Model calculations of isotope effects. VIII. Deprotonation of nitroethane

1983 ◽  
Vol 36 (8) ◽  
pp. 1513
Author(s):  
DJ McLennan
Keyword(s):  
Transition State ◽  
Isotope Effects ◽  
Deuterium Isotope Effect ◽  
Model Calculations ◽  
Kinetic Isotope ◽  
Partial Negative Charge ◽  
Hydrogen Deuterium ◽  
State Models ◽  
Experimental Values ◽  
Comparison Of The Results

Transition-state models for the base-promoted deprotonation of nitroethane have been designed, and primary and secondary hydrogen-deuterium kinetic isotope effects have been calculated. Comparison of the results with experimental values of the primary isotope effects allows no firm conclusions to be reached concerning probable transition-state structures. However, the secondary α-deuterium isotope effect comparison disqualifies from consideration those transition states in which rehybridization of Cα and delocalization of the partial negative charge by the nitro group keep pace with the extent of deprotonation. Transition-state models wherein Cα is carbanionic and essentially pyramidal yield theoretical isotope effects lying within the experimental range.

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.

Model calculations of isotope effects. VII. Deprotonation of 2-nitropropane by oxy anion bases

1983 ◽  
Vol 36 (8) ◽  
pp. 1503
Author(s):  
DJ McLennan
Keyword(s):  
Proton Transfer ◽  
Transition State ◽  
Isotope Effects ◽  
Experimental Results ◽  
Electron Delocalization ◽  
Model Calculations ◽  
Charge Delocalization ◽  
Successful Model ◽  
Deuterium Isotope Effects ◽  
State Models

Model calculations of primary and secondary deuterium isotope effects for the hydroxide-induced deprotonation of 2-nitropropane are reported. Various transition-state models have been examined in an effort to reproduce experimental results. A purely pyramidal transition state in which proton transfer has run far ahead of carbon rehybridization and charge delocalization is a successful model as far as isotope effects are concerned, but may fail on other counts. Three incipient trigonal models for the transition state have been tested, and, although none can be firmly eliminated by the resultant isotope effects, those involving the proton transfer's running ahead of electron delocalization and perhaps carbon rehybridization are favoured.

Model Calculations of Isotope Effects. III. Isotope Effects in Hydrogen Transfer Reactions, and Transition State Force Fields

1979 ◽  
Vol 32 (9) ◽  
pp. 1869
Author(s):  
DJ McLennan
Keyword(s):  
Transition State ◽  
Hydrogen Transfer ◽  
Force Fields ◽  
Isotope Effects ◽  
Semiempirical Method ◽  
Model Calculations ◽  
Surface Hydrogen ◽  
Kinetic Isotope ◽  
State Force ◽  
Better Than

Model calculations of kinetic isotope effects for the reactions: C2H6+·CH3 → ·C2H5+CH4 CH4 + ·CF3 → ·CH3+CHF3 are reported. Transition state geometries were those calculated by Dewar and coworkers using the MNDO semiempirical method. Transition state force fields were formulated from empirical expressions for stretching, bending, linear bending and interaction valence force constants by using bond orders as disposable parameters. Although it proved impossible to assign a best force field to either reaction, the calculated isotope effects generally were in satisfactory agreement with experiment, and were better than those calculated from the MNDO potential energy surface. Hydrogen tunnelling is apparently implicated in the CH4+ CF3 reaction.

Unequal primary kinetic isotope effects in the base-catalysed H–D exchange of diastereotopic protons

1980 ◽  
Vol 58 (1) ◽  
pp. 72-78 ◽  
Author(s):  
Robert R. Fraser ◽  
Philippe J. Champagne
Keyword(s):  
Transition State ◽  
Exchange Reaction ◽  
Isotope Effect ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Deuterium Isotope Effect ◽  
Kinetic Isotope

Primary kinetic isotope effects have been measured for the base-catalyzed exchange reaction of 4′,1″-dimethyl-1,2,3,4-dibenzcyclohepta-1,3-diene-6-one, 1. It was found that the isotope effects kH/kT and kD/kT for the faster exchanging protons (13.6 and 3.8 respectively) are significantly larger than the corresponding values for the slower exchanging protons (4.6 and 1.6 respectively). These differences could result from truly unequal isotope effects due to transition state differences or intrusion of a second pathway for exchange of the less reactive proton in the dedeuteration reaction. The data appear to support the latter interpretation. The secondary deuterium isotope effect was found to be 1.18.

Isotope Effect Studies on Elimination Reactions. IX. The Nature of the Transition State for the E2 Reaction of 2-Arylethylammonium Ions with Ethoxide in Ethanol

1974 ◽  
Vol 52 (5) ◽  
pp. 749-760 ◽  
Author(s):  
P. J. Smith ◽  
A. N. Bourns
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Isotope Effects ◽  
Nitrogen Isotope ◽  
Kinetic Isotope Effects ◽  
Ammonium Ions ◽  
Nitrogen Effect ◽  
Kinetic Isotope ◽  
Leaving Group ◽  
Hydrogen Deuterium

Kinetic isotope effects have been determined for the E2 reaction of some 2-arylethyltrimethyl-ammonium ions with ethoxide in ethanol at 40°. The nitrogen effect, (k14/k15 − 1)100, decreased with increasing electron-withdrawing ability of the para substituent; i.e. 1.37, 1.33, 1.14, and 0.88 for p-OCH3, p-H, p-Cl, and p-CF3, respectively. Furthermore, the primary hydrogen–deuterium isotope effects increased for the same substituents, respectively; i.e. kH/kD = 2.64, 3.23, 3.48, and 4.16. A large positive ρ value of 3.66 was found as well as a small secondary α-deuterium effect of 1.02 for p-H. In addition, the nitrogen isotope effect decreased with increasing strength of the abstracting base for the reaction of ethyltrimethylammonium ion; i.e. 1.86 and 1.41 at 60° for reaction with EtO−–EtOH and t-BuO−–t-BuOH, respectively. The results are discussed in terms of recent theoretical treatments of the effect of base, substituents, and nature of the leaving group on the nature of the transition state for an E2 process. The conclusion is reached that any structural change which causes one bond (C—H) to be weakened more at the transition state will have a corresponding effect on the other bond [Formula: see text]

A Primary Hydrogen-Deuterium Isotope Effect Study on the Carbonyl Elimination Reaction of 9-Fluorenyl Nitrate with Various Bases

1975 ◽  
Vol 53 (2) ◽  
pp. 263-268 ◽  
Author(s):  
Peter James Smith ◽  
Lorraine Marion Noble
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Isotope Effects ◽  
Effect Study ◽  
Elimination Reaction ◽  
Deuterium Isotope Effect ◽  
Nitrogen Bases ◽  
Transition State Geometry ◽  
Hydrogen Deuterium ◽  
Deuterium Isotope Effects

The carbonyl elimination reaction of 9-fluorenyl nitrate with various nitrogen bases in anhydrous ethanol at 0 °C was examined. In all cases fluorenone was formed in 100% yield indicating that there was not any substitution. A reasonable Brønsted plot was obtained for reaction promoted by structurally similar bases with β = 0.84 which suggests a product-like transition state. As well, deviations from the Brønsted plot are discussed. Primary hydrogen-deuterium isotope effects were measured for reaction promoted by 11 different amine bases. A reasonable correlation was obtained for structurally similar bases when a plot of kH/kDvs. pKa was made. The conclusion is reached that when kH/kD reaches a maximum, ∼9.2 at 0 °C, it remains unchanged and hence is a poor measure of transition state geometry. As well, very poor correlations are found when the abstracting base is tertiary which leads to the conclusion that a comparison of kH/kD values is not warranted for structurally different bases.

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.

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