Solvent effects of ionic liquids on a reaction at a phosphorus centre

by Bradley Butler

Institution: University of New South Wales
Year: 2016
Keywords: Kinetics; Ionic Liquids; Chemistry; NMR; Phosphates
Posted: 02/05/2017
Record ID: 2105840
Full text PDF: http://handle.unsw.edu.au/1959.4/55951


The work described in this thesis aims to expand the predictive framework for the use of ionic liquids as solvents for chemical processes. Rather than considering reactions at carbon centres, which have been comparatively well-considered in ionic liquids, this thesis focuses on the reaction of ethanol with diethyl chlorophosphate; this is a nucleophilic substitution reaction at an activated phosphorus centre. Initially the effect of the proportion of a representative ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, in the reaction mixture on both the rate constant and activation parameters for this reaction was investigated. It was found that the addition of the ionic liquid increased the rate constant for the reaction, with this increase being due to a decrease in both of the activation parameters. It was proposed that this decrease in the activation parameters was due to stabilisation of, and ordering of the ionic liquid about, the transition state of the reaction. The balance of the enthalpic and entropic effects determined varied with the proportion of the ionic liquid in the mixture. The addition of salts with a highly coordinating cation or anion in the place of the ionic liquid demonstrated the importance of minimising interactions with the nucleophile and increasing ion-pairing between components of the added species in maximising the ionic liquid solvent effect. Given these initial studies, the effect of changing the structure of the cation of the ionic liquid on the reaction outcomes was subsequently investigated. A range of structural changes to the cation was considered, including steric bulk, charge localisation and planarity. Once again, for each of the ionic liquids used, the effect of the proportion of the ionic liquid in the reaction mixture on both the rate constant and the activation parameters for the ethanolysis of diethyl chlorophosphate was determined. It was found that variation of the cation of the ionic liquid did not significantly change the observed solvent effects, the exception being the cases with extreme steric bulk, with the rate enhancements and dependence on the ionic liquid content paralleling that seen previously. The microscopic origins of the observed solvent effects also did not change significantly with the nature of the cation. The effect of changing the anion of the ionic liquid was also investigated. Particularly, how increasing the coordination strength, using the hydrogen bond accepting ability as a measure of such, of the anion affected the reaction outcome was investigated. As previously, the effect of the proportion of these ionic liquids on the rate constant of the reaction of ethanol with diethyl chlorophosphate was investigated and the microscopic origin of any changes probed through the determination of activation parameters for the process. Notably, it was found that an increase in the coordination strength of the anion of the solvent used greatly affected both the rate constant and activation parameters for the reaction. A systematic… Advisors/Committee Members: Harper, Jason, Chemistry, Faculty of Science, UNSW.