- The Role of Single-Electron Transfer in SN2-Type Substitution Reactions of Anions with Alkyl Halides
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Rate constants (kobsd) for reactions of electrophiles with: (a) various carbanions, including 9-substituted fluorenide ions (9-GFl-), 9-substituted xanthenide ions, α-cyano carbanions, anb β-diketo enolate ions, (b) phenothiazinide (PTZ-) and carbazole (Cb-) nitranions, (c) 4-substituted phenoxide ions, and (d) thiphenoxide ions, have been campared with rate constants for single-electron transfer (kSET) calculated using a Marcus-type equation.For both 9-GFl- carbanion and 2-GPTZ- nitranion families reacting with SET acceptors, such as 1,1-dinitrocyclohexane, the kobsd/kSET ratio is near unity.For 9-GFl- carbanions reacting with Ph2CHCl the kobsd/kSET ratios range from 2 to 105, those for 2-PTZ- nitranions from 370 to 1.3*104, and those for Cb- nitranions from 108 to 1010.For reactions of 4-GC6H4O- oxanions with Ph2CHCl the ratios range from 103 for G = Me2N to 1013 for G = CN; for PhS- ion reacting with n-BuBr the ratio is 1017.The significance of these results with respect to the role of an outer sphere SET mechanism in substitution reactions of anions with alkyl halides is discussed.
- Bordwell, F.G.,Harrelson, John A.
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p. 4893 - 4898
(2007/10/02)
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- Flash-Photolysis Generation and Reactivities of Triarylmethyl and Diarylmethyl Cations in Aqueous Solutions
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A series of 18 triarylmethyl cations and 10 diarylmethyl cations have been generated by nanosecond laser flash photolysis of cyanide, 4-cyanophenyl ether, and acetate precursors in acetonitrile/water (AN/W) solutions and first-order rate constants for their reaction with the solvent (ks) have been directly measured following the decay in their optical density.In the standard solvent employed, 1:2 AN/W, the triarylmethyl cations which were studied had ks values at 20 deg C ranging from 1E1 s-1 (for the 4,4',4''-(MeO)3-substituted ion) to 9*1E6 s-1 (4,4'-(CF3)2), while diarylmethyl cations had ks values ranging from 1E5 s-1 (4,4'-(MeO)2) to 3*1E7 s-1 (4,4'-Me2).The parent diphenylmethyl cation and its derivative with one 4-methyl substituent were too short-lived (s of varying the amounts of acetonitrile were investigated for several cations.As water is added to 100percent acetonitrile, ks increases significantly, but at around 15percent by volume water, there is a leveling and from that point to 100percent water, ks is almost unchanged, decreasing by about 20percent.A plot of log ks versus ?+ constructed for the triarylmethyl cations shows significant deviations from linearity for the points for the ? donors, in the direction which indicates that ?+ is underestimating the stabilizing effect of these substituents for a fully formed cation.A plot versus ?C+, a parameter obtained from the analysis of NMR spectra of solutions of carbocations, is reasonably linear.A two-parameter correlation indicates that polar andresonance interactions of substituents do not proceed in parallel along the reaction coordinate, the addition of water to cation resulting at the transition state in the loss of 73percent of the equilibrium resonance effect but only a 33percent loss of the polar effect.A rate-equilibrium plot (log ks versus pKR) was constructed which covers 23 pKR units.A single line of slope 0.64 can be drawn to include the entire set of data for both triarylmethyl and diarylmethyl cations.From a small extrapolation the ks value for the tert-butyl cation in water is obtained as 1E10.5 s-1.
- McClelland, Robert A.,Kanagasabapathy, V. M.,Banait, Narinder S.,Steenken, Steen
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p. 3966 - 3972
(2007/10/02)
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