Interaction Between Hydrogen Bromide or Dibromine and Solid Supports

Stevenson, Paul Robert (1992) Interaction Between Hydrogen Bromide or Dibromine and Solid Supports. PhD thesis, University of Glasgow.

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Organobromine compounds have a wide range of uses in the chemical industry, which vary from petrol additives and drilling fluids to dyestuffs and pharmaceuticals. The production of these compounds under more environmentally and industrially friendly conditions is desirable. In this work the ability of y-alumina and montmorillonite K10 to adsorb bromine onto the surface, and the ability of these adsorbed species to enhance bromination reactions, are investigated. These investigations include the bromination of the supports using, dibromomethane, hydrogen bromide and dibromine. The action of each of these individual reagents upon the supports results in its own distinct adsorbed bromine species on the surface. These investigations also include the hydrobromination of alkenes and the Hell Volhard Zelinsky reaction (bromination of the carbon a to the carboxylic acid group). The alkenes chosen for this work include 1,9-decadiene and butenes, as these give rise to both liquid and gas phase interactions with the solid supports. For the Hell Volhard Zelinsky reaction hexanoic acid was used. The techniques employed to analyse these reactions include: I) Transmission FTIR spectroscopy: to identify the volatile products of the reactions. U) DRIFTS and PAS: to investigate the surface of the supports after the reactions had occurred. HI) 27Al-MAS-NMR: for analysis of the supports after reactions. IV) Neutron activation analysis: for determination of the halogen content of the supports. V) [82Br]-Bromine labelled hydrogen bromide and dibromine: for in situ studies of the bromination reactions. A major part of this work involves developing the necessary methods for the preparation, handling and counting of radiolabelled bromine compounds in heterogeneous systems. The interaction of dibromomethane and tribromomethane (bromoform) results in the formation of at least two types of bromine species on the surface of the supports. One arises from the direct interaction of the halomethane with the support the other from the dissociative adsorption of hydrogen bromide, which forms during the reaction. These interactions do not occur to any appreciable extent below 523K. An indication as to the possible reason for this high temperature is provided by the formation of carbon monoxide. This suggests that the bromination process involves the substitution of surface oxygen species with bromine, a process which requires a high activation energy. Determination of the bromine content after these interactions indicates contents of between 0.5 and 1.2 mg atom Br g-1. There is no indication, however, that the bromination of the supports with halomethanes enhances the Lewis acidity of the supports to any great extent. The [82Br]-bromine labelled hydrogen bromide tracer studies indicate that the room temperature interaction of hydrogen bromide with the supports is rapid, resulting in a bromine content of approximately 1.0 mg atom g-1. The bromine uptake values increase in the presence of unsaturated hydrocarbons. These radiotracer experiments also indicate that the adsorption of hydrogen bromide onto montmorillonite KIO results in at least two types of bromine species. One of these species is labile to room temperature exchange with unlabelled hydrogen bromide, the other inert. The effectiveness of acidic supports in enhancing hydrobromination of alkenes, appears to be related to the uptake of hydrogen bromide onto the supports. The regioselectivity of hydrobromination is greater where the overall reaction process occurs to a less extent. Results from the [82Br]-bromine labelled dibromine interactions with the supports show these interactions to be much slower than those observed for hydrogen bromide. These interactions, like those of the previous halogenating reagents, result in more than one type of bromine species. The [82Br]-bromine radiotracer experiments indicate a bromine content much greater than observed for the other brominating reagents. Most of the bromine is removed, however, if the support is degassed. This results in a bromine content similar to that observed for the previous reagents, 1.0 mg atom Br g-1. A consequence of this ability of dibromine to adsorb onto the surface, in such large quantities, is that if dibromine is added to the support first then no surface mediated reactions occur due to the swamping of these sites by bromine, and the order of addition of reagents, therefore, determines the reaction products obtained.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: J M Winfield
Keywords: Organic chemistry, Inorganic chemistry
Date of Award: 1992
Depositing User: Enlighten Team
Unique ID: glathesis:1992-75832
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Nov 2019 17:56
Last Modified: 19 Nov 2019 17:56

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