Carbon Disulphide, CS₂

Carbon disulphide is formed when carbon and sulphur are heated together, and is consequently produced when coal containing iron pyrites is distilled. In this way it was discovered accidentally by Lampadius in 1796, and again by Clement and Desormes in 1802, who, after at first supposing it to be a compound of hydrogen and sulphur, subsequently proved that it contained only carbon and sulphur. It remained, however, for Vauquelin fully to elucidate the nature of this compound by showing that when its vapour is passed over heated copper, carbon and copper sulphide result. Besides occurring in small quantity in coal-gas, from which it should be eliminated, carbon disulphide is also found in crude petroleum and in mustard oil.

Carbon disulphide is prepared by passing sulphur vapour over red-hot charcoal. This may be done on a small scale by heating pieces of charcoal contained in a combustion tube placed in a furnace slightly tilted. To the lower end of the tube a Liebig's potash bulb is attached, and is immersed in ice. Small pieces of sulphur are introduced into the upper end of the tube, which is then closed with a cork. Sulphur vapour passes over the red-hot charcoal and impure carbon disulphide, containing sulphur in solution, is gradually formed and collects in the cooled receiver. The reversible reaction

C + 2S ⇔ CS₂

has been studied at 800°-1100° C. by Koref.

For the manufacture of carbon disulphide on a large scale the charcoal is contained in a large vertical cast-iron cylinder 10 to 12 ft. high and 1 to 2 ft. in diameter. This is surrounded by brickwork and heated by a fire beneath, the sulphur being introduced through a hopper connected with a side tube at the base of the cylinder. The carbon disulphide vapour is led away from the top of the cylinder by a pipe, the end of which dips under water, where most of the product condenses. Beyond the water-condenser is a series of tubes in which the condensation is completed and the compound thus freed from hydrogen sulphide, which is subsequently absorbed in slaked lime.

In a recent American process carbon disulphide is prepared in an electric furnace which produces 3000 kilograms per diem.

Crude carbon disulphide has a very offensive odour due to the presence of organic sulphur compounds; sulphur, also, is contained in solution, which is left behind on redistillation. Organic impurities are eliminated by distilling over fat, which retains them. Contact with mercury, corrosive sublimate, solid potassium permanganate, and fuming nitric acid serves the same purpose.

Pure carbon disulphide is a colourless, highly refractive liquid with a pleasant aromatic smell resembling that of chloroform. Its density at 0° C., according to Thorpe, is 1.2923; according to Wtillner the density at t° can be calculated from the formula D₀^t = 1.29366 – 0.001506t. The vapour density of CS₂ is 2.68, whilst that corresponding to the molecular weight would be 2.62.

The boiling-point at 760 mm. pressure is 46.25° C.; according to Regnault the vapour pressures at different temperatures are as follow:

Temp. °C.	Vapour Pressure mm.	Temp. °C.	Vapour Pressure mm.	Temp. °C.	Vapour Pressure mm.
-20	47.3	40	617.53	100	3325.15
-10	79.44	50	857.07	110	4164.06
0	127.91	60	1164.51	120	5148.79
10	198.46	70	1552.09	130	6291.60
20	298.03	80	2032.53	140	7603.96
30	434.62	90	2619.08	150	9095.94

The constants for van der Waals' equation are:

a = 0.02166; b = 0.003209, and the critical temperature and pressure, respectively: 277.68° C. and 78.14 atmospheres.

At very low temperatures CS₂ solidifies to a crystalline mass which melts at -110° C. or -108.6° C. or -112.8° C. The fusion curve, showing the connection between pressure and melting-point, has been determined by Tammann.

The total heat of vaporisation (λ) of CS₂ at 0° into vapour at t° is given by the expression:

λ = 89.5 + 0.16993t – 0.0010161t² + 0.0000033245t³; Calories per kg.

whilst the latent heat of vaporisation (r) of liquid at t° into vapour at t° is given thus:

r = 89.5 – 0.06530t – 0.0010979t² + 0.0000034245t³; Calories per kg.

The constant K for the molecular rise of boiling-point of carbon disulphide is 23.7. The specific heat of liquid carbon disulphide is:

C_liq. = 0.2352 + 0.000162t,

and of the vapour at 86°-190° C. is:

C_vap. = 0.1596 (Regnault),

whilst the ratio Cp/Cv at 99.7° C. is 1.234.

Carbon disulphide possesses high optical refractive and dispersive power, in which it is exceeded only by methylene iodide, bromonaphthalene, and phenyl-mustard oil. On this account it is used for filling hollow glass prisms for producing spectra.

The following are the refractive indices for lines of different wave lengths of the visible spectrum at 0° C. and 20° C.:

Wave Length.	Refractive Index at 0° G.	Refractive Index at 20° C.
589.31 μμ (D)	1.64362	1.62761
533.85 μμ (D)	1.65508	1.63877
480.01 μμ (D)	1.67131	1.65466
441.59 μμ (D)	1.68850	1.67135
394.41 μμ (D)	1.71989	1.70180

Carbon disulphide is an endothermic compound, its heats of formation 6 as vapour from rhombic sulphur and amorphous carbon and diamond being respectively:

C (amorphous) + 2S = CS₂ (vap.) - 25,430 calories.

C (diamond) + 2S = CS₂ (vap.) - 26,000 calories.

Carbon disulphide is an excellent solvent for fats and resins, and is employed technically for the extraction of vegetable fats and oils, and for removing fat from wool. It also dissolves rubber, camphor, and other organic substances, as well as iodine, sulphur, and phosphorus.

Carbon disulphide is slightly soluble in water, its solubility diminishing with rising temperature like that of a gas. One hundred cubic centimetres of water dissolve the following quantities of carbon disulphide at the corresponding temperatures:

Temp. ° C	0°	10°	20°	30°	40°	49°
Gram CS2	0.204	0.194	0.179	0.155	0.111	0.014

100 c.c. of carbon disulphide dissolve 0.974 c.c. of water at 22° C.

Carbon disulphide is not easily decomposed by heat, and no change is observed when it is passed through a tube at 400° C. Decomposition may be started by detonation with mercury fulminate, but is not propagated through the vapour. Dissociation according to the reaction

CS₂ ⇔ C + S₂,

which is considerable at high temperature, is promoted by the presence of metals with which the sulphur can combine, the carbon separating as graphite. A lower sulphide of carbon, C₂S₃ or CS, has been supposed to be formed.

The electric arc, electric sparks, and the silent electric discharge decompose carbon disulphide; in the latter case it has been shown by Dewar and Jones that the lower sulphide CS is formed, together with free sulphur.

Carbon disulphide burns in the air with a blue flame, producing carbon dioxide and sulphur dioxide. Moisture is not necessary to combustion. The vapour undergoes phosphorescent combustion in oxygen, thus showing "chemiluminescence" below its temperature of ignition. This phenomenon may begin at 230° C. and persist until the temperature falls below 200° C., although the temperature of ignition is about 260° C. Owing, however, to its gradual or "silent" combustion, with the accompanying chemiluminescence, no definite ignition temperature can be assigned to this substance. In this property it resembles carbon monoxide, but differs from the hydrocarbons. The flame of carbon disulphide shows a continuous spectrum and is strongly actinic.

The following facts with regard to the combustion of carbon disulphide have been established by Dixon and Russell. The combustion is not preceded by a decomposition of the substance into its elements, so that no carbon is separated; the reaction cannot be expressed by a single equation. With excess of oxygen the products are carbon dioxide, sulphur dioxide, sulphur trioxide, and free sulphur; and in the explosion wave there occur carbon monoxide, carbonyl sulphide, and unchanged carbon disulphide. With insufficient oxygen the products are: carbon dioxide, sulphur dioxide, carbon monoxide, carbonyl sulphide, and carbon disulphide; limitation of oxygen reduces the sulphur dioxide formed, but even the minimum amount of oxygen is always divided between the carbon and sulphur.

Carbon disulphide forms an explosive mixture with air or oxygen. The explosive limits with oxygen are 1 volume of oxygen to 1½ volume of carbon disulphide vapour, and 6-7 volumes of oxygen to 1 volume of carbon disulphide vapour.

When a mixture of carbon disulphide vapour and air is burnt in a Smithell's separator the interconal gases are found to consist of equal volumes of sulphur dioxide and carbon monoxide, with sulphur vapour and some unchanged carbon disulphide, together with small quantities of carbonyl sulphide and carbon dioxide.

Apart from combustion, carbon disulphide can be oxidised and reduced, and also made to undergo additive reactions, the most prominent of which is its combination with alkalis to form thio- or sulpho- salts.

Oxidation of Carbon Disulphide

Hypochlorite solution converts carbon disulphide into carbonate and sulphate, thus:

CS₂ + 8KOCl + 6KOH = 2K₂SO₄ + K₂CO₃ + 8KCl + 3H₂O.

In the absence of alkali, oxidation - as, for instance, by permanganate solution, bromine water, nitric or iodic acid - involves the separation of sulphur. Sulphur trioxide produces carbonyl sulphide, thus:

CS₂ + 3SO₃ = COS + 4SO₂.

Water and aqueous alkalis hydrolyse carbon disulphide at 150° C., thus:

CS₂ + 2H₂O = CO₂ + 2H₂S;

and baryta water, when heated with carbon disulphide in an atmosphere of nitrogen, brings about a similar change:

CS₂ + 2Ba(OH)₂ = BaCO₃ + Ba(SH)₂ + H₂O,

whilst the hydrosulphide is converted by contact with air into sulphate.

Reduction of Carbon Disulphide

Just as by the reducing action of hydrogen on carbon dioxide formic acid, formaldehyde, methyl alcohol, and methane may be directly or indirectly produced, so similarly from carbon disulphide the following compounds should result:

Thioformic acid: S=CH-SH;

Thioformaldehyde: S=CH-H;

Methylmercaptan: H₃CSH;

Methane: H₄C;

Thioformic acid is unknown, but thioformaldehyde, polymerising to trithioformaldehyde, (H₂CS)₃, results from the reduction of carbon disulphide by nascent hydrogen; methyl mercaptan cannot be obtained directly from carbon disulphide, but methane is commonly prepared by passing its vapour, together with hydrogen sulphide, over heated copper:

CS₂ + 2H₂S + 8Cu = 4Cu₂S + CH₄.

Carbon disulphide, like the dioxide, is the anhydride of a feeble acid: thiocarbonic acid, H₂CS₃. Moreover, between carbonic and thiocarbonic acids a number of intermediate acids are capable of existence. They are as follow:

Carbonic acid: HO-CO-OH
Thioncarbonic acid: HO-CS-OH
Thiolcarbonic acid: HS-CO-OH
Thiolthioncarbonic acid: HS-CS-OH
Dithiolcarbonic acid: HS-CO-SH
Thiocarbonic acid: HS-CS-SH

It was discovered by Berzelius that when carbon disulphide is brought into contact with sodium sulphide solution the former dissolves, producing a solution from which alcohol precipitates sodium thiocarbonate, Na₂CS₃, as a yellowish-brown oil. From this salt dilute hydrochloric acid separates the free acid, H₂CS₃, as a yellow oil, which possesses a very disagreeable odour and is decomposed by heat into CS₂ and H₂S. Thus it is noteworthy that H₂CS₃ is more stable than H₂CO₃, doubtless because CS₂ is a liquid while CO₂ is a gas.

Decomposition of Potassium Thiocarbonate

It has been shown by Tarugi and Magri that when a solution of potassium thiocarbonate is boiled in an atmosphere of nitrogen the following changes take place:

K₂CS₃ = K₂S + CS₂; K₂S + 2H₂O = 2KOH + H₂S.

In presence of air or oxygen, however, the reaction is as follows:

2K.2CS₃ + 2H₂O + 4O = K₂S₂O₃ + K₂CO₃ + CS₂ + 2H₂S;

and in an atmosphere of carbon dioxide:

K₂CS₃ + CO₂ + H₂O = K₂CO₃ + CS₂ + H₂S.

These authors deny that the reaction

K₂CS₃ + 3H₂O = K₂CO₃ + 3H₂S,

inherently improbable, can take place.

Salts and esters of the intermediate acids are known, being prepared from carbon disulphide or carbon oxysulphide.

Thiolcarbonic acid, HS-COOH, yields carbon oxysulphide, COS, by decomposition; the esters yield alcohols or mercaptans when saponified, according to whether the alkyl group is attached to oxygen or to sulphur; by this means the constitutions of the acids are established.

It may be observed that thion- and thiol-carbonic acids are isomeric, as well as thiolthion- and dithiol-carbonic acids. Ethyl thiolthion-carbonic acid is xanthic or xanthogenic acid. Its potassium salt, which is yellow and has a disagreeable smell, is prepared by the action of alcoholic potash on carbon disulphide:

CS₂ + C₂H₅OH + KOH = KS-CS-OC₂H₅ + H₂O

The free acid decomposes at 24° C. into ethyl alcohol and carbon disulphide; hence its constitution is known to be:

HS-CS-OC₂H₅ and not HO-CS-SC₂H₅

Carbon disulphide also combines with tertiary amines and phosphines, forming crystalline substances, the most important of which is a red compound with triethylphosphine, CS₂P(C₂H₅)₃, to which the constitution is attributed.

Carbon disulphide may be detected by means of the red, crystalline compound it forms with triethylphosphine, and the white compound with phenylhydrazine:

C₆H₅-NH-NH-CS-S-NH₃-NH-C₆H₅;

also by its conversion into thiocarbonate, which gives a yellow precipitate with silver nitrate, and a red precipitate with lead nitrate, each of which quickly turns black by conversion into sulphide.

Minute quantities of carbon disulphide can also be detected by producing one of the dithiotrimercuric salts of the type HgX₂'.2HgS, which form characteristic crystalline precipitates when dilute aqueous solutions of mercuric salts are heated on the water-bath with carbon disulphide.

Carbon disulphide is estimated by solution in alcoholic potash with formation of xanthate, followed by titration with standard copper sulphate, or with permanganate solution, which oxidises the xanthate to sulphate. A third method depends upon the fact that ammonia converts carbon disulphide into a mixture of hydrosulphide and thio- cyanate, thus:

CS₂ + 2NH₃ → H₂N-SC-SNH₄ + NH₃ → NH₄HS + NH₄CNS

The hydrosulphide may then be titrated with ammoniacal zinc solution.

Carbon disulphide may be estimated gravimetrically by treatment with baryta water, so that sulphide is produced, which is then oxidised and weighed as BaSO₄.

Besides its employment as a solvent, to which reference has been made, carbon disulphide is used in the vulcanising of indiarubber, for the preparation of ammonium thiocyanate, and of numerous organic compounds, including a number of dyes containing sulphur, as well as for the prevention of phylloxera in vines.