Supervisor : Professor Ross Stewart In the last decade there has been considerable interest in strongly basic solutions and Hammett H_ acidity functions have been defined for various solvent systems. However some doubts still remain about the ionization modes of certain indicators which were used to determine these H_ functions, and the pKa values assigned to weak carbon acids. A careful anchoring of the H_ scale for the system dimethylsulfoxide-water-0.011 molar tetramethylammonium hydroxide with 2,4,4'-trinitrodiphenylamine was undertaken. The lower region of this scale correlates well with the recently published H_ function for the more basic regions. To study the dependence of H_ functions on indicator structure, the ionization of nitrogen and oxygen acids were compared in strongly basic systems. The solvent systems used in the study were dimethylsulfoxide-methanol-0.01 molar sodium methoxide and dimethylsulfoxide-ethanol-0.01 molar sodium ethoxide. The shallow rise of the function for phenols as compared to that of the HN for diphenylamine is believed to be caused by the smaller degree of charge delocalization in the phenolate anions than in the amide ions. The dissociation constants of 15 substituted nitrodiphenyl-amines were determined in the two previously mentioned alcoholic dimethylsulfoxide systems, and in the aqueous dimethylsulfoxide system. The "similar colour indicator" postulate was elaborated on the observation that the relative acidity of Hammett-type indicators remains constant through changes in solvent environment only if their spectral characteristics are similar. Linear free energy relationships between the pK values of 30 substituted diphenylamines and the substituent constant, σ°, were drawn. The ρ value in excess of 4 for monosubstituted diphenylamines compared with ρ values of the order of 2 for nitrosubstituted diphenylamines, results from the low degree of conjugation between the central N atom and the ring substituent whenever a nitro group is present in the other ring. The base-catalyzed rates of detritiation of various hydrocarbons were determined in dimethylsulfoxide mixtures. The logarithm of these rates were correlated with the thermodynamic basicity of the medium as measured by the H N function and gave excellent straight lines with slopes around 0.8. The theoretical basis of such correlations is discussed, and it was concluded that the logarithm of the rates should be correlated with H_ + log aROH. In the case of methanolic dimethylsulfoxide the activity of methanol has been determined, giving corrected correlations with slopes around 0.9. The qualitative limitation due to the use of HN instead of HC functions is also discussed. The activation parameters for these detritiations have been determined. The enthalpies of activation were found to increase with decreasing acidity of the hydrocarbon. These variations in enthalpy of activation, coupled with variation in the kinetic isotope effect and the observed Bronsted relationship have been interpreted in terms of a mechanism involving a rate-determining pre-equilibrium step with a transition state of increasing asymmetry. A critical examination of the HC scale for carbon acids in dimethylsulfoxide-ethanol-0.01 molar sodium ethoxide was made. The failure of the scale probably results from the fact that, for a number of indicators, proton abstraction is followed by a reaction between the carbanion and the neutral molecule to give a radical and radical anion. The carbanions of three nitrophenylmethanes were prepared in hexamethylphosphoramide and unequivocally identified by high resolution nuclear magnetic resonance. The spectral characteristics of the carbanions were recorded and compared with previously published values. The radical anions of these nitrophenylmethanes were generated electrochemically and their e.s.r. spectra measured. The variation in ¹⁴N hyperfine coupling constant (aN) of these radical anions indicated a large solvent shift in the absorption maxima. / Science, Faculty of / Chemistry, Department of / Graduate

Thesis (Ph.D.(Chemistry))--University of Pretoria, 1992. / Includes abstract in English and Afrikaans. Includes bibliographical references.

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Acompanha: Sequência didática - Ensino de ácidos e bases: da experimentação ao jogo numa abordagem contextualizada / Esta pesquisa teve como objetivo avaliar a contribuição de uma sequência didática, para a aprendizagem de estudantes, no ensino de funções ácidos e bases, em que se utilizou da experimentação, e, de um jogo, numa abordagem contextualizada. A abordagem metodológica foi qualitativa de natureza interpretativa, com observação participante, de sua natureza aplicada. Participaram da pesquisa, 26 estudantes, do primeiro ano do ensino médio, do turno matutino, de um colégio da rede estadual de ensino da Região Centro Sul do Paraná, Brasil. A análise de dados foi realizada, a partir de categorias, e subcategorias, estratificadas do conteúdo das respostas dos estudantes, no programa MAXQDA. A análise e interpretação, dos dados foram realizadas a partir do desenvolvimento da sequência didática, que integrou diferentes atividades, em oito momentos, articulada desde uma experimentação problematizadora, a um jogo didático, numa abordagem contextualizada. Indicou contribuições da atividade experimental problematizadora, como uma estratégia eficiente, para problematização, contextualização, e, o estímulo a questionamentos de investigação. Possibilitou a realização de discussões, reflexão, registros, e, levantamento de hipóteses a respeito do tema, que foram fundamentais para geração de dados, a respeito das concepções iniciais dos estudantes, sobre ácidos e bases. Despertou a curiosidade dos estudantes para os acontecimentos experimentais, fazendo com que agissem como protagonistas na construção de seu conhecimento na busca de respostas, numa atitude cientifica. O desenvolvimento do jogo didático elaborado estimulou de forma unânime a motivação dos estudantes, potencializou o interesse, a curiosidade, participação ativa nas atividades, melhorando o relacionamento entre estudantes, e como consequência a evolução conceitual, social e emocional. De forma geral, a sequência didática, contribuiu para que os estudantes estabelecessem relações entre os conteúdos científicos de ácidos e base, e o seu contexto, sendo instigados na busca pelo saber. De forma que, foi possível observar contribuições nas dimensões de abrangência conceitual, procedimental, e atitudinal, previstas nas intenções educacionais. / The aim of this research was to evaluate the contribution of a didactic sequence for the learning of students in the teaching of acid functions and bases, in which experimentation and a game in a contextualized approach were used. The methodological approach was qualitative of an interpretative nature, with participant observation, of its applied nature. Twenty-six students participated in the study, from the first grade of high school, of the morning term, of a state school in the Southern Center Region of Paraná, Brazil. The data analysis was carried out, from categories, and subcategories, stratified of the content of the students' answers, in the MAXQDA program. The analysis and interpretation of the data were obtained from the development of the didactic sequence, which integrated different activities, in eight moments, articulated from a problematizing experimentation, to a didactic game, in a contextualized approach. It indicated contributions of problematizing experimental activity, as an efficient strategy, for problematization, contextualization, and the stimulation of research questions. It made possible the accomplishment of discussions, reflection, registers, and hypotheses about the subject, which were fundamental for data generation, regarding the initial conceptions of the students about acids and bases. It aroused the curiosity of the students for the experimental events, causing them to act as protagonists in the construction of their knowledge in the search for answers in a scientific attitude. The development of the elaborated didactic game unanimously stimulated students' motivation, enhanced interest, curiosity and active participation in activities, improving students’ relationships and as consequence a conceptual, social and emotional evolution has arisen. In general, the didactic sequence contributed to the students establishing relationships between the scientific contents of acids and base and their context, being instigated in the search for knowledge. Thus, it was possible to observe contributions in the dimensions of conceptual, procedural, and attitudinal scope, foreseen in educational intentions.

Enquanto que na produção do cimento Portland é feita a extração de matérias-primas não renováveis e o lançamento de CO2 de forma excessiva; a fabricação dos cimentos álcali ativados não envolve a calcinação dos materiais e pode utilizar como matéria-prima subprodutos da indústria. Nesse contexto, os cimentos álcali ativados surgem como uma nova classe de materiais aglomerantes de bom desempenho e baixo impacto ambiental, os quais podem ser sintetizados unicamente com resíduos, como por exemplo, escórias de alto forno ativadas com diferentes produtos alcalinos, gerando materiais com características interessantes para aplicações na construção civil. Em função da composição química, as escórias de alto forno geradas em fornos a coque são empregadas como adição ao cimento Portland; entretanto, aquelas oriundas de fornos a carvão vegetal não encontram a mesma destinação pois normalmente não se adequam aos módulos de basicidade prescritos nas normas, implicando assim um passivo ambiental. Esta pesquisa tem por objetivo à obtenção de cimentos álcali ativados (CAT) visando a valorização da escória A (ácida). Também foi utilizada uma escória proveniente de alto forno a coque (B) como matéria-prima, a fim de comparar a composição química de ambas. Foram elaboradas composições iniciais de CAT com hidróxido de sódio (NaOH) como ativador, as quais foram submetidas a dois processos de cura (úmida e imersa) para testes iniciais. A cura imersa se mostrou menos propícia no uso da escória como ligante, uma vez que o contato com a água lixivia o material diminuindo sua superfície de contato, o que traz consequências no desenvolvimento mecânico e da microestrutura. Já em cura úmida, com 5% de NaOH como ativador, as argamassas de escória A obtiveram ótimo desempenho mecânico, atingindo valores próximos a 45MPa aos 28 dias; sendo que o desenvolvimento da fase de C-S-H e hidrotalcita pôde ser observada a partir dos difratogramas de raios-X e análises térmicas. A escória B desenvolveu menor resistência na presença do ativador, mas as mesmas fases foram observadas em ambas as composições; constatando-se então que a composição química da matéria-prima tem forte influência no desempenho dos CAT’s. / While production of Portland cement is made the extraction of non-renewable raw materials and the release of CO2 excessively; the manufacture of alkali activated cement does not involve the calcination of materials and can be used as raw material byproducts of industry. In this context, the alkali activated cements emerge as a new class of binder materials of good performance and low environmental impact, which can be synthesized only with waste, such as slag in blast furnace activated with different alkalis generating materials with characteristics interesting for applications in construction. Depending on the chemical composition, blast furnace slag generated in coke ovens are used as addition to Portland cement; however, those from charcoal ovens are not the same destination as usually is inappropriate the basicity modules prescribed in the rules, thus implying an environmental liability. This research has the purpose to obtain activated alkali cements (CAT) aimed at the valorisation of the slag (acid). Also used was a blast furnace from coke (B) as raw material, in order to compare the chemical composition of both. CAT initial compositions with sodium hydroxide was prepared (NaOH) as an activator, which were subjected to two curing processes (wet and immersed) for initial testing. The immersed cure was less benign in the use of slag as a binder caused a lower compressive strenght, since the contact with the water solubilizing the material reducing its contact surface. In wet cure, with 5% NaOH as an activator, the slag mortars A obtained high mechanical performance, arriving values close to 45MPa at 28 days; the development of C-S-H and hydrotalcite phase was observed from the XRD patterns of X-rays and thermal analysis. Slag B developed less strenght in the presence of the activator, but the same phases were observed in both compositions. It was possible to verify that the chemical composition of raw material had strong influence on the performance of the CAT’s.

This work describes the application of a variety of Lewis bases, encompassing predominantly N-heterocyclic carbenes (NHCs), but also the use of imidazoles, aminopyridines, amidines and isothioureas, as effective catalysts in the dearomatisation of heterocyclic carbonates, predominantly the rearrangement of oxazolyl carbonates to their C-carboxyazlactone isomers by means of the Steglich rearrangement. This rearrangement reaction has been investigated extensively, with the development of simplified reaction procedures and the invention of domino cascade protocols incorporating this transformation. In an attempt to understand the mechanism of this O- to C-carboxylation process, a number of interesting observations have been made. Firstly, the class of NHC has an important factor in promoting the rearrangement, with triazolinylidenes being the most effective. Secondly, an interesting chemoselectivity has been delineated using triazolium-derived NHCs, prepared using weak bases (typically Et₃N) or strong metallated bases; both alkyl and aryl oxazolyl carbonates undergo smooth rearrangement with triazolinylidenes derived from strong metallated bases such as KHMDS, while only aryl oxazolyl carbonates undergo rearrangement using Et₃N. Extensive effort has focused towards the development of asymmetric variants of these protocols, primarily towards the design, synthesis and evaluation of chiral NHC precatalysts. To this end, a number of chiral azolium salts have been prepared, encompassing a number of different NHC classes, including C₁- and C₂-imidazolinium salts, C₂-imidazolium salts and a range of triazolium salts. Efforts towards the asymmetric catalysis of the Steglich rearrangement of oxazolyl carbonate substrates have given an optimal 66% ee. Similar rearrangements have been demonstrated with the related furanyl heterocyclic substrate class, producing a mixture of α- and γ-carboxybutenolides. In contrast to the analogous oxazolyl carbonates, the regioselectivity of this rearrangement is dependent upon the nature of the Lewis base employed. Amidines and aminopyridines give a mixture of the α- and γ- regioisomers with generally the α-regioisomer being preferred, while a triazolium-derived NHC gives rise to predominantly the thermodynamically more stable γ-carboxybutenolide. Using amidines or aminopyridines, this rearrangement has been shown to proceed via an irreversible C-C bond-forming process, but in contrast, the rearrangement using the NHC proceeds via an equilibrium process with an optimised regioselectivity of >98:2 for the γ-carboxybutenolide regioisomer over the α-regioisomer. Whilst the asymmetric variant using chiral NHCs has proven unfruitful, rearrangements using a chiral isothiourea have given high levels of regioselectivity towards the α- regioisomer and with excellent levels of enantiodiscrimination (77–95% ee).