Thesis - Open Access
Master of Science (MS)
William S. Wardsworth
Many attempts have been made to elucidate the various mechanism by which phosphates act in biological and other systems. There are many suggestions regarding mechanisms all of which fit into one of two broad categories. 1. Associative. 2. Dissociative. Of these two, interest to the associative type displacement has been of most investigators and this pathway has been established with certainty. On the other hand, less work has been done and there has been much controversy surrounding the dissociative mechanism. The dissociative mechanism is believed to be important because it has been thought that it represents the actual biological processes by which life utilizes phosphaphate esters. The schemes in fig. 5, represent the two types of displacement at the phosphorus atom. There has been a number of proposals which suggest a dissociative mechanism in displacements warrants an intermediate with a reduced coordination number. In the phosphorus esters this intermediate would be a phosphacylium ion analogous to a carboacylium ion generated from an acylhalide, fig. 6. Among several structural possibilities, a phosphacylium ion could retain the geometry of the starting material. This pathway then would produce a stereospecific product where the leaving group is replaced by the nucleophile while retaining the absolute configuration around the phosphorus atom, fig. 7. For the second possibility, the intermediate phosphacylium ion could attain planarity. If this is the case the nucleophilic substitution should give upon substitution a mixture of isomers. The phosphorus atom would be prone to be attacked by nucleophile from both sides, perpendicular to the plane of the ion, fig. 8. The ratio of isomers of the product may be equal or different depending on thermodynamic and kinetic parameters.
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South Dakota State University
No Copyright - United State
Ali, Mohammed Hashmat, "Chemistry of Phosphate Esters :Stereochemical Evidence for a Dissociative Mechanism" (1983). Electronic Theses and Dissertations. 4190.