- Alkyl Substituent Effect in the Deprotonation of Unsymmetrical Ketones
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The effect of various degrees of alkyl substitution on the relative rates of deprotonation from the two distinct sites in several unsymmetrical ketones in the gas phase is examined.The infrared multiple photon activation of an appropriately deuterium-labeled alkoxide ion generates the ion-molecule complex for the half-reaction of the bimolecular proton transfer process between an alkyl anion and an unsymmetrical ketone with one deprotonation site selectively deuterated.The resulting products are enolate ions generated by the removal of either a deuteron or a proton and, thus, are distinguishable by mass.The measurement of the enolate ion product ratios, along with an independent measurement of the kinetic isotope effect, allowed the kinetic effect of the alkyl environment on the relative proton transfer rates to be determined.The primary and secondary isotope effects are also estimated from the enolate ion product ratios.By examining the magnitude of the kinetic alkyl effect, the primary isotope effect, and the secondary isotope effect, we learn about the transition state for proton transfer.
- Johnson, Cris E.,Sannes, Kristin A.,Brauman, John I.
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- On the Mechanism of Base-Induced Gas-Phase Elimination Reactions of Thioethers
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The base-induced gas-phase elimination reactions of diethyl sulfide have been studied by using the method of Fourier transform ion cyclotron resonance mass spectrometry.Reaction with the nitrogen bases NH2-, EtNH-, and Me2N- preferentially proceeds via an α',β-elimination mechanism, where α' proton abstraction of the sulfide results in the formation of an α-thio carbanion, which undergoes an intramolecular syn elimination to generate the ethyl thiolate anion.Reaction with OH-, MeO-, and F- exclusively proceeds via an E2 mechanism.However, for the reaction with OH-, rapid exchange is observed within the reaction complex between the α-hydrogens of the sulfide and the hydroxide hydrogen prior to E2 elimination.For the E2 eliminations the α- and β-deuterium isotope and leaving group effects have been determined as a function of the base strength.As in our previous study of the mechanism of base-induced gas-phase elimination reactions of ethers, the isotope and leaving group effect data are interpreted in terms of a variable E2 transition-state structure.Combining the results of our previous study with those of the present study has led to the conclusion that the perturbation of the transition-state character effected by changing the leaving group or the base follows the rules established for condensed phase β-elimination reactions.For the nearly thermoneutral elimination reaction of diethyl sulfide induced by F- it is concluded that two reaction mechanisms are operative, both characterized by a bent proton transfer: an E2C mechanism involving a two-side attack of F- on the β-hydrogen and α-carbon of the sulfide, yielding free ethyl thiolate anions and a syn elimination involving a two-side attack of F- on the β-hydrogen and the leaving group, generating HF solvated ethyl thiolate anions.
- Berkel, Werefridus W. van,Koning, Leo J. de,Nibbering, Nico M. M.
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- On the Pyrolysis of Pent-1-ene-4,4,5,5,5-d5
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The distribution of deuterium in the main products of thermal decomposition of pent-1-ene-4,4,5,5,5-d5 at 550 to 650 deg C was studied and interpreted.The results include conclusions on kinetic isotope effects, on relative reactivities of different C-H-bonds, on the proportion of terminal and nonterminal addition of methyl radicals, and on the importance of a radical isomerisation reaction.It was shown that the molecular decomposition (Retro-En-Reaction) cannot successfully compete with the radical way in the temperature range above 600 deg C.
- Kopinke, F.-D.,Bach, G.,Ondruschka, B.,Zimmermann, G.
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p. 699 - 707
(2007/10/02)
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