Advanced Propellant Chemistry

14 Reactions of Difluoramine with Lewis Acids J.N.K E I T H , R. J. D O U T H A R T , W. K. S U M I D A , and I. J. S O L O M O N 1

Downloaded by CORNELL UNIV on September 28, 2016 | http://pubs.acs.org Publication Date: January 1, 1966 | doi: 10.1021/ba-1966-0054.ch014

IIT Research Institute, Chicago, Ill.

The acid-base chemistry of difluoramine, HNF , and its methyl derivative is being studied to establish the existence of difluorammonium compounds and to obtain new oxidizers containing difluoramino groups. The protic acids, hydrogen chloride and trifluoroacetic acid, do not form compounds with difluoramine in the condensed state; methyldifluoramine, however, may form a weak complex with hydrogen chloride. Trimethylaluminum, a strong Lewis acid, reacts with difluoramine liberating methane. The postulated intermediate, has not yet been isolated. Although trimethylgallium and difluoramine do not form a compound at low temperatures, methane is slowly evolved. Both difluoramine and methyldifluoramine form 1:1 adducts with sulfur trioxide. NMR analysis of the difluoramine adduct indicates that the proton and fluorine atoms are no longer adjacent, possibly indicating that it is difluoramidosulfamic acid. 2

/ ^ r a i g (2) has recently reported the results of studying the reactions of ^ s e v e r a l nitrogen-fluorine compounds with Lewis acids. A s expected he found that all the compounds he studied were extremely weak bases, difluoramine and its methyl derivative being among the strongest. Here we report some of our data on the reactions of the latter compounds with other strong acids. To obtain new nitrogen-fluorine type oxidizers, we have been studying the possibility of preparing difluorammonium salts and introducing d i fluoramino groups into compounds of light metals such as A 1 ( N F ) 3 . T o this end, the reactions of difluoramine and methyldifluoramine with cer tain acids have been studied. 2

1

Present address:

University of Illinois, Urbana, Ill.

14 1

Holzmann; Advanced Propellant Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1966.

142

ADVANCED PROPELLANT

CHEMISTRY

Experimental Apparatus and Materials. A standard Stock-type high vacuum line was used, except for experiments with S 0 or C H N F . I n these cases stopcocks lubricated witn K e l F-90 grease were used, and pressures were measured with a Bourdon gage. The commercially available materials, HC1, ( C H ) A 1 , and C F C 0 H , were purified b y trap-to-trap distillation i n the vacuum une. S 0 was generated as needed by passing the vapors from a sample of Sulfan Β over a P 0 column. Trimethylgallium was prepared b y the reaction of gallium with dimethylmercury at 130° C . Difluoramine was prepared toy the thiophenol reduction of tetrafluorohydrazine ( 3 ) . Methyldifluoramine was obtained b y fluorinating sodium N-methyl sulfamate. Pressure Composition Studies. Pressure composition studies were performed i n small, calibrated U-traps fitted w i t h a manometer or Bourdon gage. A measured sample of the less volatile component was condensed in the trap, and successive small amounts of the more volatile component added. T h e trap was then brought to the appropriate temperature and allowed to equilibrate until the pressure became constant (usually 15-30 minutes were required). T h e data were plotted vs. the composition of the liquid phase. The results are summarized i n Table I. 3

3

2

3

3

3

2

3


2

5

Table 1. Pressure-Composition Studies Temperature °C.

System HNFr-HCl CHsNFr-HCl

- 1 1 2 to ·-138 - 9 5 , - 112 -127

HNFt-CFsCOsH HNF2-S0

-45. 2 -63. 5 0

3

CH,NFr-S0

- 6 3 .5 -45. 2 -80 25 -80 - 7 8 . 5 , --45.2

8

HNFHCH,),A1 CHtNFHCHiigAl HNFHCHi^Ga

Result Miscible, with positive deviations Miscible, with negative deviations Slight plateau up to 0.5 more fraction HC1, Ρ — 6 mm. Solid, slightly soluble in H N F Components readily separated by distillation Minimum 14 mm. at 0.4 mole fraction H N F 2 , inflection at 0.5, homogeneous liquid 0-0.6 Slight solubility 1:1 adduct v.p., 30 mm. m.p., —10° C . 1:1 adduct slowly loses CH4 2 moles H N F consumed, yielding 2 CH4 1 mole H N F j + 1 (CH,)»A1 gave 2 C H - f H C N Solution positive deviation, C H 4 at room tem perature 2

2

4

The H N F 2 - S O 3 System. Because of the corrosive nature of sulfur trioxide and the tendency of the low melting form to undergo transition to more highly polymerized forms, it is a rather difficult material to use i n an equilibrium process. It was not possible to obtain reproducible satura tion pressures for this system b y adding H N F to S 0 at l o w temperatures. However, at 0° C . a liquid was obtained over the whole composition range studied (up to χ = 0.6 mole fraction), and equilibrium could be obtained, although slowly. T h e pressure-composition diagram ( Figure 1 ) shows a minimum at X N F = 0 . 4 , a n d an inflection at about 0.5, with a sharp increase i n pressure beyond that. Possibly, a 2:1 as well as a 1:1 com pound is formed. The proton and F NMR spectra of the 1:1 mixture without solvent at 0° C . indicated only single frequencies for hydrogen and fluorine, 2

H

3

2

1 9


14.

200

1 43

Difluoramine with Lewis Acids

ΚΕΓΓΗ E T A L .

_

0 150

-


i

I

I

I

0

0.2

0.4

,

I

Mole Fraction H N F

Figure 1.

The HNF SO r

I

0.6

1

0.8

1.0

2

system at 0°C.

s

neither of which was split, indicating that the hydrogen is no longer on the nitrogen atom. These observations are consistent w i t h the structure F N S 0 O H but not w i t h the coordination compound, H N F : S 0 . Slow decomposition of the product occurs at room temperature, the volatile products being N F and S 0 . The reaction is probably the following: 2

2

2

2

4

3

2

2 NF2SO2OH

N2F4 +

SO2 +

(5)

H2SO4

The H N F < r - ( C H ) A l System. W h e n attempts were made to study the pressure-composition curve of this system at —80° C . explosions occurred whenever liquid H N F was present. However, no explosions occurred w i t h gaseous difluoramine at low pressures. Several attempts were made to obtain a pressure-composition curve by adding very small amounts of H N F , but even under these conditions methane was slowly evolved. The following procedure was used to determine the stoichiometry of the reaction: 19.3 cc. H N F were condensed i n the tip of a small reactor, and 7.7 cc. of ( C H ) A 1 (calculated as monomer) were con densed i n a ring above it. The mixture was thawed to —80° C . for 30 min. during which the pressure rose to 45 mm. (v.p. of H N F = 25 m m . ) ; 2.3 cc. of methane (v.p., 10 mm. at —196° C.) and 14.2 of difluoramine (v.p., 1 mm. at —127° C.) were obtained by distillation from the reactor at —80° C . A n additional 2.5 cc. of methane were obtained on thawing the 3

3

2

2

2

3

3

2


1 44

ADVANCED PROPELLANT CHEMISTRY

reactor. Additional treatment with the recovered difluoramine produced further evolution of methane, as tabulated below: HNF

+

2

(GH ) A1 3

3

—

CH

4

+

(Me AlNF ) 2

2

19.3 c c . 7.7 cc. 30 m i n . - 8 0 ° G . — 1 4 . 2 cc. t h a w e d 571 cc.

2.3 cc. 2.5 cc. 4 . 8 cc.

overnight - 8 0 ° C . t h a w e d residue with H N F , 25 ° C , 2 hr. 19.3 cc. with H N F , 2 5 ° C , overnight - 3 . 9 cc. w i t h H N F , 2 5 ° C , 3 days

0.7 0.9 2.2 6.2 05

2

2

2


1 5 . 4 cc.

cc. cc. cc. cc. cc.

15.3 cc.

W h e n the methane production had essentially ceased, only 3.9 cc. of condensable gas remained, consisting of H N F contaminated w i t h a small quantity of N F as shown b y its infrared spectrum. Assuming this i m purity to be negligible, the agreement is rather good: 15.4 cc. H N F was consumed, producing 15.3 cc. of methane. This corresponds to the re placement of two methyl groups per aluminum atom (2 X 7.7 = 15.4). The product first formed at —80° C . was a clear, colorless, viscous liquid, and remained so at room temperature. O n prolonged standing, however, it gradually changed to a pale yellow powder. In a similar experiment, a mixture of 6.0 cc. ( C H ) A 1 and excess H N F were allowea to stand for 1 hour at —80° C , then overnight at 25° C . 12.7 cc. H N F were consumed, yielding 12.9 cc. methane. The hydrolysis of the nonvolatile products of these reactions i n neutral or acidic iodide solution liberated only traces of iodine but was accompanied by evolution of methane. The H N F 2 - ( C H ) G a System. H N F and ( C H ) G a were com bined i n a small reactor and thawed. T h e reaction to form methane was very slow, about / mole being produced overnight at room temperature. No explosion occurs, even i n the presence of large excess of H N F . A preliminary pressure-composition study d i d not reveal an adduct at —78.5° or —45.2° C . Distillation of the mixture through a —80° C . trap separated the components almost completely i n one pass. 2

2

4

2

3

3

2

2

3

2

3

2

3

3

3

2

Results and Discussion Difluoramine, H N F , and methyldifluoramine, C H N F , form isolable complexes with B F , B C 1 , and S 0 , and i n some cases dissociation data could be obtained. It is evident from Craig's work that usable thermo dynamic data w i l l be available only for the strongest acids. T h e problem is further complicated by the tendency for irreversible decomposition to occur i n most of these systems. T h e acids chosen for this study included the protic acids, hydrogen chloride and trifluoroacetic acid, the alkyl metals, trimethylaluminum and trimethylgallium, and sulfur trioxide, one of the strongest, gaseous Lewis acids known. Pressure-composition studies were made of these systems to detect adduct formation or other condensed phase interactions. 2

3

3

3

2

2


14.

KEITH E T A L .

1 45


The pressure-composition diagrams of the difluoramine-protic acid systems d i d not indicate compound formation. H N F was miscible i n all proportions with HC1, with large positive deviations from Raoults L a w while C F C 0 H was partially soluble i n H C 1 , giving an ideal solution. The stronger base, methyldifluoramine, was also miscible w i t h H C 1 i n a l l proportions, with large negative deviations from Raoults L a w . A t the lowest temperature, —127° C , a plateau was obtained, indicating the formation of a weak 1:1 adduct. 2

2


3

Hole Fraction CH3NF2

Figure 2.

The CH NF SO s

r

s

system at

-45.2°C.

The pressure-composition curves shown in Figures 1 and 2 indicate the formation of 1:1 adducts between S 0 and the fluoramines. Contrary to what might be expected of the coordination compounds H N F - S 0 and C H N F S 0 , the H N F adduct is much less volatile than the C H N F adduct (although C H N F is probably the stronger base). Furthermore, it was extremely difficult to obtain equilibrium pressures i n the H N F case, but no difficulty was experienced with C H N F . This fact suggests that something more than a simple addition reaction occurs i n the former. 3

2

3

2

3

2

3

3

8

2

2

2

3

2


1 46

ADVANCED PROPELLANT CHEMISTRY

The proton and F N M R spectra of the H N F 2 - S O 3 adduct at 0° C . showed only a single line with no evidence of the splitting w h i c h should occur if the fluorine and hydrogen atoms were both still bound to nitrogen. A possible explanation of this may be that the H N F adduct is not the coordination compound, but N,N-difluorosulfamic acid, H O S 0 N F . The reaction between ( C H ) A l and H N F appears to produce a 1:1 adduct at —80° C. which liberates C H slowly at this temperature. This elimination reaction, typical of the behavior of ( C H ) A l with sec ondary amines and related bases ( J ), probably occurs via the coordination compound, which, although not isolated, appears to be moderately stable at —80° C . The mass balance seems to indicate that only one mole of H N F can react at this temperature. In fact even at room temperature only two moles of methane are displaced by H N F . The assumed reac tion agrees w e l l with the H N F - C H balance: 1 9

2

2

3

3

2

2

4


3

3

2

2

2

4

(CH ) Ai + H N F 3

3

(CH ),Al:HNF t

(CH,) A1NF 2

2

2

=

2

=

(CH ) A1:HNF 3

3

(CH,) A1NF 2

+ HNF, =

2

(1)

2

+ CH

GH A1(NF ) + C H 3

2

2

(2)

4

4

(3)

The reaction of equimolar amounts of methyldifluoramine and with trimethylaluminum produces two moles of methane and a small amount of H C N . The reaction is evidently the decomposition of the amine: CH NF 3

2

=

HCN + 2 HF

(4)

followed by reaction of the H F with trimethylaluminum, liberating methane. The difluoramine-trimethylgallium system was studied briefly i n an attempt to isolate a coordination compound. N o evidence was obtained for such a compound from the pressure-composition data, and the mixture obtained was easily resolved into its components by simple trap-to-trap distillation. A t room temperature, however, one mole of methane was slowly evolved from one mole of ( C H ) G a , producing a colorless, viscous liquid. Work i n progress on the IR and N M R data of the aluminum and gallium compounds w i l l be reported elsewhere. 3

3

Acknowledgment This research was supported by the Advanced Research Projects Agency under A R P A Order N o . 350-62, Project Code N o . 9100. Techni cal direction was provided by the Director of Engineering Sciences, S R E P , A i r Force Office of Scientific Research Contract N o . A F 49(638)-1175.


14.

ΚΕΓΓΗ ET AL.


14 7

Literature Cited (1) Coates, G. Ε., "Organometallic Compounds," John Wiley and Sons, New York, 1956. (2) Craig, A . D., Inorg. Chem. 3, 1628 (1964). (3) Freeman, J. P., Kennedy, Α., Colburn, C. B., J. Am. Chem. Soc. 82, 5304 (1960).


RECEIVED April 23, 1965.


Advanced Propellant Chemistry

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