Model Calculations of Kinetic Isotope Effects in Nucleophilic Substitution Reactions

1974 ◽  
Vol 52 (6) ◽  
pp. 903-909 ◽  
Author(s):  
Jan Bron
Keyword(s):  
Transition State ◽  
Isotope Effects ◽  
Benzyl Bromide ◽  
Kinetic Isotope Effects ◽  
Substitution Reactions ◽  
Model Calculations ◽  
Kinetic Isotope ◽  
Proposed Model ◽  
Standard Reaction ◽  
Deuterium Isotope Effects

The results of calculations indicate that a previously proposed model for the transition state in "borderline" substitution reactions can be generalized and, as a result, the observed differences in the carbon-13 and deuterium isotope effects of SN1, SN2, and "borderline" reactions rationalized. Although the conclusions may apply more generally, the standard reaction investigated is the solvolysis of benzyl bromide. The importance of resonance interaction with the phenyl ring, the significance of the product- or reactant-like character of the transition state, and the influence of the magnitude of force constants in determining isotope effects are examined. The temperature dependence of kinetic isotope effects in solvolysis is also investigated.

Secondary Kinetic Isotope Effects in Bimolecular Nucleophilic Substitutions. V. The Role of the Solvent in the Reaction Between Methyl Iodide and Pyridine

1972 ◽  
Vol 50 (7) ◽  
pp. 982-985 ◽  
Author(s):  
K. T. Leffek ◽  
A. F. Matheson
Keyword(s):  
Transition State ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
State Structure ◽  
Transition State Structure ◽  
Initial State ◽  
Nucleophilic Substitutions ◽  
Kinetic Isotope ◽  
Deuterium Isotope Effects

Secondary kinetic deuterium isotope effects are presented for the reaction of methyl-d3 iodide and pyridine in four different solvents. Calculations on mass and moment of inertia change with deuteration in the initial state and an assumed tetrahedral transition state, together with internal rotational effects, are used to rationalize the inverse isotope effects. It is concluded from the variation of the isotopic rate ratio, that the transition state structure varies with solvent.

Isotope effects in nucleophilic substitution reactions. III. The effect of changing the leaving group on transition state structure in SN2 reactions

1979 ◽  
Vol 57 (11) ◽  
pp. 1354-1367 ◽  
Author(s):  
Kenneth Charles Westaway ◽  
Syed Fasahat Ali
Keyword(s):  
Transition State ◽  
Nucleophilic Substitution ◽  
Isotope Effects ◽  
Heavy Atom ◽  
Substituent Effects ◽  
Kinetic Isotope Effects ◽  
Substitution Reactions ◽  
Nucleophilic Substitution Reactions ◽  
Kinetic Isotope ◽  
Leaving Group

The nucleophilic substitution reactions of a series of 4-substituted phenylbenzyldimethyl-ammonium ions with thiophenoxide ions at 0 °C in N,N-dimethylformamide have been used to demonstrate how a change in the leaving group alters the structure of the SN2 transition state. Heavy atom (nitrogen) kinetic isotope effects, secondary α-deuterium kinetic isotope effects and Hammett ρ values provide qualitative descriptions of both the nucleophile–α-carbon and α-carbon–leaving group bonds in the transition states of these reactions. The results indicate that changing to a better leaving group causes the bond between the α-carbon and the nucleophile to be much more fully formed while the bond to the leaving group is essentially unchanged. The results are discussed in the light of current theories of substituent effects on SN2 reactions and a possible explanation for the surprising results (i) that the greatest effect is in the bond more remote from the point of structural change and (ii) that more nucleophilic assistance is required to displace a better leaving group is given.

Isotope Effect Studies on Elimination Reactions. X. The Nature of The Transition State for the ECO2 Reaction of Para- substituted Benzyl Nitrates with Base in 90% by Volume Ethanol–Water

1975 ◽  
Vol 53 (9) ◽  
pp. 1319-1326 ◽  
Author(s):  
Peter James Smith ◽  
Carol Audrey Pollock ◽  
Arthur Newcombe Bourns
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Isotope Effects ◽  
Nitrogen Isotope ◽  
Kinetic Isotope Effects ◽  
The Other ◽  
Theoretical Treatment ◽  
Kinetic Isotope ◽  
Hydrogen Deuterium ◽  
Deuterium Isotope Effects

Kinetic isotope effects have been determined for the Eco2 reaction of para-substituted benzyl nitrates with ethoxide in 90 vol.% ethanol–water at 20°. The nitrogen isotope effect, (k14/k15−1)100 decreased with increasing electron-withdrawing ability of the para-substituent; i.e. 2.26, 1.95, 1.60, and 0.84 for p-CH3, p-H, p-CF3, and p-NO2, respectively. Furthermore, the primary hydrogen–deuterium isotope effects increased also for electron-withdrawing substituents; i.e. kH/kD = 5.78, 6.06, 6.40, 6.67, and 7.05 for p-CH3, p-H, p-Br, p-CF3, and p-NO2, respectively. The results are discussed in terms of a recent theoretical treatment dealing with the effect of substituents on the nature of the transition state for a concerted E2 process. The conclusion is reached that any structural change which causes one bond (carbon–hydrogen) to be weakened more at the transition state will have a corresponding effect on the other bond (oxygen–nitrogen).

Solvent effects on nucleophilic substitution reactions. III. The effect of adding an inert salt on the structure of the SN2 transition state

1996 ◽  
Vol 74 (12) ◽  
pp. 2528-2530 ◽  
Author(s):  
T.V. Pham ◽  
K.C. Westaway
Keyword(s):  
Ionic Strength ◽  
Transition State ◽  
Isotope Effects ◽  
Nitrogen Isotope ◽  
Kinetic Isotope Effects ◽  
Substitution Reactions ◽  
Kinetic Isotope ◽  
Ionic Transition ◽  
Hydrogen Deuterium ◽  
Inert Salt

The nitrogen and secondary α-hydrogen–deuterium kinetic isotope effects found for the SN2 reaction between thiophenoxide ion and benzyldimethylphenylammonium ion at different ionic strengths in DMF at 0 °C indicate that the structure of the transition state changes markedly with the ionic strength of the reaction mixture. In fact, a more reactant-like, more ionic, transition state is found at the higher ionic strength. This presumably occurs because a more ionic transition state is more stable in the more ionic solvent. Key words: transition state, ionic strength, secondary α deuterium kinetic isotope effects, nitrogen isotope effects, SN2.

Isotope effects in nucleophilic substitution reactions. II. Secondary α-deuterium kinetic isotope effects: a criterion of mechanism?

1979 ◽  
Vol 57 (9) ◽  
pp. 1089-1097 ◽  
Author(s):  
Kenneth Charles Westaway ◽  
Syed Fasahat Ali
Keyword(s):  
Transition State ◽  
Nucleophilic Substitution ◽  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Substitution Reactions ◽  
Nucleophilic Substitution Reactions ◽  
Kinetic Isotope ◽  
Steric Crowding

A very large secondary α-deuterium kinetic isotope effect of 1.179 ± 0.007 (1.086 ± 0.003 per α-deuterium) has been observed for the SN2 reaction of thiophenoxide ion with benzyldimethylphenylammonium ion in DMF at 0°C. This large isotope effect which is far outside the range reported for SN2 reactions, is attributed to the fact that the extraordinarily large steric crowding around the Cα—H bonds in the substrate is reduced in the SN2 transition state. The structure of the transition state is shown to be consistent with this hypothesis.

Isotope effects in nucleophilic substitution reactions. IX. The effect of bond strength on substituent effects on the structure of SN2 transition states

1993 ◽  
Vol 71 (12) ◽  
pp. 2084-2094 ◽  
Author(s):  
Kenneth Charles Westaway
Keyword(s):  
Bond Strength ◽  
Transition State ◽  
Isotope Effects ◽  
Transition States ◽  
Substituent Effects ◽  
Kinetic Isotope Effects ◽  
Substitution Reactions ◽  
Kinetic Isotope ◽  
Leaving Group ◽  
Made In

The effects of substituents on the structure of SN2 transition states suggested by kinetic isotope effects and Hammett ρ values are often different and, moreover, often do not agree with substituent effects predicted by current theories whether the change in substituent is made in the nucleophile, in the leaving group, or at the α-carbon. The importance of the strength of the reacting bonds in determining the effects of substituents on transition-state structure is investigated. A bond strength hypothesis that suggests there will be a significant change in the weaker reacting bond but little or no change in the stronger reacting bond in an SN2 transition state when a substituent in the nucleophile, the substrate, or the leaving group is altered in an SN2 reaction, predicts a high percentage of the experimental results.

Model calculations of isotope effects. IX. Origin of large hydrogen and carbon isotope effects in the deprotonation of 2-nitropropane by pyridine bases

1983 ◽  
Vol 36 (8) ◽  
pp. 1521
Author(s):  
DJ McLennan
Keyword(s):  
Transition State ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Bond Orders ◽  
Model Calculations ◽  
Kinetic Isotope ◽  
Pyridine Bases ◽  
State Models ◽  
Experimental Findings ◽  
Large Primary

The abnormally large primary hydrogen and carbon kinetic isotope effects found in the deprotonation of 2-nitropropane by hindered pyridine bases are investigated by means of model calculations. Transition-state models have been varied between tight and loose extremes, and between carbanion-like and nitronate-like structures. The only models that reproduce the experimental findings are those in which the sum of the bond orders to the transferring proton is less than unity (loose transition states) and which are subject to tunnelling corrections.

The Importance of Anharmonicity of The Vibrational Excited States in Chemical Kinetics

1975 ◽  
Vol 53 (20) ◽  
pp. 3069-3074 ◽  
Author(s):  
Jan Bron
Keyword(s):  
Transition State ◽  
Excited States ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Vibrational States ◽  
Vibrational Anharmonicity ◽  
Kinetic Isotope ◽  
Large Correction ◽  
Lower Temperature ◽  
Lower Temperature Region

The corrections to rate constants for an harmonicity of vibrational excited states have been evaluated over the temperature range of 200–1100 K. The reaction O2 + X, where X is H or D, has been chosen as the model system. Only the influence of vibrational anharmonicity of the triatomic transition state has been determined. Two geometric shapes for the transition state, bent and isosceles configurations, have been investigated in detail by the bond order method.It is found that the correction can be large, depending upon the geometry and force field of the transition state and the temperature. The magnitude of the correction for anharmonicity of the vibrational excited states depends mainly, at a particular temperature, on the strength of the O—X bond in the transition state. In the case of a large correction, anharmonicity may lead to a nonlinear Arrhenius plot.Because of cancellation effects, the correction for anharmonicity of the excited vibrational states in kinetic isotope effects can be ignored in the lower temperature region. It has also been found that anharmonicity of the vibrational groundstate can explain unexpected large isotope effects.

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