Arsenic Tribromide, AsBr₃

Arsenic Tribromide, AsBr₃, was first obtained in 1828 by Serullas, who added dry powdered arsenic in small quantities to bromine contained in a glass retort until no further action occurred. On contact with the bromine the arsenic inflamed brilliantly, and on distillation of the product, arsenic tribromide condensed in the receiver in the form of prismatic crystals. A more convenient method is to pass bromine vapour over an excess of arsenic contained in a hard glass tube. By repeated distillation of the product, very pure arsenic tribromide may be obtained.

The bromide may also be prepared (1) by adding arsenic powder to a mixture of carbon disulphide and bromine (2:1 by weight) and agitating the liquid until decolorised; on evaporation, crystals of the bromide remain; (2) by heating a mixture of arsenious oxide, potassium bromide and acetic acid at 100° C.; (3) by heating a mixture of arsenious oxide and sulphur in a current of bromine vapour.

Arsenic tribromide is a solid at ordinary temperatures, crystallising in beautiful colourless rhombic prisms which possess a feebly aromatic odour and are stable in dry air. In the presence of moisture slight fuming occurs. The crystals melt sharply at 31° C. The density at 15° C. is 3.66; after fusion and resolidification, the product has density 3.54 at 25° C. The density of the liquid at various temperatures may be obtained from the expression

D₄^θ = 3.3972 – 0.002822(θ - 25) + 0.00000248(θ - 25)²

The surface tension, a, and molecular surface energy, μ, of the liquid at various temperatures have been found to be:

t, °C	49.6	74.5	90.0	121.0	149.6	165.0	179.7
σ, dynes per cm.	49.6	46.6	44.8	41.0	38.2	37.0	36.1
μ, ergs per sq. cm.	1029.5	980.5	947.8	884.1	835.6	815.6	801.6
The parachor is 253.5

The molecular volume at the ordinary temperature is 111 c.c., and the ratio of this value to the sum of the atomic volumes of the constituent elements is 1.21. From measurements of density and coefficient of expansion at -194° C. the molecular volume at 0° Abs. has been calculated to be 76.1. The molecular weight determined by ebullioscopic and cryoscopic methods with bromine as solvent agrees with the formula AsBr₃. The solution in bromine is a non-conductor of electricity; on the other hand, the solution in liquid sulphur dioxide is a conductor, the equivalent conductivity of 1 mole of AsBr₃ in 101.4 and 925.3 litres of the solvent being respectively 0.249 and 2.347. The solution in liquid hydrogen sulphide also conducts electricity. Arsenic tribromide itself may be used as a solvent in molecular weight determinations; the cryoscopic constant has been given values ranging from 189 to 206. It behaves similarly to arsenic trichloride in forming mixed halides with halides of the elements of Groups IV and V. The ionising power of arsenic tribromide is somewhat less than that of the trichloride. The liquid boils at 221° C. (760 mm.); at lower pressures the following temperatures have been observed: 109° C. (20 mm.), 92° C. (14 mm.).

The heat of fusion is 8.93 calories per gram. The heat of formation is:

As(cryst.) + 3Br(liq.) = AsBr₃(cryst.) + 45,500 calories

The following values for the refractive index for sodium, lithium and thallium light, respectively, have been obtained:

Temperature, ° C.	nNa	nLi	nTl
24	1.7849	1.7716	1.7983
30	1.7815	1.7678	1.7939

The value for the electron polarisation, is 41.97, and the stable form of the molecule is a three-sided pyramid. The dipole moment is 1.66×10^-18 e.s.u. The dielectric constant at 20° C. is 3.4, and at 35° C. 9.3.

Arsenic tribromide dissolves in many organic liquids, such as carbon disulphide, methylene iodide and benzene; in the last named the decomposition voltage is 0.50 volt. In solutions in diethyl ether the electrical conductance at 18° C. rapidly increases with increase in the bromide concentration up to 95 per cent., after which it quickly falls to the value for the pure bromide. For high concentrations the temperature coefficient is positive, but decreases with falling concentration and for concentrations below 75 per cent, is negative. This effect does not appear to be due to interaction between solvent and solute; if the molecular conductivity is calculated with reference to the bromide as electrolyte, the conductivity-concentration curve shows two maxima, but if the ether is taken as the electrolyte a normal curve results. For solutions in methylethyl ether1 the specific conductivity and its temperature coefficient are at a maximum for solutions containing the two components in equimolecular proportions, indicating the formation of the compound AsBr₃.CH₃OC₂H₅. The solution of arsenic tribromide in anisole is non-conducting. The tribromide also dissolves in boron bromide and in phosphoryl chloride.

Arsenic tribromide is itself a good solvent for certain halides. The following freezing points and densities at 15° C. of saturated solutions have been determined:

Solute.	Freezing Point, °C.	Density.
SbBr3	47	3.685
SbI3	40	3.720
ZnI3	15	3.73

Arsenic triiodide also dissolves, the saturated solution at 15° C. having density 3.661. Other soluble halides are potassium bromide, anhydrous ferric and aluminium chlorides and tetramethyl ammonium iodide; but the iodides of rubidium, cadmium, manganese and cobalt, also mercuric and stannic iodides, and cobalt and stannic bromides, are insoluble or only very slightly soluble in arsenic tribromide. The liquid also dissolves phosphoryl bromide and, very slightly, ammonium thiocyanate. In the mixed solutions of halides, the components may react chemically, but such is not always the case; for example, with antimony tribromide a continuous series of solid solutions is formed.

Arsenic tribromide reacts with water in small quantity to form the oxybromide, AsOBr; in larger quantity arsenious and hydrobromic acids are formed. One part of the tribromide dissolves in three parts of boiling water, and on cooling the solution deposits crystals of arsenious oxide. The tribromide is less soluble in aqueous hydrobromic acid. It dissolves in bromine, but does not combine. Oxygen reacts with arsenic tribromide vapour at red heat to form arsenious oxide, arsenic oxybromide, AsOBr, and free bromine. Hydrogen sulphide passed over the tribromide at 150° C. yields the bromosulphide, AsSBr (m.pt. 118° C.). Ammonia gas is absorbed by arsenic tribromide to yield a pale yellow solid of composition AsBr₃.3NH₃; if the bromide is in benzene solution the product, according to Landau, is a white precipitate of composition 2AsBr₃.7NH₃. The latter can be crystallised from hot absolute alcohol; it is soluble in cold water, but the solution decomposes on warming, as also does the dry ammine, losing ammonia. The pale yellow triammino-compound also decomposes on heating yielding, at 300° C., nitrogen, ammonium bromide and arsenic. Organic amines react similarly with arsenic tribromide. Phosphine yields hydrogen bromide and arsenic phosphide, AsP.

Carbon tetrachloride is partially transformed into the bromide by prolonged heating with arsenic tribromide at 150° to 200° C. Acetylene, in the presence of aluminium chloride or bromide as catalyst, forms the bromovinylarsines (CHBr:CH)AsBr₂ and (CHBr:CH)₂AsBr, and with an increased quantity of the catalyst (CHBr:CH)₃As. When a mixture of arsenic tribromide and cyanogen bromide is slowly heated in an autoclave to 180° C. and then cooled and kept for an hour at 120° C., the additive compound AsBr₃.2BrCN is formed. It is decomposed when heated above 190° C.

Certain compounds with metals and metallic salts have been described. Thus, by heating finely divided silver or copper with arsenic tribromide in a sealed tube, substances of composition 3Ag.AsBr₃ and 7Cu.2AsBr₃ have been isolated, the latter probably containing uncombined copper in solid solution. Similar products are obtained by the action of arsenic vapour on silver bromide or cuprous bromide at 500° C. Double decomposition reactions occur when arsenic tribromide is heated with stannic iodide or germanium iodide, the reactions being complete. Sodium azide reacts with the tribromide in acetone or methyl alcohol solution to form a white compound of composition Na₈[AsBr₃(N₃)₈]. Rubidium and caesium salts of composition 3RbBr.2AsBr₃ and 3CsBr.2AsBr₃ have been obtained in the form of trigonal crystals by a method analogous to that used for the corresponding chlorides.