Criterion of Mechanism

The Hammond postulate is a valuable criterion of mechanism, because it allows a reasonable transition state structure to be drawn on the basis of knowledge of the reactants and products and of energy differences between the states (i.e., AG and AG°). Throughout this chapter we have located transition states in accordance with the Hammond postulate. 

Hi) Reaction selectivity substituent effects). This property can be used as a diagnostic criterion of mechanism since a bimolecular attack is expected to be sensitive to the structure of the substrate to an extent depending on the role of the bond-making step. The importance of the... 

Kresge and Chiang15 showed that the dependence of rate coefficient upon the Hammett acidity function was not a safe criterion of mechanism and also explained how different dependencies of rate upon acidity function for different compounds could arise. Rate coefficients for detritiation of [3H]-2,4,6-tri-methoxybenzene followed the acidity function h0 in perchloric acid up to 3 M, i.e. k = 0.484 (Ao)1 07. so that a plot of log k versus —H0 would have a slope of 1.07, which implies the A-l mechanism. (New acidity function values479 would make this slope 1.14.) However, the diagnostic value of such a correlation was... 

As pointed out above, values of KTS are obtainable from rate data without making any assumptions about the reaction mechanism. Therefore, one may use KTs and its variation with structure as a criterion of mechanism, in the same way that physical organic chemists use variations in other kinetic parameters (Brpnsted plots, Hammett plots, etc.). For present purposes, the value of Kts can be useful for differentiating between the modes of binding in the S CD complex and the TS-CD transition state, between different modes of transition state binding, and hence between different types of catalysis (Tee, 1989). 

In other cases a label may be transferred from A into P or from B into Q. Information on such exchanges has provided a valuable criterion of mechanism which is considered in Chapter 12, Section B,4. 

A criterion of mechanism based on the Hammett acidity function, H0 (Section 3.2, p. 130),has long been used to decide the type of question raised by the choice between Mechanisms I and II in Scheme 8. Since in strongly acidic media the concentration of the protonated substrate should be proportional to h0, the reaction rate for a unimolecular decomposition of this protonated substrate (Mechanism I) should also be proportional to h0, whereas if a water molecule is required (Mechanism II), the rate should follow H30+ concentration instead. This test, known as the Zucker-Hammett hypothesis,76 when applied to acetal and ketal hydrolysis, appears to confirm the A-l mechanism, since a linear relationship is found between rate constant and h0 at high acidity.77 Inconsistencies have nevertheless been found in application of the Zucker-Hammett hypothesis, for example failure of the plots of log k vs. — H0 to have the theoretical slope of unity in a number of cases, and failure to predict consistent mechanisms for forward and reverse reactions the method is therefore now considered to be of doubtful validity.78 Bunnett has devised a more successful treatment (Equation 8.45), in which the parameter to measures the extent of... 

Entropies of activation as a criterion of mechanism. Substitution by mechanism Se2... 

The volume of activation, A V, which is — RT 8 lnk)Jdp, has been suggested as a criterion of mechanism (Whalley, 1964). Known volumes of activation for A-SB2 reactions, measured near 25°, seem to be limited to olefin hydration (—12 cm3 mole-1 for isobutene at 35°, Baliga and Whalley, 1965) and allylmercuric iodide cleavage ( — 11 cm3 mole-1 at 25°, Halpem and Tinker, 1965). It is impossible to generalize from so few examples, but, in principle, it seems possible that AV is less dependent on structural ramification than AS, and therefore easier to interpret. Against this must be weighed the experimental difficulties in... 

Hi) the relative rate of 2-adamantyl tosylate and bromide (OTs/Br ratio) is similar to that of tertiary substrates (Fry et al., 1970a) this criterion of mechanism was proposed by Hoffmann (1965), but it now appears that OTs/Br ratios in secondary and tertiary systems are determined largely by steric effects (Slutsky et al., 1974) ... 

The criterion of mechanical stability reveals that (dV/dp)T, aii v, < 0, as discussed... 

Spinodal points represent the boundary between positive and negative curvature of A-V isotherms. An equilibrium state on the spinodal curve is defined by (9p/9V)7 ,au JV = 0. Regions between the spinodal points are intrinsically unstable and violate the criterion of mechanical stability. 

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