Nucleophilicity ratio, plot

Presumably less nucleophilically assisted solvolyses could show higher a-deuterium isotope effects, and there is a linear relationship between the magnitude of nucleophilic solvent assistance (Table 2) and the a-deuterium isotope effect for solvolyses of 2-propyl sulpho-nates (Fig. 7). Another measure of nucleophilic assistance is the ratio k2 (OH )/, where k2 is the second-order rate constant for nucleophilic attack by OH and kx is the first-order rate constant for reaction with the solvent water, and a linear correlation was obtained by plotting the ratio versus the experimentally observed isotope effects for methyl and trideuteriomethyl sulphonates, chlorides, bromides and iodides (Hartman and Robertson, 1960). Using fractionation factors the latter correlation may also be explained by a leaving group effect on initial state vibrational frequencies (Hartshorn and Shiner, 1972), but there seems to be no sound evidence to support the view that Sn2 reactions must give a-deuterium isotope effects of 1-06 or less. 

The rate constant kgaiv for solvolysis is assumed to reflect the stability and reactivity of (i.e., faster solvolysis gives a more stable cation, which, therefore, reacts more slowly with nucleophiles). The ratio kt /k , measured by product distribution studies, is a measure of selectivity with respect to the nucleophiles azide ion and water. The plot of log k oiv against log (kn/k ) is messy and rather confusing, containing regions of constant selectivity, RSP behavior, and possibly anti-RSP behavior. The present point is that the measures of reactivity and selectivity may affect the conclusions drawn. The role of solvent effects can be important in modifying RSP behavior. ... 

For neutral nucleophiles, we have utilized a series of ring-substituted N,N-dimethylanilines. The second-order rate coefficients should now be independent of nucleophile concentration, and this was confirmed by showing that log (k/k0 obtained from the product ratios, was independent of the amine concentration for 0.008 to 0.08 M N,N-dimethyl-p-toluidine. The log (k/k0) values could also be conveniently determined for m-CH3-, H-, p-Br-, and m-Cl-substituted derivatives (equation 13). For the m-N02 derivative, even at 0.32 M, the dominant reaction is solvolysis and only an approximate value for log (k/k0) could be obtained. A Hammett plot against the tabulated a values (43) (omitting the approximate m-N02 data) led to a linear plot and a slope (p value) of —2.77 0.15 (r = —0.996). This value is similar to values for reaction with other ethyl derivatives, derived from kinetically determined k values —3.60 for reaction with ethyl iodide in nitrobenzene at... 

In the majority of reactions studied, a constant selectivity relationship does not apply. In the addition of morpholine, glycine, and MeO- to CH2=CHX systems, X=PO(OEt)3, CONH2, CN, C02Me, S02Me, COMe, COPh, and CHO plots of log k versus pKa (MeX) are linear but not parallel (28) thus, the reactivity ratios for each pair of nucleophiles are not constant. 

In certain types of reactions involving nucleophiles, it has been found that rate constants for nucleophiles that have an atom bearing a lone pair of electrons adjacent to ( alpha to ) the nucleophihc atom, when inclnded in a Brpnsted-type plot, lie above the line, that is to say, their nucleophilic strength is enhanced above what is expected for a normal nucleophile of the same base strength. Examples of such supemucleophiles are illustrated by the filled circles in Figure 3.3. This enhancement is termed the alpha effect (See, e.g., Buncel etal., 1982). It is measured by the ratio (alphaNu)/Mnormal Nu). Note that the normal Nn is one (which may be hypothetical) with the same as the alpha Nu. Commonly found alpha nucleophiles contain as the alpha atom ... 

The inhibition of human leukocyte elastase by the ynenol lactone was irreversible in the presence of the nucleophiles )6-mercaptoethanol and hydroxylamine and after size exclusion chromatography. The partition ratio r was evaluated in two different ways. Titration of the enzyme by suicide substrate using the plot shown in Fig. 13.3 gave r = 1.7 0.5. The partition ratio was also determined from the ratio of rates 3/ 4= 1.5. 

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