Reaction diffusion substituent effects

One facet of kinetic studies which must be considered is the fact that the observed reaction rate coefficients in first- and higher-order reactions are assumed to be related to the electronic structure of the molecule. However, recent work has shown that this assumption can be highly misleading if, in fact, the observed reaction rate is close to the encounter rate, i.e. reaction occurs at almost every collision and is limited only by the speed with which the reacting entities can diffuse through the medium the reaction is then said to be subject to diffusion control (see Volume 2, Chapter 4). It is apparent that substituent effects derived from reaction rates measured under these conditions may or will be meaningless since the rate of substitution is already at or near the maximum possible. 

The proximity of the diffusion limit also inhibits a detailed discussion of the data in Table 7, but a significant difference to the substituent effects discussed in Section III.D.4 is obvious. Whereas the reactivities of terminal alkenes, dienes, and styrenes toward AnPhCH correlate with the stabilities of the new carbenium ions and not with the ionization potentials of the 7r-nucleophiles [69], the situation is different for the reactions of enol ethers with (p-ClC6H4)2CH+ [136]. In this reaction series, methyl groups at the position of electrophilic attack activate the enol ether double bonds more than methyl groups at the new carbocationic center, i.e., the relative activation free enthalpies are not controlled any longer by the stabilities of the intermediate carbocations but by the ionization potentials of the enol ethers (Fig. 20). An interpretation of the correlation in Fig. 20 has not yet been given, but one can alternatively discuss early transition states which are controlled by frontier orbital interactions or the involvement of outer sphere electron transfer processes [220]. 

Very rapid desulfurization of thiiranes by a bimetallic complex was reported late in 1995. Bergman and co-workers found that Cp2Zr(/t-A -f-Bu)IrCp, where Cp is cyclopentadienyl and Cp is pentamethylcyclopenta-dienyl, desulfurizes thiiranes via a bridging sulfido complex 121, for which a crystal structure was obtained <1995JA10041>. Small substituent effects in the thiiranes and retention of substituent stereochemistry suggested that ring-opened intermediates are not involved and that the reaction rate might approach the diffusion-controlled limit. 

The nucleophilic substitution reactions of anilines with ero-2-norbomyl arenesulfonates, 2, present an interesting example of the preassociation mechanism55 (Scheme 1). The rate is faster with 2-exo (k2 = 15.9 x 10 4 and 3.24 x 10-5 M 1 s 1 when X = Z = H in MeOH and MeCN at 60.0 °C, respectively) than with 2-endo (k2 = 0.552 x 10 5 M 1 s 1 with X = Z = H in MeOH at 60.0 °C). These reactions are characterized by a large pz (1.8 and 1.2 for 2-exo and 2-endo) coupled with a small magnitude of px (—0.21 and —0.15 for 2-exo and 2-endo). The pz values for the aniline reactions are even larger than those for the SY 1 solvolysis in MeOH (pz =1.5 and 1.0 for solvolysis of 2-exo and 2-endo). Thus the abnormal substituent effect in the anilinolysis of 2 can only be accounted for by the preassociation mechanism of Scheme 1. The upper route is the normal S/v 1 pathway, and the lower route is the preassociation pathway. The preassociation step, Xass, and association of the Nu to the ion pairs, kn. occur in a diffusion limited or fast process and k is the rate-limiting step. This mechanism leads to second-order kinetics and therefore is an SY/2 process, but structural effects on rates are very similar to those of S l reactions, since the R+ Z pair consists essentially of the two free ions. 

The rate constant A is a composite parameter, k = ELk, where E is the effectiveness factor, L the concentration of active sites on the surface of the catalyst, and k the actual rate constant of the transformation of the adsorbed species. The effectiveness factor which can attain values from zero to one is a measure of retardation of the reaction by diffusion of reactants or products into or out ofthe pores of the catalyst. For our purpose it should have a value of one or near to one and with careful experimentation this can be achieved. According to Thiele (14) the effectiveness factor is a function of reaction rate and effective diffusion coefficient. Both these parameters depend on the structure of the reacting compound and therefore the effectiveness factor will tend to change with the nature of the substituents. The effect of structure on reaction rate is more critical than on diffusion coefficient and if the reactivity within the series of investigated compounds will vary over some orders there is always danger of diffusional retardation in the case of the most reactive members of the series. This may cause curvature of the log kva a plot. 

The final product ArCH ONO is formed in further oxidation of ArCH/ to ArCH/ by CAN and the subsequent reaction with NOj . For toluene derivatives with electron-donating substituents such as the methoxy group, the electron transfer reaction (Equation 4.73) was confirmed by the laser flash photolysis method [44]. For toluene, there is a probability for direct H-atom abstraction (Equation 4.72) with a highly polar transition state. Furthermore, for toluene derivatives with electron-withdrawing substituents, the addition ability of NO3 to phenyl 7t-bonds can be considered on the basis of data for reactions with phenols [41] and furan [45]. To clarify the interchanges in the reaction paths by the substituent in toluene, reaction rate constants for various toluene derivatives were evaluated by flash photolysis [44]. The substituent effect of the rate constants for toluene derivatives was correlated with ionisation energies (lEs) of these substances. The reaction rate for anisole is too fast to obtain accurate rate constants, and only lower limits of the rate constants are obtained (anisole) >310 M -s h For p-nitrotoluene, the rate constant is 2.3T0 M -s IE = 9.5 eV. A deuterium kinetic isotope effect of 1.6 was observed for the reaction of NO3 with toluene and toluene - dg. This indicates that NO3 predominantly abstracts the H atom from methyl groups. In the case of p-xylene, the deuterium isotope effect was not observed [43]. The rate constant forp-xylene (> 2 x 10 M/s) is close to the diffusion-controlled limit in acetonitrile, and consequently selectivity becomes low. 

Although reaction 6 is essentially a diffusion-controlled process for all kinds of substituents, the small differences observed in the rate constants through the series alkane-, amino- and alkoxy-sulfonyl chlorides have been attributed to the increased importance of polar effects to the transition state11. 

The very small p- and m-values observed for the fast bromination of a-methoxystyrenes deserve comment since they are the smallest found for this electrophilic addition. The rates, almost but not quite diffusion-controlled, are amongst the highest. The sensitivity to polar effects of ring substituents is very attenuated but still significant that to resonance is nil. These unusually low p-values for a reaction leading to a benzylic carbocation are accompanied by a very small sensitivity to the solvent. All these data support a very early transition state for this olefin series. Accordingly, for the still more reactive acetophenone enols, the bromination of which is diffusion-controlled, the usual sensitivity to substituents is annulled. 

FIGURE 12. Plots of the corrected rates of reaction of (a) silicon-substituted silenes and (b) carbon-substituted silenes vs. substituent parameter functions, accounting for diffusion effects on the overall rate constants... 

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