On the determination of the reaction rate constant and selectivity in gas and liquid-phase organic reactions: temperature and solvent effects
Creators: name: Diamanti, Aikaterini
Publisher: name: Imperial College London
Year: 2016

description: Chemical reactions occur abundantly in nature and the rates at which they proceed are critically influenced by factors such as the temperature and the solvent medium in which they take place. These two factors do not only affect the rate of a given reaction but may also have a significant impact on other metrics, such as the selectivity and the catalytic activity, as well as on the overall performance of a process application. Faced with such a broad scope of considerations, the identification of an optimum reaction environment remains a challenge. This thesis provides an in-depth investigation of gas and liquid-phase reactions in the context of predicting selected reaction metrics under different thermodynamic and media conditions. In particular, the effect of temperature on the rate constant of a gas-phase reaction and the effects of solvent media on the rate constant and selectivity of a liquid-phase reaction are considered. A gas-phase hydrogen abstraction reaction between ethane and the hydroxyl radical is studied in a broad range of temperatures. A thorough computational investigation is performed of the temperature dependence of the reaction rate constant, assessing several ab initio and density functional theory methods with various basis sets. A novel hybrid strategy is proposed for the development of correlative kinetic models that incorporate information from the quantum-mechanical calculations and experiments into classical Arrhenius expressions. The hybrid models derived bring new insight into the value and contribution of the data obtained via quantum-mechanical calculations and via measurements. The benefits of such models in the context of accuracy, statistical significance, applicability and practical importance on the study of reactions with scarce experimental data, are highlighted. A regioselective Williamson reaction between sodium β-naphthoxide and benzyl bromide is selected for the investigation of solvent effects on the reaction rate constant and product selectivity. The solvent medium has a key impact on the selectivity of the reaction for alkylation at two possible sites (an oxygen or a carbon atom) resulting in O- and C-alkylated products. For this reaction, a systematic study is performed combining detailed kinetic experiments and density functional theory calculations to determine the reaction rate constant in a set of solvents. The challenges in conducting reliable experiments using NMR spectroscopy are highlighted, and the performance of the various computational methods is scrutinized. Good agreement is obtained between computational predictions and experimental data for the reaction rate constants as well as for the ranking of solvents in terms of the product selectivity ratios for a number of the theoretical methods considered. These promising results pave the way for future computer-aided molecular design tools for the identification of solvents for improved reaction performance.