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Utah State University DigitalCommons@USU All Graduate Theses and Dissertations Graduate Studies 5-2012 The Kinetics and Mechanisms of Some Fundamental Organic Reactions of Nitro Compounds Zhao Li Utah State University Follow this and additional works at: https://digitalcommons.usu.edu/etd Part of the Organic Chemistry Commons, and the Physical Chemistry Commons Recommended Citation Li, Zhao, "The Kinetics and Mechanisms of Some Fundamental Organic Reactions of Nitro Compounds" (2012). All Graduate Theses and Dissertations. 1407. https://digitalcommons.usu.edu/etd/1407 This Dissertation is brought to you for free and open access by the Graduate Studies at DigitalCommons@USU. It has been accepted for inclusion in All Graduate Theses and Dissertations by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. THE KINETICS AND MECHANISMS OF SOME FUNDAMENTAL ORGANIC REACTIONS OF NITRO COMPOUNDS by Zhao Li A dissertation submitted in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY in Chemistry Approved: _________________________ _________________________ Vernon D. Parker, Ph.D. Lance C. Seefeldt, Ph.D. Major Professor Committee Member _________________________ _________________________ Alvan C. Hengge, Ph.D. Roger A. Coulombe, Jr, Ph.D. Committee Member Committee Member _________________________ _________________________ Cheng-Wei Tom Chang, Ph.D. Mark R. McLellan, Ph.D. Committee Member Vice President for Research and Dean of the School of Graduate Studies UTAH STATE UNIVERSITY Logan, Utah 2012 ii Copyright © Zhao Li 2012 All Rights Reserved iii ABSTRACT The Kinetics and Mechanisms of Some Fundamental Organic Reactions of Nitro Compounds by Zhao Li, Doctor of Philosophy Utah State University, 2012 Major Professor: Dr. Vernon D. Parker Department: Chemistry and Biochemistry The central topic of this dissertation is to seek the answer to the question: Is the single transition state model appropriate for (1) the proton transfer reactions of nitroalkanes and (2) the aromatic nucleophilic reactions of trinitroarenes? If not, what are the real mechanisms? This objective has been accomplished by careful kinetic and mechanistic studies which take advantage of modern digital acquisition of absorbance – time data, combined with extensive new data analysis of results from pseudo-first-order kinetic measurements. Several new analysis procedures for pseudo-first-order kinetics that are capable of distinguishing between single-step and multi-step mechanisms have been introduced and intensively confirmed during the application in the kinetic study of the target reactions. The procedures include (1) half-life dependence of apparent rate constant, (2) sequential linear pseudo-first-order correlation, (3) approximate instantaneous rate constant analysis, and (4) time-dependent apparent kinetic isotope effects. iv Various conventional and nonconventional pseudo-first-order kinetic analyses of the reactions of nitroalkanes in aqueous solutions revealed that the reactions are complex and involve kinetically significant intermediates. The spectral evidence for the formation of reactive intermediates was obtained by carrying out the kinetic experiments at the isosbestic points where changes in reactant and product absorbance cancel. The apparent deuterium kinetic isotope effects studies indicated that the deuterium kinetic isotope effects are not associated with the formation of the intermediates. The observations of anomalous Brønsted exponents previously found for this reaction series could be rationalized well with the complex mechanisms proposed in this work, which indicate that the nitroalkane anomaly does not exist, but arises from an interpretation based upon the incorrect assumption that the reactions follow a simple one-step mechanism. Simulations revealed that the generally accepted competitive mechanism was not appropriate to describe the Meisenheimer complex formation during the reaction of 2,4,6-trinitroanisole with methoxide ion in methanol. This conclusion is supported by the conventional pseudo-first-order kinetic analysis. A reversible consecutive mechanism that accounts for the kinetic behavior has been proposed, which involves an intermediate dianion complex that is formed reversibly from the initial 1,3-σ-complex, and then eliminates methoxide ion to form the 1,1-σ-complex product. (296 pages) v PUBLIC ABSTRACT The Kinetics and Mechanisms of Some Fundamental Organic Reactions of Nitro Compounds Zhao Li, Ph.D. Department of Chemistry and Biochemistry Utah State University, 2012 In chemistry, kinetics is the study of the rates of chemical reactions. A reaction mechanism describes in detail exactly what takes place at each stage of an overall chemical reaction from reactants to products. The work outlined in this dissertation was carried out in order to find out the true mechanisms of some fundamental organic reactions of nitro compounds by means of a series of novel kinetic techniques and analysis methods. The target reactions include (1) the proton transfer reactions of nitroalkanes and (2) the aromatic nucleophilic reactions of trinitroarenes, both of which are undeniably among the most elementary and significant processes in organic chemistry and biochemistry. Rigorous kinetic investigations have found that both reactions actually take place by multi-step mechanisms involving kinetically significant intermediates. This result challenges the long-held assumption that the reactions follow a simple one-step mechanism. Although the assumption of the one-step mechanism can be applied to evaluate the magnitude of the rate of the overall reaction, it fails to provide a detailed vi description of chemical reactions, especially those behaving abnormally. This is obvious in the conundrum found in the description of the proton transfer reactions of nitroalkanes which is commonly known as the nitroalkane anomaly. However, adopting the multi-step mechanisms proposed in this dissertation should end this chapter of organic chemistry. Reaction mechanism is a primary topic for organic chemical studies that provides a framework for appropriate application in the chemical and pharmaceutical industry, and in agricultural applications. An essential objective of this work is to point out that more attention should be paid to the importance of reaction mechanism in the field of organic chemistry. A broad impact of this work on academic studies may be achieved via promoting a revision of the mechanisms believed to be established over the past seventy years. It will have a broad effect in other branches of chemistry as well as in biochemistry. This work is a sub-project of the three-year $665,000 international cooperative project of “Is the single transition state model appropriate for the fundamental reactions of organic chemistry?” initiated by Professor Vernon D. Parker at USU, collaborating with Professors Jin-Pei Cheng and Xiao-Qing Zhu in China’s Nankai University. This international project, supported by both NSF (National Science Foundation) and NSFC (National Natural Science Foundation of China), has been fostering cooperation and good will between the scientific communities of the United States and China. Contact: [email protected] vii ACKNOWLEDGMENTS The primary acknowledgment goes to my advisor, Dr. Vernon D. Parker, for his consistent guidance, support, and help during my time at Utah State University. His breadth and depth of knowledge in physical organic chemistry was a constant source of support to encourage my independent and original thought, and to provide me a model of what a successful scientist should be. I am honored that I could have an opportunity to study in his group. What I have learned from him is invaluable for me and my future career development. I would also like to express my appreciation to the other professors in my supervisory committee, Dr. Alvan C. Hengge, Dr. Cheng-Wei Tom Chang, Dr. Lance C. Seefeldt, and Dr. Roger A. Coulombe, Jr, for their support, guidance, and helpful suggestions. I am grateful for their experience in their respective fields. I especially thank Dr. Jin-Pei Cheng, our collaborator and my former advisor at Nankai University, for providing me the opportunity to join the USU-Nankai International Cooperative Graduate Program, as well as for his extensive help and encouragement over the past five years. I am also grateful to the other collaborator at Nankai University, Prof. Xiao-Qing Zhu, for his efforts in the funding application and helpful suggestions in my research. Great thanks to Dr. Donald Bethell, professor at the University of Liverpool, for his help in the study of isosbestic point. I wish to express my appreciation to my colleagues, Dr. Weifang Hao for her numerous helps throughout my educational experience at USU; Xiaosong Xue for his viii cooperation in calculations; Chad Jacobs for helping with the kinetic study of SNAr reactions; Dr. Rui Cao, Xiaozhen Han, and Russell Scow for their helpful discussion and friendship. I am profoundly grateful to the Parker couple (Vernon and Monica) and the Zhang couple (Tao and Weifang). Their friendship and considerate help made me feel part of a large family. Deep gratitude also belongs to my parents, Lanlin Li and Xueping Wang, for their constant strength, love, and faith in me. I also owe many thanks to all my friends and relatives, whose names are not mentioned here. Your love and support mean a lot to me. Last but not least, I would express my appreciation
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