Reaction rates results

Although both stereoisomers yield 4 tert butylcyclohexene as the only alkene they do so at quite different rates The cis isomer reacts over 500 times faster than the trans The difference in reaction rate results from different degrees of rr bond develop ment in the E2 transition state Since rr overlap of p orbitals requires their axes to be parallel rr bond formation is best achieved when the four atoms of the H—C—C—X unit he in the same plane at the transition state The two conformations that permit this are termed syn coplanar and anti coplanar... 

Direct esr evidence for the intermediacy of radical-cations was obtained on flowing solutions of Co(III) acetate and a variety of substituted benzenes and polynuclear aromatics together in glacial acetic acid or trifluoroacetic acid solution . A p value of —2.4 was reported for a series of toluenes but addition of chloride ions, which greatly accelerated the reaction rate, resulted in p falling to —1.35. Only trace quantities of -CH2OAC adducts were obtained and benzyl acetate is the chief product from toluene, in conformity with the equation given above. 

In several experiments we have followed the exit conversion as a function of the inlet CO concentration. For a Pd catalyst, decreasing inlet CO concentration increases the reaction rate resulting in a jump to the upper steady state. Obviously the lower steady state disappears. On the... 

The change in absorbance at the maximum in the 255 - 270 nm region of films baked for various periods of time at 140, 110 and 90 °C are shown in Figure 7. All of the data were fit to curves of the form of Equation 5. The reaction rate results are summarized in Table I for both the early (up to twenty hours) and the latter portions of the curves (beyond twenty hours). 

Deviations resulting from the diffusion control of termination at low conversions of monomer to polymer are the relatively weak effects discussed in the preceding subsection. By contrast, changes in reaction rate resulting from hindered diffusion at high conversions are very important in most radical polymerizations. Figure 6-3 shows rate curves for the polymerization of methyl methacrylate in benzene at 50°C [17]. At monomer concentrations less than about 40 wt % in this case, the rate is approximately as anticipated from the standard kinetic scheme described in this chapter. Rp decreases gradually as the reaction proceeds and the concentrations of monomer and initiator are depicted. 

Fig. 11. The gradual transition of slope in the Arrhenius plot of reaction rate resulting from diffusion effects.

The empirical rate law in Eq. 23 holds only for the initial rates. Tamura et al. (1976) observed an autocatalytic effect of the ferric precipitates produced in the reaction. Sung and Morgan (1980) identified y-FeOOH as the primary oxidation product at neutral pH and confirmed its autocatalytic effect. Adsorbed Fe(II) seems to compete in an additional parallel reaction with the dissolved ferrous species. Fast surface reaction rates resulted from a fit of the kinetic data. Examples of these constants are included in Fig. la for comparison. They represent only estimates of an order of magnitude because Tamura et al. (1976) did not determine the surface concentration of Fe(II). However, Figure 2 shows qualitatively that the ferrous ion is adsorbed specifically to mineral surfaces. 

Commercial light soda ash from a freshly opened container is desirable, but a laboratory grade of anhydrous sodium carbonate can be used if it is first heated at 150 to 200° for at least 1 hour. Light soda ash is preferred to the dense material because of its small particle size. The completely dry solid reacts slowly, but an optimum reaction rate results when ca. 10% by weight of water is present. 

The two global decomposition reactions are nearly thermally neutral at temperatures around 600 K. Subsequent reactions among the products of (Rl) and (R2) may occur and provide the energy to sustain pyrolysis. Brill [12] examined several plausible secondary reactions, such as CH2O + NO2, CH2O + N2O, and HCN + NO2, and their corresponding reaction rates. Results indicated that the following reaction... 

For a less-complicated consideration, aU the tautomers, rotamers, as well as resonance structures are not specifically considered here. For example, although these equilibriums can be included in the kinetic analyses, the outcome reaction rate results in the same kinetic effects on [HA] or [A] as described in the text. In order to have more focused discussion in the text, detailed kinetic analyses of all other possible mechanisms, which can be done as described in the text, are not hsted here. 

In general, the hollow cylinder or average microenvironment model fits the NH data extraordinarily well with only minor variation in rj, the effective density of animals. The required variation in r- presumably comes about because of the uneven distribution of animals within sediment (Jumars et al., 1977) and mobility. It may also be apparent variation due to the arbitrary requirement that r, remain fixed. The variation forced into r2 by this restriction could actually reflect changing boundary conditions at r, for example, inhibition to diffusion by burrow linings or a reduction in irrigation activity. A part of the discrepancy between the model and measured profiles could also reflect changes in the production rate of NH/ because of factors other than temperature. For example, fall profiles may show a smaller maximum than predicted, due to a lower reaction rate resulting from a depletion of substrate (see Nixon et al., 1980). 

For a less-complicated consideration, all the tautomers, rotamers, as well as resonance structures are not specifically considered here. For example, although these equilibriums can be included in the kinetic analyses, the outcome reaction rate results in the same kinetic effects on [HA] or [A] as described in the text. In order to have more focused discussion in the text, detailed kinetic analyses of all other possible mechanisms, which can be done as described in the text, are not hsted here. For recent reviews, see (a) Tidwell, T.T. Ketenes lohn Wiley Sons New York, 1995. (b) Wentrup, C. Heihnayer, W. KoUenz, G. Synthesis 1994, 1219. (c) Tidwell, T.T. Acc. Chem. Res. 1990, 23, 273. For leading references, see (a) Gong, L. McAUister, M.A. Tidwell, T.T. J. Am. Chem. Soc. 1991, 113, 6021. (b) Lien, M.H. Hopkinson, A.C. J. Org. Chem. 1988, 53, 2150. (c) Armitage, M.A. Higgins, M.J. Lewars, E.G. March, R.E. J. Am. Chem. Soc. 1980, 102, 5064. 

Fig. 7.12 Analysers for (a) direct and (b) differential kinetic determinations. The determination carried out on (a) (glucose) is based on two measurements made at two different times that performed with (b) (transaminases) relies on the simultaneous measurement of a signal increment arising from the difference in reaction rate resulting from a difference in temperature between the two channels. (Reproduced from [15] with permission of the American Chemical Society).

Combining Eqs. (8) and (10), a simpliiled expression for reaction rate results ... 

Above pH 7, the rate of reaction of THP appears to be controlled by diffusion of the reagent into the fibers [73] and, like the reaction of mercap-tans with hair, increases rapidly with increasing pH in the vicinity of pH 9 to 12. Presumably, this increase in reaction rate results from increased swelling of the keratin substrate with increasing pH. 

A power law can often be used for the concentration-dependent term (A...), so that a simple expression for the reaction rate results (Equation 3.1.3-7) ... 

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