grenade Powder for hand grenades and trench-mortar shells which helped to relieve this shortage. Then, before World War 11, he was asked by the Army to develop a demolition explosive, millions of pounds of which were used in the last war and which is known as Trojan or Nitrostarch Demolition 33(1944) Explosive. It has the following Starch nitrate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Barium nitrate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aluminum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Graphite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Paraffin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. ........................... ...........................

..........................

48-52 3841 2 4 1-3 1-3 2-4 0.5-1.5 0.5-1.5%

This explosive was very thoroughly tested, in comparison with T N T blocks, by the U. S. Engineer Corps in the late ’~OS,with very favorable results for the starch nitrate688 69. A recent paper from Pakistan by Ahmad and Karimullah’o disclosed the use of starch nitrate in propellants as an adjunct to cellulose nitrate. Starch nitrate is a fascinating product, as anyone who has ever made it will enthusiastically agree. (68) G . Linkswiler, Military Eng., 28, 266 (1936). (69) J . M. Young, Military Eng.,31, 11 (1939). (70) I. Ahmad and Karimullah, Pakistan J. Sci. Research, 6 , 133 (1954) ; Chem. Abstracts, 49, 16436 (1955).