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|Title:||Photochemical and thermal interconversions of protonated phenols and bicyclo(3.1.0)hexenones|
|Authors:||George, Edward Baha|
|Description:||The work described in this thesis involves a quantitative study of the photoisomerizations of a series of protonated phenols and the reverse, thermally induced ring opening reactions of protonated bicyclo[3.1.0]hexanones. In the first chapter the importance of these reactions is established in terms of their potential use in photochemical latent heat solar energy storage systems. The second and third chapters consist of the results and discussion of this present work. In the first place the protonation of polymethyl and chlorophenols in CF₃SO₃H (room temperature) and FSO₃H (low temperature) have been investigated using ¹H NMR and UV spectroscopy. Apart from 2,3,4,5-tetramethylphenol protonation at C₄ was observed for all methylphenols in CF₃SO₃H. In the case of 2,3,4,5-tetramethylphenol, both C₄ (95%) and C₆ (5%) protonation was observed. With 2-chloro- and 3,5-dichlorophenols 85 and 87 in FSO₃H O- and C₄-protonation was observed. One difference in this work to that previously reported is the use of CF₃SO₃H as a strong acid medium. Overall, the same cations were produced in CF₃SO₃H as in FSO₃H. However, in certain cases there were some differences noted in the ¹H NMR spectra of these ions as compared to those obtained at low temperature in FSO₃H. These differences are discussed in terms of an exchange of the ring protons of some of these protonated phenols with CF₃SO₃H. The photochemical reactions of the C₄ protonated phenols in stong acid media (i.e. CF₃SO₃H and FSO₃H) have been examined. Most of these protonated phenols were found to undergo clean photoisomerizations to give protonated bicyclo[3.1.0] hexanones. Neutralizations of the irradiated acid solutions gave substituted bicyclo[3.1.0]hexanones. The quantum yields for the photoisomerization of a number of protonated phenols to the bicyclo[3.1.0]hexanones were measured in CF₃SO₃H. It was found that substituents have a profound effect upon the efficiency of these reactions. Methyl groups in the two ortho positions were found to increase the efficiency of the reaction, while methyl substituents in the meta position were found to reduce the efficiency dramatically. This substituent effect can be understood in terms of a two step mechanism with the initially formed intermediate undergoes competitive thermal reactions. The relative rate of these two processes would seem to depend on the positioning of the methyl substituents. A quantitative investigation of the ground state isomerizations of a series of methyl substituted bicyclo[3.1.0]hexanones in CF₃SO₃H was also undertaken. In every case the protonated bicyclo[3.1.0]hexanones isomerized to give a protonated phenol. The activation energies for these processes were sufficiently high in several instances to necessitate heating the solutions to 100°C for several hours in order for the reactions to be completed. An analysis of the products of these reactions showed that the ring opening could take place by three different mechanisms. The factors which determines the preferred isomerization route are discussed in the thesis. In the concluding part of the thesis the quantitative results found in this work for the protonated methyl substituted phenol/bicyclo[3.1.0]hexanones interconversion are discussed in terms of their use for solar energy storage.|
|Appears in Collections:||Open Access Dissertations and Theses|
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