Masters Degrees (Chemistry and Polymer Science)

Permanent URI for this collection

https://scholar.sun.ac.za/handle/10019.1/3622

Browse

Browsing Masters Degrees (Chemistry and Polymer Science) by Author "Applewhite, Malcolm"

Now showing 1 - 1 of 1
  • Thumbnail Image
    Item
    A study towards the synthesis of dithiadiazolyl functionalised calix[4]arenes
    (Stellenbosch : Stellenbosch University, 2012-03) Applewhite, Malcolm; Arnott, Gareth E.; Haynes, Delia A.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: Dithiadiazolyl heterocyclic radicals form part of an ever-growing research field in the quest for organic magnets and conducting materials due to the lone electron located within the heterocyclic ring. These compounds could potentially be used as electrical current conductors and transistors which may be developed into a molecular switch or other types of molecular devices. This thesis describes the successful synthesis of three nitrile functionalised calix[4]arenes, mono-, distal- and tetra-, as bulky scaffolds to be used as precursors in the synthesis of dithiadiazolyl functionalised calix[4]arenes. The crystal structures of these three nitriles are reported herein for the first time. Dithiadiazolyls tend to dimerise in the solid state, thus by selecting a calix[4]arene as a bulky R-group, it was hoped to inhibit dimerisation. Furthermore, synthesis of the radicals on different positions of the calix[4]arene may also inhibit dimerisation in the solid state. The typical reaction in the synthesis of dithiadiazolyls involves the reaction between a nitrile and lithium bis(trimethylsilyl)amide followed by the formation of the dithiadiazolylium chloride salt by the addition of sulfur dichloride. The salt is subsequently reduced giving the desired dithiadiazolyl. However, the addition of lithium bis(trimethylsilyl)amide to mono-nitrile calix[4]arene in the synthesis of the mono-dithiadiazolyl functionalised calix[4]arene was unsuccessful. To investigate the incompatible nature of the electrophile and nucleophile, computational and physical experiments were conducted on the mono-nitrile calix[4]arene derivative. These avenues were pursued to determine whether the unreactive nature of the nitrile in this case is due to electronic effects. Changes in the reaction conditions (i.e. temperature, solvent and the equivalents of nucleophile) were all varied, but this proved to be unsuccessful. Computationally, through charge calculations, it was determined that the electronic properties of the nitrile were similar to that of nitrile examples known to work in the literature. Therefore, it was established that steric effects of the calix[4]arene are playing a critical role in the unreactive nature of the nitrile. There are two non-degenerate LUMO orbitals for the nitrile as one is conjugated and the other is planar (LUMO + 1) to the aromatic system. It has been established that nucleophilic attack through the lowest energy LUMO would result in a high energy transition state due to the loss of conjugation and attack through the higher energy LUMO + 1 would result in a lower energy transition state. However, this was not possible due to the steric conditions surrounding its position relative to the nitrile. The results obtained from this study were, therefore, able to confirm that the normally suitable reaction procedure seems to be limited to less sterically encumbered nitriles.