| dc.description.abstract | The present status of knowledge about the chemistry of thiophosphoryl fluoride has been reviewed in the introductory chapter. Taking this background into consideration, a line of investigation has been undertaken which is broadly indicated in the scope of the present investigation.
Most of the methods available in literature for the preparation of thiophosphoryl fluoride give mixed fluorohalides along with the thiophosphoryl fluoride. In the present investigation, a method has been standardised for the preparation of pure thiophosphoryl fluoride. This method involves the fluorination of thiophosphoryl chloride by sodium fluoride in boiling acetonitrile medium. It is possible to get thiophosphoryl fluoride in good yield (80%). The purity of the gas (99.9%) is checked by IR spectrum and chemical analysis.
Reagents required for the present investigation have been purified wherever necessary by standard methods. Physico?chemical data have been obtained with standard commercial instruments.
During the present study, it became necessary to standardise new analytical procedures for the estimation of fluoride and sulphur. An elegant, rapid spectrophotometric method has been evolved for the estimation of fluoride, making use of the bleaching action of fluoride ion on the yellow colour of cerium (IV) solution in sulphuric acid medium.
A rapid and reliable method for the estimation of elemental sulphur has been standardised. This method involves the reduction of elemental sulphur with hydrazine hydrate to give hydrogen sulphide quantitatively. Standard, well?established analytical procedures are adopted in other cases.
Although thiophosphoryl fluoride is known to be unstable towards moisture, no detailed accounts are available in literature about its hydrolytic reactions. Hydrolytic reactions of thiophosphoryl fluoride have therefore been studied both in alkaline and acid media. It is interesting to point out that along with phosphate and fluoride, different oxygenated sulphur species such as sulphite and thiosulphate, in addition to sulphide and elemental sulphur, are produced. Formation of these different oxygenated sulphur species has been explained by considering an ionic structure for thiophosphoryl fluoride wherein sulphur has an average oxidation state of +3.
It is of interest to point out that an acidified solution of Chloramine?T is an efficient oxidising agent towards a variety of phosphorus?sulphur compounds, the P–S bond being ruptured and sulphur being oxidised to its maximum oxidation state of +6. During the present study the oxidation of thiophosphoryl fluoride with Chloramine?T has been investigated. Even in this case, it is observed that the P–S bond undergoes cleavage, sulphur getting oxidised to sulphate by consuming eight equivalents of Chloramine?T for every mole of thiophosphoryl fluoride.
Thiophosphoryl fluoride is observed to undergo reduction with a variety of reducing agents such as anhydrous hydrogen iodide, hydrazine hydrate, lithium aluminium hydride and sodium borohydride. In all the cases, it is found that sulphur in thiophosphoryl fluoride gets reduced to hydrogen sulphide quantitatively. Pentavalent phosphorus is also reduced to the trivalent state. However, with lithium aluminium hydride and sodium borohydride, the P–F/Cl bond is completely broken down with the simultaneous formation of P–H bond resulting in the formation of phosphine quantitatively. In the other two cases only traces of phosphine are obtained. With hydrazine hydrate, the trivalent phosphorus undergoes hydrolysis and remains in the reaction mixture as phosphite. With anhydrous hydrogen iodide, the trivalent phosphorus formed partially gets oxidised to pentavalent state by the liberated iodine. These products on hydrolysis with alkali result in the formation of phosphite and phosphate along with fluoride in the solution.
Thiophosphoryl fluoride reacts with metals producing respective metal sulphides, getting itself reduced to phosphorus trifluoride, while with silver oxide phosphoryl fluoride is formed along with silver sulphide. The metals chosen for the present study are copper, silver, nickel, iron, zinc, cadmium, mercury and aluminium. It is observed that the reaction at room temperature (25°C) is rather sluggish, so the reactions are carried out at elevated temperature (100°C). Silver oxide is found to react with thiophosphoryl fluoride even at room temperature to give phosphoryl fluoride and silver sulphide, while with other metal oxides no perceptible reaction is found even at 100°C. Above 100°C, the gas itself decomposes on the surface of glass giving rise to different products such as silicon tetrafluoride, sulphur dioxide, phosphorus pentafluoride, elemental sulphur and phosphorus. The reactions with metals and their oxides could not therefore be carried out above 100°C.
Thiophosphoryl fluoride reacts with pyridine giving rise to a 1:1 adduct. This indicates that thiophosphoryl fluoride is a Lewis acid. The adduct has been characterised and the structure has been established by spectral data, hydrolytic reactions, reduction with hydriodic acid and electrical conductivity measurements.
Thiophosphoryl chloride is also found to give an adduct with pyridine of similar nature under the same experimental conditions. This has been characterised employing the above?mentioned methods.
Lewis acids are reported to initiate the polymerisation of tetrahydrofuran. In the present investigation it is observed that thiophosphoryl fluoride does initiate the polymerisation of tetrahydrofuran. The polymer thus obtained is a high molecular weight elastomer. A cationic mechanism has been proposed for its formation.
Phosphoryl fluoride is also observed to initiate the polymerisation of tetrahydrofuran giving a high molecular weight elastomer.
From the experimental results in this thesis, it is possible to conclude that thiophosphoryl fluoride is a very reactive compound towards many reagents. The oxidation?reduction studies indicate that it is possible to employ these reagents for analytical purposes. The hydrolytic reactions show that thiophosphoryl fluoride has an ionic structure. Thiophosphoryl fluoride is observed to be a good Lewis acid as evident from the adduct formation with pyridine. This Lewis acid property has enabled it to function as an efficient initiator for the polymerisation of tetrahydrofuran. | |
