Ion-pair intermediates and extreme deuterium isotope effects in partially diastereospecific base-promoted elimination competing with base-catalyzed 1,3-proton transfer

1979 ◽  
Vol 101 (24) ◽  
pp. 7311-7318 ◽  
Author(s):  
Alf Thibblin ◽  
Per Ahlberg
Keyword(s):  
Proton Transfer ◽  
Isotope Effects ◽  
Ion Pair ◽  
Deuterium Isotope Effects

Isotope effects and activation parameters for the proton transfer reaction from 1-(4-nitrophenyl)-1-nitroethane to free anions and ion pairs of cesium n-propoxide in n-propanol solvent

1986 ◽  
Vol 64 (6) ◽  
pp. 1021-1025 ◽  
Author(s):  
Arnold Jarczewski ◽  
Grzegorz Schroeder ◽  
Przemyslaw Pruszynski ◽  
Kenneth T. Leffek
Keyword(s):  
Proton Transfer ◽  
Rate Constants ◽  
Isotope Effects ◽  
Ion Pairs ◽  
Activation Parameters ◽  
Solvation Shell ◽  
Ion Pair ◽  
Initial State ◽  
Free Ion ◽  
Deuteron Transfer

Rate constants for the proton and deuteron transfer from 1-(4-nitrophenyl)-1-nitroethane to cesium n-propoxide in n-propanol have been measured under pseudo-first-order conditions with an excess of base for four temperatures between 5 and 35 °C. Using literature values of the fraction of cesium n-propoxide ion pairs that are dissociated into free ions, separate second-order rate constants for the proton and deuteron transfer to the ion pair and to the free ion have been calculated. The cesium n-propoxide ion pair is about 2.8 times more reactive than the free n-propoxide ion. The primary kinetic isotope effects for the two reactions are the same (kH/kD = 6.1–6.3 at 25 °C) within experimental error. The enthalpy of activation is smaller for the ion-pair reaction and the entropy of activation more negative than for the free-ion reaction. For proton transfer, ΔH±ion pair = 8.3 ± 0.2 kcal mol−1, ΔH±ion = 9.6 ± 1.0 kcal mol−1, ΔS±ion pair = −12.3 ± 0.6 cal mol−1 deg−1, ΔS±ion = −10.1 ± 3.4 cal mol−1 deg−1. The greater reactivity of the ion pair relative to the free ion is interpreted in terms of the weaker solvation shell of the ion pair in the initial state.

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.

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