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- Dybala-Defratyka, Agnieszka, et al.
(författare)
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A New Interpretation of Chlorine Leaving Group Kinetic Isotope Effects; A Theoretical Approach
- 2004
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Ingår i: J. Org. Chem.. ; 69, s. 4900-4905
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Tidskriftsartikel (refereegranskat)abstract
- The chlorine leaving group kinetic isotope effects (KIEs) for the SN2 reactions between methyl chloride and a wide range of anionic, neutral, and radical anion nucleophiles were calculated in the gas phase and, in several cases, using a continuum solvent model. In contrast to the expected linear dependence of the chlorine KIEs on the Ca-CI bond order in the transition state, the KIEs fell in a very small range (1.0056-1.0091), even though the Ca-CI transition state bond orders varied widely from approximately 0.32 to 0.78, a range from reactant-like to very product-like. This renders chlorine KIEs, and possibly other leaving-group KIEs, less useful for studies of reaction mechanisms than commonly assumed. A partial explanation for this unexpected relationship between the Ca-CI transition state bond order and the magnitude of the chlorine KIE is presented.
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| 2. |
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| 3. |
- MacMillar, Susanna, et al.
(författare)
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Solvent effects on ion pairing of tetra-n-butylammonium cyanide : A conductometric study
- 2008
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Ingår i: Journal of Physical Organic Chemistry. - : Wiley. - 0894-3230 .- 1099-1395. ; 21:3, s. 237-241
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Tidskriftsartikel (refereegranskat)abstract
- Tetra-n-butylammonium cyanide (n-Bu4NCN) is a commonly used reagent, for example, for the synthesis of nitriles. Recently n-Bu4NCN has been used as the nucleophilic reagent in kinetic isotope effect studies of nucleophilic aliphatic substitution reactions. The present research concerns the aggregation status (dissociated ions, ion pairs, higher aggregates) and transport properties of n-Bu4NCN in water, dimethyl sulfoxide (DMSO), and tetrahydrofuran (THF), at 25 C as studied by means of precision conductometry. These properties are of great importance since both non-polar and dipolar aprotic solvents are commonly used in the applications. In water as solvent the equilibrium constant for ion-pair formation, K-p =10.1 and the limiting molar conductivity, Lambda(o)= 102.4 cm(2) Omega(-1) mol(-1). The corresponding values for DMSO are K-p =1.98 +/- 0.19 and Lambda(o) = 34.59 +/- 0.03 cm(2) Omega(-1) mol(-1). These data imply that the degree of dissociation, in contrast to the expectations, is higher in DMSO than in water at the same salt concentration. In THF, the conductance as a function of concentration shows a minimum typical for solvents with low relative permittivity, indicating the formation of higher aggregates. The equilibrium constant for ion-pair formation and conductivity in THF is K-p=58.4 X 10(3) and Lambda(o)=9.81 cm(2) Omega(-1) mol(-1).
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| 4. |
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| 5. |
- Westaway, Kenneth C., et al.
(författare)
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A New Insight into Using Chlorine Leaving Group and Nucleophile Carbon Kinetic Isotope Effects To Determine Substituent Effects on the Structure of SN2 Transition States
- 2007
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 111:33, s. 8110-8120
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Tidskriftsartikel (refereegranskat)abstract
- Chlorine leaving group k(35)/k(37), nucleophile carbon k(11)/k(14), and secondary alpha-deuterium [(k(H)/k(D))(alpha)] kinetic isotope effects (KIEs) have been measured for the S(N)2 reactions between para-substituted benzyl chlorides and tetrabutylammonium cyanide in tetrahydrofuran at 20 degrees C to determine whether these isotope effects can be used to determine the substituent effect on the structure of the transition state. The secondary alpha-deuterium KIEs indicate that the transition states for these reactions are unsymmetric. The theoretical calculations at the B3LYP/aug-cc-pVDZ level of theory support this conclusion; i.e., they suggest that the transition states for these reactions are unsymmetric with a long NC-C-alpha and reasonably short C-alpha-Cl bonds. The chlorine isotope effects suggest that these KIEs can be used to determine the substituent effects on transition state structure with the KIE decreasing when a more electron-withdrawing para-substituent is present. This conclusion is supported by theoretical calculations. The nucleophile carbon k(11)/k(14) KIEs for these reactions, however, do not change significantly with substituent and, therefore, do not appear to be useful for determining how the NC-C-alpha transition-state bond changes with substituent. The theoretical calculations indicate that the NC-C-alpha bond also shortens as a more electron-withdrawing substituent is placed on the benzene ring of the substrate but that the changes in the NC-C-alpha transition-state bond with substituent are very small and may not be measurable. The results also show that using leaving group and nucleophile carbon KIEs to determine the substituent effect on transition-state structure is more complicated than previously thought. The implication of using both chlorine leaving group and nucleophile carbon KIEs to determine the substituent effect on transition-state structure is discussed.
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| 6. |
- Westaway, Kenneth C., et al.
(författare)
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Determining the Transition-State Structure for Different S(N)2 Reactions Using Experimental Nucleophile Carbon and Secondary alpha-Deuterium Kinetic Isotope Effects and Theory
- 2008
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 112:41, s. 10264-10273
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Tidskriftsartikel (refereegranskat)abstract
- Nucleophile C-11/C-14 [k(11)/k(14)] and secondary alpha-deuterium [(k(H)/k(D))(alpha)] kinetic isotope effects (KIEs) were measured for the S(N)2 reactions between tetrabutylammonium cyanide and ethyl iodide, bromide, chloride, and tosylate in anhydrous DMSO at 20 degrees C to determine whether these isotope effects can be used to determine the structure of S(N)2 transition states. Interpreting the experimental KIEs in the usual fashion (i.e., that a smaller nucleophile KIE indicates the Nu-C-alpha transition state bond is shorter and a smaller (k(H)/k(D))(alpha) is found when the Nu-LG distance in the transition state is shorter) suggests that the transition state is tighter with a slightly shorter NC-C-alpha, bond and a much shorter C-alpha-LG bond when the substrate has a poorer halogen leaving group. Theoretical calculations at the B3LYP/aug-cc-pVDZ level of theory support this conclusion. The results show that the experimental nucleophile C-11/C-14 KIEs can be used to determine transition-state structure in different reactions and that the usual method of interpreting these KIEs is correct. The magnitude of the experimental secondary alpha-deuterium KIE is related to the nucleophile-leaving group distance in the S(N)2 transition state (R-TS) for reactions with a halogen leaving group. Unfortunately, the calculated and experimental (k(H)/k(D))(alpha)'s change oppositely with leaving group ability. However, the calculated (k(H)/k(D))(alpha)'s duplicate both the trend in the KIE with leaving group ability and the magnitude of the (k(H)/k(D))(alpha)'s for the ethyl halide reactions when different scale factors are used for the high and the low energy vibrations. This suggests it is critical that different scaling factors for the low and high energy vibrations be used if one wishes to duplicate experimental (k(H)/k(D))(alpha)'s. Finally, neither the experimental nor the theoretical secondary alpha-deuterium KIEs for the ethyl tosylate reaction fit the trend found for the reactions with a halogen leaving group. This presumably is found because of the bulky (sterically hindered) leaving group in the tosylate reaction. From every prospective, the tosylate reaction is too different from the halogen reactions to be compared.
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