Evidence for a rate-determining solvation change in methyl transfer to water. Solvent dependence of H/sub 2/O/D/sub 2/O kinetic isotope effects
When H/sub 2/O is a dilute (< 1 M) solute in CH/sub 3/CN or sulfolane, the rate of methyl transfer from CH/sub 3/X to H/sub 2/O is first order both in (CH/sub 3/X) and in (H/sub 2/O), but H/sub 2/O and D/sub 2/O react at the same rate (k/sub H//k/sub D/ = 1.00 +- 0.01 for X/sup -/ = OClO/sub 3//sup -/ or thiophene). The analogous methyl transfers in CH/sub 3/CN at 25/sup 0/C from the same X/sup -/'s to CH/sub 3/OH have k/sub H//k/sub D/ = 1.07 and 1.05 and to (CH/sub 3/)/sub 3/COH have 1.12 and 1.07. In CH/sub 3/CN/H/sub 2/O (or ROH) solvent mixtures, as the H/sub 2/O (or ROH) content is increased to mole fractions > 0.5, k/sub H//k/sub D/ for five of these six reactions increases sharply to the values observed in near H/sub 2/O or ROH: at 25/sup 0/C for X/sup -/ = OClO/sub 3//sup -/ and thiophene, respectively, k/sub H//k/sub D/ = 1.20 and 1.13 for H/sub 2/O, 1.22 and 1.15 for CH/sub 3/OH, and 1.08 and 1.12 for (CH/sub 3/)/sub 3/COH. Comparison of these k/sub H//k/sub D/ values to known values of deuterium fractionation factors for CH/sub 3/OH, H/sub 3/O/sup +/, and CH/sub 3/OH/sub 2//sup +/ suggests the following: (a) for CH/sub 3/X + H/sub 2/O, there is little or no positive charge (delta = 0.00 +- 0.04) on the nucleophilic oxygen in the transition state. (b) For CH/sub 3/X + CH/sub 3/OH or (CH/sub 3/)/sub 3/COH, that charge is larger than for H/sub 2/O. (c) The activation process in these methyl transfers (except for CH/sub 3/OClO/sub 3/ + (CH/sub 3/)/sub 3/COH) is predominantly a change in solvation of a preassociated H/sub 2/O (or ROH), CH/sub 3/X pair and, in the transition state, the solvation is more product-like than is the internal charge distribution.
- Research Organization:
- Washington Univ., St. Louis, MO
- OSTI ID:
- 5532553
- Journal Information:
- J. Am. Chem. Soc.; (United States), Vol. 108:11
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
HEAVY WATER
CHEMICAL REACTIONS
ISOTOPE EFFECTS
PERCHLORATES
POLYCYCLIC SULFUR HETEROCYCLES
SULFONATES
WATER
CHARGE DISTRIBUTION
CHEMICAL REACTION KINETICS
EXPERIMENTAL DATA
MEDIUM TEMPERATURE
METHYL RADICALS
SOLVATION
ALKYL RADICALS
CHLORINE COMPOUNDS
DATA
HALOGEN COMPOUNDS
HETEROCYCLIC COMPOUNDS
HYDROGEN COMPOUNDS
INFORMATION
KINETICS
NUMERICAL DATA
ORGANIC COMPOUNDS
ORGANIC SULFUR COMPOUNDS
OXYGEN COMPOUNDS
RADICALS
REACTION KINETICS
400302* - Organic Chemistry- Isotope Effects- (-1987)