Showing papers on "Transition state published in 1970"

Relationships between E2 and E1cB mechanisms of β-elimination

Abstract: It is shown that for base-catalysed β-elimination of 9-fluorenylmethyl derivatives, the stability of the 9-fluorenylcarbanion is too great to be consistent with a continuity of transition states between E2 and E1cB mechanisms. A model is formulated in which the reactants, products, and transition states for concerted elimination are represented on a common potential-energy surface with the carbonium ion and carbanion intermediates of stepwise reaction paths. The effect of energy changes in each stable species upon the energy and structure of the E2 transition state is considered within the framework of the Hammond postulate. The model provides an interpretation of the effects of substituents and reaction conditions upon β-eliminations. It is concluded that a more stable carbanion causes the E2 transition state to approach the carbanion in structure, leading to an apparent relation between E2 and E1cB mechanisms. At the borderline between the mechanisms a continuity of transition states, or operational indistinguishability between them, can be expected only when the reaction of the carbanion approaches diffusion control.

Model calculations of hydrogen isotope effects for non-linear transition states

Abstract: Calculations are reported showing that primary hydrogen isotope effects depend smoothly on transition-state geometry and yield consistently low values, kH/kD= 1–2, for sufficiently non-linear configurations. Specific calculations for 1,2-hydride shifts, borane hydrolyses, and E2C eliminations are compared with available measurements for these and for other hydrogen-transfer reactions proceeding through 3-, 4-, 5-, and 6-membered cyclic transition states. A unique temperature-independent kH/kD is noted. The low isotope effects are ascribed to the presence of a large isotopically sensitive vibration νR‡(e.g., 3500 cm.–1) in the transition state. The results are fairly insensitive to reasonable force-constant changes. The usefulness of kH/kD in defining the geometry of transition states for hydride transfer reactions is discussed.

Kinetics of reaction of metal alkyl compounds with alkenes. Part 5.—Aluminium triethyl and substituted alkenes

Abstract: The kinetics of the addition reactions of aluminium triethyl with several linear, branched and cyclic alkenes have been re-studied, using dilatometry and g.l.c. product analysis. The concentration dependence of rate is consistent with the monomeric form of aluminium triethyl alone being reactive. The Arrhenius parameters are analyzed in terms of four-centre transition states for the addition reactions.

Transition state analysis. I. Correlation between solvent stretching frequencies and C vs. O product ratios in alkylation of ambient anions. A new explanation for the observed product ratios

Abstract: A new explanation is presented for the O to C product ratios observed in alkylation reactions of ethyl acetoacetate and related enolic and phenolic compounds, based on a Hammond postulate analysis of the nature of the transition states involved. The explanation not only covers the effects of solvent and the nature of the metal cation, but also encompasses the previously unexplained effect of the alkyl halide structure. From the latter effect, ground state interaction between solvent and alkyl halide is predicted. Evidence consistent with this interaction is found in linear free energy relationships between the ΔΔG‡ (C vs. O) values for various alkyl halides and the stretching frequency of the solvent (S=O of DMSO; P=O of HMPA) when measured as alkyl halide solutions.