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Primary kinetic hydrogen isotope effects in deprotonations of carbon acids
Author(s) -
Watt C. Ian F.
Publication year - 2010
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.1671
Subject(s) - chemistry , kinetic isotope effect , deprotonation , kinetic energy , solvent , hydrogen , quantum tunnelling , base (topology) , carbon fibers , branching (polymer chemistry) , computational chemistry , medicinal chemistry , deuterium , organic chemistry , atomic physics , ion , mathematical analysis , physics , mathematics , optoelectronics , quantum mechanics , materials science , composite number , composite material
The mechanisms of deprotonation of carbon acids are reviewed to emphasise the important kinetic consequences associated with the internal and solvent reorganisation in forming the conjugate base. Some practicalities of measurements of kinetic hydrogen isotope effects are then considered to show how competing reactions, exchange and mechanistic branching might affect these measurements. Finally, isotopic rate ratios ( k H / k D ) have been collected for non‐enzymic deprotonations of carbon acids for which temperature dependences have also been determined. All examples contain nitro groups. For reactions in hydroxylic solvents, only two examples are found for which k H / k D > 8 at 25 °C (28.8 and 10.7), possibly indicating a tunnelling contribution. In both cases, values of E a D – E a Hare larger and values of A H / A D smaller than the range expected ( E a D – E a H = 4.54 kJ mol −1 and 0.7 < A H / A D < 1.4) for effects arising from zero‐point energy differences with no tunnelling contributions. For reactions in aprotic solvents, isotope effects are larger (12.1 < k H / k D < 19.6), but again, the largest of these (19.6 and 15.8) are associated with elevated values of E a D – E a Hand small values of A H / A D . It is pointed out that behaviour of this type is accommodated by corrections to the semi‐classical model with shallow tunnelling maximised by high symmetrical barriers. No examples have been found in which large effects are associated with large values of A H / A D and small values of E a D – E a H . Comparisons are made with two recently reported enzyme‐catalysed reactions involving deprotonation of nitroalkanes. Copyright © 2010 John Wiley & Sons, Ltd.