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NACA RESEARCH MEMORANDUM

- PREPARATION OF DIPENTABORYLMET}jANE BY FRIEDEL- 1 CRAFTS REACTION ByAlbert C. Antoine

Lewis Flight Propulsion Laboratory Cleveland, Ohio *

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C CLASSrFIED DOCUMENT o -' - . This material contains information affecting the National Defense of the United States within the meaning of the esptona taws, tle ii, U.S.C., Secs. 791 and 794, the ths1on or reveisUos of which in any manner to an unauthorized person is prohibited by low. NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS WASH I NGTQN April 1, 1958

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NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS

RESEARCH MEMORMIDUM

PREPARATION OF DIPENTABORYLNETEANE BY FRIEDEL- GRAFTS REACTION

By Albert C. Antoine

SUMMARY

Preparation of dipentaborylmethane by reacting pentaborane with dichloromethane in the presence of aluminum chloride gave maximum yields of less than 1 percent. No dipentaborylethane resulted from the reaction of pentaborane with l,2-dichloroethane and with l,2-dibromoethane. The reaction did produce ethylpentaborane in yields up to 47 percent. The formation of polymeric solids accompanied all reactions. These solids resulted in part from the decomposition of unstable reaction products, one of which was isolated but not identified.

INTRODUCTION

Boron hydrides are of great interest in the current search for new high-energy fuels. Two characteristics of boron hydrides present serious problems in their use as fuels: their ready flammability on exposure to air and their extreme toxicity. Alkyl derivatives of boron hydrides hold promise of reduced toxicity and reduced flammability on exposure to air while retaining high heats of combustion (ref. 1).

Several alkyl derivatives of pentaborane have been prepared (refs. 2 to 4). They have generally been prepared by a Fried.el-Crafts reaction with the boron hydride and alkyl halides. The general reaction is

Aid3 B5H9 + RX B5H8R + HX where R is the alkyl group and X is the halide. The aikyl halides previously used have been the monobalogenated alkanes such as methyl, ethyl, and propyl chlorides, bromides, or iodides. The present report describes the reaction with five dihalogenated. alkanes: d.ichloromethane, dibromomethane, diiodomethane, 1, 2-dichioroethane, and 1, 2-dibromoethane. The products expected would have two pentaboryl groups in one molecule. Th general reaction is

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Aid3 2B5119 + XCH2X B5118C112B5118 + 211X

A1C13 2B5119 + XCH2CH2X -- B5118C112C112B5118 + 211X

The heats of combustion are expected to compare favorably with those of the other alkyl derivatives (ref. i).

APPARATUS MJD PROCEDURE

The pentaborane was used. as obtained from the manufacturer, since periodic checks of the vapor pressure showed little change. The alkyl halides were commercial products. They were dried over anhydrous cal- cium sulfate for 24 hours prior to use.

Reactions of Pentaborane with Dichioromethane

AU reactions were carried out in a dry box. The solvent n-heptane was placed in a flask and aluminum chloride, pentaborane, and dichioro- methane were added in that order. The mixture was refluxed. for 5 hours. After cooling, the volatile material was rmoved under vacuum and. the residue extracted with carbon disulfide. An amorphous material was left after removal of the carbon disulfide. When this material was heated in a sublimation apparatus, a colorless crystalline material resulted. The crystals were washed with water, then recrystallized from carbon disul- fide. The melting point, 51.5° to 53.0° C, was determined in a double- bath melting-point apparatus (ref. s). Elemental analysis gave the fol- lowing results: Percent boron, 78.9; percent carbon, 8.4; percent hydrogen, 13.5. The calculated results for dipentaborylmethane are: Percent boron, 78.2; percent carbon, 8.7; percent hydrogen, 13.1. Figure 1 shows the infrared spectrum of the product. The yield was 0.35 percent.

Aluminum chloride and aluminum (20-mesh) were placed in a flask. Dichloromethane and pentaborane were added in succession. The mixture was refluxed for 4 hours. After cooling, volatile material was removed by vacuum distillation at room temperature and 20 millimeters of mercury. A second fraction was removed by boiling from 35° to 500 C at 1 to 2 millimeters of mercury. This fraction was redistilled yielding material whose boiling point was 50° C at 7 millimeters of mercury. The product was spontaneously flammable in air and readily decomposed at room temper- ature in a nitrogen atmosphere. Elemental analysis showed that the com- pound contained chlorine in addition to boron, carbon, and hydrogen. The infrared spectrum is shown in figure 2.

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Reaction of Pentaborane with Dibromomethane

and. with Diiodomethane

Pentaborane was reacted. with dibromomethane and. with diiod.omethane in the presence of aluminum chloride with n-heptane as the solvent. The reactions were carried out in a manner similar to that used with dichloro- methane. Small amounts of an alkylated. boron hydride were indicated by infrared spectra, but no product was isolated.

Reaction of Pentaborane with l,2-Dibromoethane

Aluminum chloride and l,2-d.ibromoethane were added to n-decane in a flask. The flask was shaken as pentaborane was slowly added. The flask was then slowly heated because a vigorous reaction occurs shortly after heating begins. The mixture was refluxed for 7 hours. Unreacted penta- borane was removed from the reaction mixture by distillation, and the remaining liquid was distilled under vacuum. A fraction boiling from 35° to 63° C at 19 to 20 millimeters of mercury was collected, introduced into a vacuum system (ref. 6), and fractionated. A product having a vapor pressure of 15 millimeters at 0° C, 35 millimeters at 25° C was isolated. The molecular weight was determined from the weight and volume of a sample of vapor and found to be 95.76. The literature values for ethylpentaborane are: vapor pressure, 13 millimeters at 0° C, 40 mil- limeters at 25°; molecular weight, 94.0 (ref. 2). The infrared spectrum of the product is shown in figure 3.

Reaction of Pentaborane with l,2-Dichloroethane

Aluminum chloride and l,2-dichloroetbane were placed in a flask. The flask was shaken as pentaborane was added in small portions. A moderately vigorous reaction started soon after heating began but sub- sided in a few minutes. The mixture was refluxed. for 6 hours. frac- tionation of the reaction mixture yielded pentaborane and ethylpentaborane.

RESULTS AND DISCUSSION

Dipentaborylmethane was formed in very small quantities by the reaction of pentaborane with d.ichloromethafle. Varying amounts of the reactants were recovered, together with a polymeric solid material formed from direct decomposition of the reactants, or from decomposition of the reaction products, or from a combination of the two processes. These results are summarized in table I. Dibromomethane and diiodomethane were reacted under like conditions. None of the desired product was isolated, although infrared analysis indicated the presence of some alkylated boron hydride. (See table II.)

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Reacting dichioromethane without a solvent and in the presence of aluminum did not improve the yield of dipentaborylmethane. However, an unstable liquid compound containing chlorine was also formed. Molecular- weight determinations and boron analyses indicated that the compound con- tained fewer than five boron atoms, rather than the pentaborane structure. In experiments with diiodomethane under similar conditions, a like com- pound was observed by infrared spectra (fig. 4) but was not isolated.

Reacting l,2-dichloroethane and. 1,2-dibromoethane with pentaborane yielded no disubstituted. ethane. Instead, ethylpentaborane was the only product isolated, as indicated in table III. Similar behavior has been observed in the reaction of decaborane with l,2-dichloroethane (ref. 7). The product obtained, ethylpentaborane, could arise in two ways, both of which involve a reduction by the hydride. The first method would begin with a Friedel-Crafts reaction with pentaborane and the dihalide, fol- lowed by reduction of the product. In the second method, the dihalide is reduced to the monohalide and the latter then reacts with pentaborane. The equations for the two processes are:

A1C13 B5H9 + XCH2 CH2X B5H8CH2CH2X + HX (1) A1C13 B5118 C112 C112X + B5119 - B5118 C112 CH3 + fiX + polymeric products

A1C13 B5119 + XCH2 CH2X CH3CH2X + BX + polymeric products (2) A1C13 CR3 CH2X + B5119 B5H8CH2CH3 + fiX

SUMMARY OF RESULTS

1. Yields of less than 1 percent of dipentaborylmethane were obtain- ed from a Friedel- crafts reaction between pentaborane and dichioromethane. Using dibromomethane and. diiodomethane in place of dichioromethane did not improve the yield.

2. None of the desired product was obtained from the reactions of pentaborane with l,2-d.ichloroethane and 1,2-dibromoethane. Ethylpenta- borane was obtained, with a maximum yield of 47 percent.

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3. The formation of polymeric solids accompanied all reactions, partly as a result of the decomposition of unstable reaction products. One such compound was isolated but not identified.

Lewis Flight Propulsion Laboratory National Mvisory Committee for Aeronautics Cleveland, Ohio, January 16, 1958.

1. Altshuller, Aubrey P.: calculated Heats of Formation and conibustion of Boron Compounds (Boron, Hydrogen, Carbon, Silicon). NACA RN E55G26, 1955.

2. Voegtly, R. 0., et al.: Preliminary Investigations of Methods of Preparing Alkyl Derivatives of Pentaborane-9 and Decaborane. Rep. No. CCC-1024-Th-8O, Callery Chem. Co., Dec. 27, 1954. (Contract NOa(s) 52-1024-c.)

3. Harris, Samuel: Friedel-Crafts Synthesis of Methylpentaborane-9 and Dimethyldecaborane. Tech. Rep. MCC-1023-TR-96, Olin thieson Chem. Corp., Nov. 8, 1954. (Contract NOa(s) 52-1023-C.)

4. k1e, Paul S., Pisani, Joseph A., and Tannenbaum, Stanley: Au Investigation of Some Physical Properties of n-Propylpentaborane. Tech. Rep. MCC-1023-TR-141, Olin thieson Chem. Corp., July 15, 1955. (Contract NCa(s) 52-1023-C.)

5. Morton, Adrian Avery: Laboratory Technique in Organic Chemistry. Mci^aw-Hill Book Co., Inc., 1938, pp. 27-28. 6. Sanderson, Robert Thomas: Vacuum Inipulation of Volatile Compounds. John Wiley & Sons, Inc., 1948, pp. 104-105. 7. Fidler, D. A.: Reactions of Decäborane with Vinyl Chloride, Allyl Chloride, and Alkyl Dihalides. Tech. Rep. MCC-lO23-TR-l68, Olin Mathieson Chem. Corp., Aug. 29, 1955. (Contract NOa(s) 52-1023-C.)

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TABLE II. - COMPARISON OF REACTIONS OF PENTABORANE WITH DICHORO1ETHA.NE, DIBROMOMETHANE, AND DIIODOMETHANE UNDER LIKE CONDITIONS

Reactants, Aid3, Solvent, Time, Product, moles moles n-heptane, hr B5H8dH2B5118, percent B5H9 Halide yield 0.21 CH2C12 , 0.12 0.023 30 4.5 0.35

0.29 CH2Br2 , 0.12 0.026 30. 5 (a)

0.29 CH212 , 0.12 0.026 30 5 (a)

a}duct observed in infrared spectra but not isolated.

TABLE III. - REACTION OF PENTABORANE WITH 1, 2-DICRLOROETIIANE

AND l,2-DIBROMOETHANE

Reactants, Solvent, Time, Reactants Residue, Product, moles n-decane, hr recovered, percent B5H8CH2CH3, ml moles weight of B5H9 Halide ercent reactants yield B5H9 Halide 0.27 C1CH2CH2C1, 0.12 6 0.07 -- None 45 20

0.40 BrCH2CH2Br, 0.10 50 7 0.07 None 23 47

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