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Lookup NU author(s): Dr Peter Miatt
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The effect of solvent composition in aqueous ethanol, trifluoroethanol and hexafiuoropropan-2-ol on the rate constant and activation parameters for the uncatalysed hydrolysis of 2-(p-nitrophenoxy)tetrahydropyran (1) was investigated, and the m(TOTs) value is 0.60. This appreciable but less than maximal value is in accordance with an SN1 mechanism with rate-limiting ionization. The α-deuterium secondary kinetic isotope effect (α-kie) for the uncatalysed hydrolysis of 1 is 1.17 in water (46 °C), 1.15 in aqueous trifluoroethanol (50% mole fraction, 70.6 °C) and 1.13 in aqueous ethanol (50% mole fraction, 70.6 °C). These values correspond to about 1.19 at 25 °C, which is characteristic of rate-limiting ionization in an SN1 reaction and appreciably higher than values for enzyme-catalysed glycolysis. The α-kie is smaller under aqueous acidic conditions (1.07, 0.1 mol dm-3 hydrochloric acid, 20.2 °C) when 1 hydrolyses with acid catalysis. The previously reported α-kie for the hydrolysis of 1 in buffered aqueous dioxan (1.063, 25 °C) is now seen to correspond to acid-catalysed hydrolysis. These new results for 1 indicate that transition structures in enzyme-catalysed glycolyses with α-kie values of less than about 1.15 at 25 °C involve a lower degree of carbenium ion character than has hitherto been assumed. Copyright © 2004 John Wiley & Sons, Ltd.
Author(s): Ahmad IA, Birkby SL, Bullen CA, Groves PD, Lankau T, Lee WH, Maskill H, Miatt PC, Menneer ID, Shaw K
Publication type: Article
Publication status: Published
Journal: Journal of Physical Organic Chemistry
Year: 2004
Volume: 17
Issue: 6-7
Pages: 560-566
ISSN (print): 0894-3230
ISSN (electronic): 1099-1395
Publisher: John Wiley & Sons Ltd
URL: http://dx.doi.org/10.1002/poc.803
DOI: 10.1002/poc.803
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