Studies on the chemical behaviour of elemental sulphur and some of its compounds

Abstract

The present investigation, the hydrolytic reactions of elemental sulphur, heptasulphurimide, tetrasulphur tetraimide, tetrasulphur tetranitride, dimethyl and diethyl thiosulphites, tetraethyl dithiodiamine, dithiopiperidine, disulphur dichloride and disulphur monoxide have been investigated in a homogeneous reaction medium, in presence of alkali, acid, thiophilic reagents such as sulphite and thiosulphate and an oxidising agent, viz., chloramine T. A common feature which underlines all these reactions is that the highly unstable lower oxyacids of sulphur such as H?SO, H?SO? and H?S?O? (which have not been isolated so far even in solution) probably function as intermediate products in these reactions. The hydrolysis of elemental sulphur is an extremely slow reaction at ordinary temperatures. The action of water on sulphur is negligible even at high temperatures; thus, G.N. Lewis et al. have shown that for the reaction, S? + 8H?O ? (H?S)?(SO?)? the equilibrium constant K =- has a value of only 0.00154 at 500°C. Alkaline hydrolysis of elemental sulphur is more feasible; thus when sulphur is boiled with alkali, it is converted into polysulphide and thiosulphate. But at ordinary temperatures, alkaline hydrolysis is very slow. It was found that when finely powdered elemental sulphur was left in contact with aqueous alkali (0.5N) for 24 hours, it suffered hydrolysis only to a negligible extent, viz., less than 0.5%. Also, when a solution of elemental sulphur in an organic solvent such as carbon tetrachloride was kept in contact with aqueous alkali (0.5N) for 24 hours, there was only negligibly small reaction. During the present investigation, it was observed that the alkaline hydrolysis of elemental sulphur could be carried out rapidly and quantitatively at ordinary temperatures (25 ± 2°C) using dilute alkali (0.1 to 0.5N) if a homogeneous medium were employed for the reaction. The procedure adopted was as follows: sulphur was dissolved in an organic solvent such as benzene, carbon tetrachloride or chloroform, and was added to the aqueous alkali. The immiscible liquids were then homogenized by the addition of sufficient ethyl alcohol and shaking. The solution became intense yellow on homogenization and the colour faded gradually. The entire sulphur was found to be hydrolysed quantitatively into a mixture of sulphide, thiosulphate and small quantities of sulphite. A homogeneous reaction medium could also be had by dissolving sulphur in dioxan and adding to aqueous alkali, since dioxan is completely miscible with water. The products of alkaline hydrolysis of sulphur could be qualitatively and quantitatively accounted for in terms of the intermediate formation of highly reactive and unstable species such as dihydrogen sulphoxide (H?SO), sulphoxylic acid (H?SO?) and thiosulphurous acid (H?S?O?). Sulphur is quantitatively converted into thiosulphate by the action of sulphite in homogeneous medium. This reaction has been used as the basis of an elegant method of estimation of elemental sulphur in organic solvents. The enhanced reactivity of elemental sulphur in a homogeneous medium is to be attributed to the increased facility for opening up of the S? ring under these conditions. This ring opening is an essential primary step in the usual reactions of elemental sulphur. The polysulphenyl radicals, which are produced where the S? ring is opened, are known to be much more reactive than the S? molecule. The intermediate formation of the unstable lower oxyacids of sulphur in the hydrolysis of elemental sulphur is postulated on the basis of a mechanism involving ring opening to give polysulphenyl chains which are subsequently degraded into the simpler species. An interesting oxidation reaction further pointed out the enhanced reactivity of elemental sulphur in a homogeneous medium. Chloramine T is a versatile oxidizing agent capable of rupturing S–H, S–S and O–S bonds and of oxidizing all the sulphur in such compounds into sulphuric acid. But chloramine T has no action on elemental sulphur under normal conditions. It was, therefore, of interest to find that, in an acidic homogenized medium, chloramine T oxidised elemental sulphur quantitatively into sulphuric acid. Dioxan was used as the solvent for sulphur in the chloramine T oxidation studies, since dioxan did not consume chloramine T as seen from blank experiments. The procedure of dissolving sulphur in benzene or carbon tetrachloride, adding to aqueous chloramine T and homogenizing by addition of alcohol was found to be unsuitable for the chloramine T oxidation experiments, since considerable chloramine T was consumed in the corresponding blank experiments. With dioxan, the blank corrections were less than 0.1 ml of decinormal chloramine T. It was found that 48 equivalents of the oxidant were consumed per mole of S? in agreement with the equation: S? + 8H?O + 24(O) ? 8H?SO? The technique of carrying out hydrolytic reactions in homogeneous media was extended to other sulphur ring systems. The imides of sulphur suggested themselves immediately, since they are closely related structurally to elemental sulphur. Thus heptasulphurimide and tetrasulphur tetraimide, S?(NH) and S?(NH)?, may be regarded as formally derived from elemental sulphur, S?, by the replacement of one or four of the sulphur atoms respectively by the isoelectronic (with respect to outer shell) NH groups. The sulphur imides (especially S?NH) also show great physical and chemical similarity to elemental sulphur. The hydrolytic reactions of sulphur imides were investigated in a homogeneous medium. Heptasulphurimide, S?NH, was found to be hydrolysed by alkali in homogeneous medium giving sulphide, sulphite and thiosulphate, the proportion of sulphite being more and of sulphide being less than in the case of elemental sulphur. Acid (4N) hydrolysed S?NH in homogeneous medium to give elemental sulphur mainly (92%) and smaller amounts of sulphur dioxide (6%) and hydrogen sulphide (2%). S?NH was found to react with sulphite in a neutral homogeneous medium giving 6 moles of thiosulphate and one mole of trithionate per mole of the imide. The hydrolytic reactions of S?NH were impressive on account of their chromogenic nature. Thus during alkaline hydrolysis, the solution became intense bluish purple fading to light orange, clear yellow and colourless. During the sulphite reaction, the solution became intense blue and faded to light blue and colourless. These colour formations are likely to be due to the formation of polysulphenyl radicals which are known to be chromogenic. Tetrasulphur tetraimide was found to be somewhat resistant towards nucleophilic attack. Thus it could be hydrolysed only with difficulty by dilute alkali and only at elevated temperatures, the main product under these conditions being sulphite. No reaction was observed between S?(NH)? and sulphite in homogeneous medium even on keeping for half an hour at the end of which period, all the sulphite taken remained unchanged. Acid (4N), however, hydrolysed S?(NH)? in homogeneous medium giving mainly sulphur dioxide (46%) and elemental sulphur (52%) with traces of hydrogen sulphide. Both sulphur imides were found to be quantitatively oxidized by chloramine T in an acidic homogeneous medium (dioxan–water), the entire sulphur being converted into sulphuric acid. During the oxidation, 40 and 16 equivalents of the oxidant are consumed per mole of S?NH and S?(NH)? respectively in accordance with the equations: S?NH + 8H?O + 20(O) ? 7H?SO? + NH? S?(NH)? + 4H?O + 8(O) ? 4H?SO? + 4NH?