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Selective Heating of Polar Solutes in a Homogeneous Solution

Title: Selective Heating of Polar Solutes in a Homogeneous Solution: Evidence of Microwave-Specific Effects and a Method to Quantify These Effects.
Name(s): Rosana, Michael R., author
Dudley, Gregory B., professor directing dissertation
Gunjan, Akash, university representative
Saha, Sourav, committee member
Alabugin, Igor, committee member
Stiegman, Albert, committee member
Dorsey, John, committee member
Department of Chemistry and Biochemistry, degree granting department
Florida State University, degree granting institution
Type of Resource: text
Genre: Text
Issuance: monographic
Date Issued: 2014
Publisher: Florida State University
Place of Publication: Tallahassee, Florida
Physical Form: computer
online resource
Extent: 1 online resource
Language(s): English
Abstract/Description: Microwave reactors offer convenience in sealed tube, high temperature reactions that can lower reaction time, increasing the value of certain synthetic procedures. Benzylation of alcohols, carboxylic acids and arenes using 2-benzyloxy-1-methylpyridinium triflate (BnOPT) translates smoothly from conventional heating to a microwave-heated method in trifluorotoluene in at 120 °C. Use of BnOPT avoids typical highly acidic and highly basic conditions of Benzylation, and compared to the analogous reaction in an oil bath is completed in 20 minutes as opposed to 24 hours. In addition to shortening reaction times, 2-benzyloxy-1-methylpyridinium tetrakis(3, 5-bistrifluoromethylphenyl) borate (BnOPB), an analog of BnOPT, strongly supports the existence of microwave effects in homogeneous solutions despite widespread skepticism. Inspired by several unexplained observations in the area of microwave chemistry, we hypothesized that microwave effects would most likely be observed with a thermally-activated, highly polar (microwave-absorbing) reagent dissolved in a largely nonpolar (microwave transparent) solvent. Applied microwave energy is selectively absorbed by BnOPB leading to Friedel-Crafts benzylation of the solvent (toluene or xylene) that is not dependent solely on observed bulk temperature and which cannot be reproduced in an oil bath. A reflux-controlled reaction circumvents the fallibility of temperature probes which demonstrated an enhanced reactivity in a microwave reactor versus an oil bath (36% versus 67% after 30 minutes). We have coined the term microwave-actuated organic reactions: reactions for which there is clear benefit under microwave-heated conditions as compared to an oil bath reaction of the same temperature. Though using a solvent's physical property of boiling point, our data and conclusions are still met skeptically. In a debate style series of essays in Angewandte Chemie International Edition, we defended our work providing further data to support our conclusions and pointing out fallacies in the opposing essay denouncing our work. Three main points were addressed in our rebuttal essay: 1) our conclusions were misstated by the opposing author, 2) our experiments were inappropriately replicated and 3) the text of the authors essay and their supporting information are inconsistent with one another. Each of these points is addressed in full detail below, but the biggest issue raised by Kappe, et al. was that our refluxing toluene temperature was actually higher than literature-reported boiling point under microwave-heating, which could certainly undermine our conclusions. A quick analysis of the boiling point using a NIST-calibrated fiber-optic probe under several conditions proved to liberate us from these claims. It is clear to us that microwave effects exist, yet our desire to distance our work from the controversy associated with the nebulous terms of "microwave effects", "athermal microwave effects" and "specific microwave effects" has led us to develop a new term: microwave-specific thermal effects (MSTE). We define MSTEs as reactivity enhancements that are thermal in nature, but cannot be replicated outside of a microwave reactor. It is our belief that selective absorption of microwave energy by BnOPB in nonpolar solutions creates a temperature differential between the bulk solvent and the solvent cage surrounding the reagent. Reactivity should thus be based on the temperature of the microenvironment surrounding the reagent, not the bulk solvent temperature. Having shown that MSTEs exist, it became our goal to begin quantifying the extent of their effect. First order kinetic plots were created from conversion data of xylene to benzylxylene in the presence of BnOPB at varying temperatures from which Arrhenius plots were derived. An activation energy of 30.2 kcal/mol and a pre-exponential value of 9.64 x 1013 sec-1 were calculated. In conjunction with the Arrhenius equation (k = Ae(-Ea/RT)), kinetic data allows us to relate how observed changes can affect different varaiables of the Arrhenius equation that may approximate the extent of observed MSTEs. Best results were obtained using external cooling of a reaction vessel heated at high microwave power where we observed about a seven-fold increase in reaction rate under microwave heating. Success was shown in demonstrating MSTEs in another reaction: the conversion of allyl p-nitrophenyl ether to 2-allyl-4-nitrophenol by Claisen rearrangement. The system was examined under constant temperature, constant power and pulsed microwave heating. Constant temperature conditions nearly completely hid MSTEs. MSTEs were clearly observed under constant power conditions showing about a 4-fold rate enhancement. Best results were observed under pulsed microwave heating which showed about a 9-fold rate enhancement.
Identifier: FSU_migr_etd-9081 (IID)
Submitted Note: A Dissertation submitted to the Department of Chemistry and Biochemistry in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
Degree Awarded: Summer Semester, 2014.
Date of Defense: July 8, 2014.
Bibliography Note: Includes bibliographical references.
Advisory Committee: Gregory B. Dudley, Professor Directing Dissertation; Akash Gunjan, University Representative; Sourav Saha, Committee Member; Igor Alabugin, Committee Member; Albert Stiegman, Committee Member; John Dorsey, Committee Member.
Subject(s): Chemistry
Persistent Link to This Record:
Owner Institution: FSU

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Rosana, M. R. (2014). Selective Heating of Polar Solutes in a Homogeneous Solution: Evidence of Microwave-Specific Effects and a Method to Quantify These Effects. Retrieved from