Rates and isotope effects in the proton transfers from 2-nitropropane to pyridine bases

Abstract

The reaction between pyridine bases and 2-nitropropane in the presence of iodine is uncomplicated in t-butyl alcohol-water mixtures. In the absence of excess iodide ion and with only a small fraction of the pyridine converted to its conjugate acid, the reversibility can be neglected, and the reaction is apparently free from solvent and lyate ion catalysis. Steric retardation from 2-and 6-alkyl groups in the pyridine is significant, and comparable in magnitude to the acceleration resulting from the increase in base strength. Isotope effects measured when using 2-nitropropane-2-d are large and variable, increasing not only with base strength but also with steric hindrance from a value of kH/kD = 9.8 at 24.88' for pyridine catalysis to kdkD = 24.2 for 2,4,6-trimethylpyridine catalysis at the same temperature. Reasons for the unusually large isotope effects are considered; both tunneling and extensive loss of zero-point energy are invoked.he proton transfer reaction is one of the most im-Py + (CHB)ZCHNO? + Iz ----f PyH' + I-+ (CH&CINO? (1)

Influence of steric hindrance in molecules of substituted 2-phenyl-1,1,3,3-tetramethylguanidines on the kinetics and isotope effects of their proton transfer reactions with 4-nitrophenylnitromethane in acetonitrile solvent

“…0.12 for the 2-ClPhTMG and 1.22 + 0.19 for 2-BrPhTMG, indicate a small participation of tunnelling in these reactions that increases smoothly with steric crowding around the basic site. These results are in good agreement with those of Lewis and Funderburk for the reaction of 2-nitropropane with pyridine bases (24,25) and with our previous results (26,27).…”

Secondary Kinetic Isotope Effects in Bimolecular Nucleophilic Substitutions. VI. Effect of α and β Deuteration of Alkyl Halides in their Menschutkin Reactions with Pyridine in Nitrobenzene

“…Secondary P-deuterium isotope effects in SN2 reactions present a problem in that some amount of E2 side reaction usually occurs concurrently and the elimination of a /I-hydrogen (or deuterium) in the E2 reaction introduces a primary deuterium isotope effect of large magni- tude, typically a factor of 4 to 8 for the rate ratio (18,19) although much larger values have been observed (20)(21)(22). While the percentage of E2 to the total reaction is expected to be small for the quaternization reaction (23), it is apparent that even a small percentage could give rise to misleading, small, overall normal isotope effects.…”

Equilibrium and kinetic study of the proton transfer reactions between nitroalkanes and strong organic bases — phosphazenes in tetrahydrofuran solvent

“…Initially, it was believed that steric hindrance leads to increased primary deuterium kinetic isotope effects by making the energetic barrier steeper [19,20] or by desolvation of the transition state [15]. However, there are examples of kinetic isotope effects insensitive to the bulk of substituent or even decreasing with steric hindrance either on C-acid or on the base [15].…”