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Mechanisms of Nucleophilic Substitution Mechanisms o f Nucleophilic Substitution MECHANISMS OF NUCLEOPHILIC SUBSTITUTION by STUART WILLIAM PAINE A thesis submitted to the University of London in partial fulfilment of the requirements for the degree of Doctor of Philosophy in the Faculty of Science. 1993 The Christopher Ingold Laboratories, Department of Chemistry, University College London, 20, Gordon Street, London WCl HOAJ. 1 ProQuest Number: 10044486 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest. ProQuest 10044486 Published by ProQuest LLC(2016). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code. Microform Edition © ProQuest LLC. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 Mechanisms o f Nucleophilic Substitution Mechanisms o f Nucleophilic Substitution ABSTRACT The n.m.r. studies of various reactions that are believed to proceed via nucleophilic radical chain mechanisms did not show a C.I.D.N.P. effect. The reaction of sodium nitrite with p-nitrocumyl bromide in dimethyl sulphoxide is a new and interesting system. No C.I.D.N.P. effect was observed for this reaction when followed by ^^N n.m.r. spectroscopy. The reaction of sodium nitrite with p-nitrocumyl bromide in dimethyl sulphoxide proceeds via a heterolytic mechanism to give the products a,p-dinitrocumene, p-nitrocumyl alcohol and p-nitro-a-methylstyrene respectively. The overall disappearance ofp-nitrocumyl bromide can be expressed as follows: -(dERBrlt/di) = &g[RBr]( + *3[RBr]t [N 02lt^ The unimolecular component of the overall rate of reaction can be attributed to the elimination of p-nitrocumyl bromide in dimethyl sulphoxide. The termolecular component is probably due to a variation in the rate constant for a bimolecular process caused by the addition of sodium perchlorate. There is strong evidence to support this premise as a very large termolecular rate constant was obtained when sodium perchlorate was excluded. The reaction of lithium azide with p-nitrocumyl bromide in dimethyl sulphoxide proceeds via a heterolytic mechanism to give the products p-nitrocumyl azide, p- nitrocumyl alcohol and p-nitro-a-methylstyrene respectively. The overall disappearance of p-nitrocumyl bromide can be expressed as follows: -(4RBr]t/t/0 = ^g[RBr]t + 4[RBr]t [Ngl^ Moreover, the reaction of tetrabutylammonium azide with p-nitrocumyl bromide in dimethyl sulphoxide also obeys, within experimental error, concurrent first and second order kinetics. Mechanisms o f Nucleophilic Substitution The unimolecular component of the overall rate of reaction can be attributed to the elimination of p-nitrocumyl bromide in dimethyl sulphoxide. The bimolecular kinetic component is best explained by a "loose" S%^2/E2C mechanism. The lack of an isotope effect for the bimolecular component indicates that there is very little carbocation character in the rate determining step. The large ratio obtained for this reaction is indicative of S]^2 reactions in a dipolar aprotic solvents. Mechanisms o f Nucleophilic Substitution *My senses may deceive me. Perhaps my life is only a dream; even my knowledge of mathematics may be perverted by a malignant demon. But there is one thing I cannot doubt: think, therefore I am''; If I think at all, there must be something that is thinking.* René Descartes ’The important thing is to not stop questioning.* Albert Einstein ’Baloney is flattery so thick that it cannot be true and blarney is flattery so thin that we like it.* Bishop Fulton J, Sheen Mechanisms o f Nucleophilic Substitution Mechanisms of Nucleophilic Substitution ACKNOWLEDGEMENTS The author would like to thank the following: Professor J.H. Ridd for his guidance, encouragement and patience throughout the course of the work. Dr J.P.B. Sandall, Dr R.G. Coombes, Dr B.P. Roberts and Dr K.J. Hale for their useful discussions. The Ridd, Garratt, Hale, Best and Thiru groups, especially Rob Claridge, Rupert Austin, Joe Christofi, Simon Thom, Akbar Salam and Philip Tregenna-Piggot, for the great times we had together. Sukh Bhangoo for his loyal and true fraternity. All the fnends and acquaintances who have come and gone during my stay at University College London. The technical staff of the Chemistry Department, especially Dave Bowman and Dick Waymark of the Electronics section. And my family for their endless love, support and encouragement. Mechanisms o f Nucleophilic Substitution Mechanisms o f Nucleophilic Substitution TABLE OF CONTENTS ABSTRACT...............................................................................................................3 ACKNOWLEDGEMENTS....................................................................................... 7 TABLE OF CONTENTS.......................................................................................... 9 CHAPTER 1............................................................................................................. 19 (Introduction) 1.1 Nucleophilic Substitution................................................................................19 1.2 Nucleophilic Substitution at a Saturated Carbon............................................ 19 1.2. 1 The S^2 Mechanism .........................................................................20 1.2.2 The S]sfl Mechanism .........................................................................21 1.2.3 Ion Pairs in the S ^l Mechanism ....................................................... 23 1.2.4 Ion Pairing and the Reactivity of Nucleophilic Anions ..................... 24 1.2.5 The Effect of the ReactionMedium ...................................................26 1.2.6 Mixed S ^l and S%^2 Mechanisms ......................................................27 1.3 Rearrangements .............................................................................................. 28 1.3.1 Bimolecular Mechanism, S%\^2'...........................................................28 1.3.2 Unimolecular Mechanisms, S ^ l'........................................................29 1.3.3 The S^i Mechamsm .......................................................................... 29 1.4 p-Elimination ...................................................................................................30 1.4.1 Unimolecular Mechanism, E l ............................................................30 1.4.2 Bimolecular Mechanism, E 2..............................................................31 1.5 Nucleophilic Substitution at a Vinylic Carbon................................................ 32 1.6 Aromatic Nucleophilic Substitution................................................................ 33 1.6.1 The S^Ar Mechanism .......................................................................33 1.6.2 The S ^l Mechanism .........................................................................34 1.6.3 The Benzyne Mechanism ...................................................................35 1.7 Radical Nucleophilic Substitution................................................................... 36 1.7.1 The SrjvH Mechanism .......................................................................36 1.7.2 The Sr n 2 Mechanism .......................................................................41 1.7.3 The Se t 2 Mechanism.......................................................................44 Mechanisms o f Nucleophilic Substitution 1.7.4 7i-Complex/Charge Transfer Complex .............................................. 44 CHAPTER 2 .............................................................................................................47 (The Investigation of Various Radical Nucleophilic Substitution Reactions using N.M.R. Spectroscopy) 2.1 Introduction................................................................................................... 47 2.2 l^N N.M.R. Spectroscopy ........................................................................... 47 2.3 Chemically Induced Dynamic Nuclear Polarisation .......................................48 2.3.1 Interaction of a Nucleus with One Electron...................................... 49 2.3.2 Systems of Two Electrons................................................................ 50 2.3.3 Kaptein's Rule....................................................................................52 2.4 Nucleophilic Radical Chain Mechanisms ........................................................ 52 2.4.1 Chain Length ....................................................................................53 2.5 1% N.M.R. Studies............................... 54 2.6 40 MHz 1 ^N N.M.R. Study of the Reaction of the ^ ^N-Labelled Lithium Salt of 2-Nitropropane withp-Dinitrobenzene in Dimethyl Sulphoxide 55 2.6.1 Conclusion.........................................................................................58 2.7 40 MHz 1 ^N N.M.R. Study of the Reaction of Sodium Thiophenoxide with l^N-Labelled p-Nitrocumyl Chloride in a Dimethyl SulphoxideiMethanol Solvent System..............................................................................................62 2.7.1 Conclusion.........................................................................................64 2.8 40 MHz l^N N.M.R. Study of the Reaction
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