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|Title:||A study of the vapour phase pyrolysis and alkaline hydrolysis of chloroform|
|Authors:||Andrews, Laura Elizabeth|
|Keywords:||Kinetic investigations;Induction period;Rate of disappearance;Negative salt effect;Yagil approach|
|Abstract:||The decomposition of chloroform at 510˚C was investigated in a continuous flow system, using nitrogen as the carrier gas. The main products of the reaction were hydrogen chloride, tetrachloroethylene, pentachloroethane and hexachloroethane; hydrogen and chlorine could not be detected. Neither the addition of a radical initiator (azobisisobutyroni trile) nor an inhibitor (phenol) affected the breakdown, and so we have rejected the idea that the main reaction mechanism is of a radical nature. From the results obtained by varying the surface to volume ratio of the reactor, the reaction appeared to be catalysed by the surface of the reactor, but pretreating the tube by steaming, soaking in water or EDTA solution did not affect the pyrolysis. The reaction appeared to be virtually unaltered when a tube made from very pure silica was used. Kinetic investigations showed that the reaction was first order with respect to chloroform, and exhibited an induction period. Carrying out the reaction in a carbonised tube resulted in a faster reaction, and an increase in the induction period. To account for the results obtained, a mechanism is suggested which involves the formation of a carbon polymer on the silica reactor. Chloroform could then be adsorbed onto the polymer where reaction could occur. In a carbonised tube a graphite-like carbon structure may begin to take over as the catalyst. The alkaline hydrolysis of chloroform was studied in aqueous 1,4-dioxane (32 % w/v), at 360C. The rate of disappearance of chloroform was followed by gas-liquid chromatography, whilst the sodium hydroxide concentration was determined by titration with dilute hydrochloric acid. The reaction was second order overall, first order with respect to each of the reactants. Data obtained at 25˚C, 31˚C, 36˚C and 41˚C was used to determine the activation parameters. Increasing the concentration of dioxane in the solvent decreased the reaction rate. The reaction showed a definite negative salt effect, sodium chloride exhibiting a greater effect than potassium nitrate. The reaction between deuterochloroform and sodium deuteroxide in deuterium oxide/dioxane was faster than the reaction in the corresponding proton system. Application of the Yagil approach suggested that the transition state is associated with seven water molecules. This led us to suggest an alternative mechanism for the reaction, involving nucleophilic attack by water on the trichloromethyl anion, formed by loss of a proton from chloroform. In order to apply the Yagil criterion the hydration number of diolane had to be determined; a value of 2.25 was obtained.|
|Description:||This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.|
|Appears in Collections:||Brunel University Theses|
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