Effects of structure on the acidity of heterocyclic carbon acids: Towards correlations with catalytic efficiency

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Copyright: Dunn, Michelle
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Abstract
This thesis reports the determination of the pKa values of 56 azolium salts in dimethyl sulfoxide using the bracketing indicator method, where the change in concentration of a fluorene indicator of known pKa value was monitored using UV-Vis spectroscopy upon addition of aliquots of heterocyclic carbon. This methodology is explored within this thesis, along with the synthesis, purification and testing of the required fluorene indicators. Each following chapter focuses on the synthesis, measurement and subsequent analysis of a different class of carbon acid, with a particular emphasis on systematically modifying structural and electronic properties of the carbon acid to understand the subsequent effects on acidity. The first series investigated were the imidazolium salts containing N-alkyl and N-aryl substituents. The particular salts chosen examined the effects of attaching electron withdrawing and electron donating groups directly to the azolium ring, the effect of extending the π system and the effect of changing the electronic and steric nature of the N-substituent. The effect of systematically changing the electronic nature of the 4'-substituent was also quantified through utilisation of Hammett parameters. The effect of different anions on the N-aryl system was also considered. These studies on imidazolium salts were followed by consideration of their more saturated counterparts with differing heterocylic ring sizes and bulkier N-substituents, along with quantification of the effect of the 4'-substituent. Azolium salts containing different heteroatoms were also considered. A set of triazolium salts was investigated with an emphasis on the effect of changing the N-aryl and azolium ring substituents. While thiazolium salts were examined in terms of the effect of electron donating groups attached directly to the ring, changing the 2',4' and 6'-aryl substituents. This work also endeavours to make a direct correlation between pKa values determined in dimethyl sulfoxide to those determined in water and using computational methods. Correlation between acidity and existing data that quantify the electronic nature of the deprotonated carbon acid (carbene) when coordinated to a metal was also attempted. Preliminary studies are also presented that show a correlation between acidity of a heterocyclic carbon acid and catalyst efficacy of its corresponding carbene.
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Author(s)
Dunn, Michelle
Supervisor(s)
Harper, Jason
Cole, Marcus
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Publication Year
2014
Resource Type
Thesis
Degree Type
PhD Doctorate
UNSW Faculty
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