Eyring plot isotope

This was cleanly first-order, kobs = 5.6 X 1(T6 s 1 at 294 K an Eyring plot between 294 and 327 K gave activation parameters AH = 17 kcal/mol and AS = -25 cal/(mol-K). A primary kinetic isotope effect of approximately 5 was seen for the deuteroxide at 294 K. No rate inhibition was seen in the presence of added alkyne, leading to the conclusion that preequilibrium loss of alkyne did not occur and that the rate-determining step was in... 

In a report describing acid-promoted Ritter reactions involving a-methylene-)5-hydroxyesters, an I -type process was found to be the preferred mechanism. For compound (104), both Ritter reaction products (105 and 106) are obtained. In order to rule out 5 2 or Sf 2 mechanisms, kinetic analysis was performed, kinetic isotope effects were evaluated, and both Hammett and Eyring plots were done. The mechanistic studies were consistent with an I -type process being preferred with initial formation of the oxonium cation, loss of water, and formation of the allylic acarbocation (107). DFT calculations indicated nucleophilic attack at the terminal carbon (107b, 5 10 was favored by about 2.6kcalmol over attack at the benzylic position (107a, 5 1) (Scheme 23). 

Figure 16. Summary plot of activation parameters (enthalpy of activation, AH entropy of activation, AS ) for oxygen isotope exehange in siUeates reacted with either pure water or salt solntions. The Eyring-Polanyi relationship (Eqn. 102) was nsed with data described by Cole et al. (1983), Cole and Ohmoto (1986), Cole et al. (1987, 1992), and in Table 2 (see Appendix). Note that data falling on a hnear trend generally indieate that a conunon leaetion meehanism predominates, ab = albite, san = sanidine, eels = celsian, qtz = qnartz, mnsc = muscovite, bio = biotite, chi = chlorite, parag = paragonite, woll = wollastonite, diop = diopside, gran = granite, bas = basalt.

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