Intrinsic barrier substituents

The data in Table 2 suggest that the intrinsic barriers for the reactions of X-[6+] increase with increasing stabilization of these carbocations by resonance electron donation from the ring substituent. This complements the indirect evidence for changes in these intrinsic barriers that has been described in recent reviews.8,15 The data in Table 2 and Fig. 5 provide support for the following generalizations about the reactions of X-[6+] ... 

The coupling of cyanides ions with aryl radicals is an interesting example where quantitative kinetic data are available.30 The forming bond is strong, but this favorable factor is counteracted, in terms of driving force, by the fact that °./x- [second term in equation (3.24)] is very positive (in other words, CN is a hard nucleophile). In addition, the large value of Drx r +x is unfavorable in terms of the intrinsic barrier. Overall, the presence of electron-withdrawing substituents is necessary to allow the... 

Intrinsic barriers define the intrinsic velocity by which an ET (or any other reaction) proceeds. Although they are tailored to the particular acceptor considered, chemical intuition suggests that if the main reaction site is kept constant, e.g. the S—S bond, the intrinsic ET rate should be some function of the substituents at the atoms of concern. Let us consider first the data... 

It should also be noted that while an increase in the number of fluorine substituents leaves the rate of reaction of the cation with water unaffected, the reverse reaction is profoundly affected. In the latter direction the full equilibrium effect of the substituent is felt on the rate. This is because now the effects of changes in thermodynamic driving force and intrinsic barrier complement each other. 

Richard has also shown that intrinsic barriers for carbocation reactions depend not only on the extent of charge delocalization but to what atoms the charge is delocalized. In a case where values of pifR for calculation of A were not available, comparisons of rate constants for attack of water kH2o with equilibrium constants for nucleophilic reaction with azide ion pKAz for 65-67 showed qualitatively that delocalization to an oxygen atom leads to a lower barrier than to an azido group which is in turn lower than to a methoxyphenyl substituent.226... 

One of the consequences of the imbalanced nature of the transition state is that the polar effect of a remote substituent may either increase or decrease the intrinsic barrier whether there is an increase or decrease depends on the location of the substituent with respect to the site of charge development. Let us consider a reaction of the type shown in Equation (4). In this situation an electron-withdrawing substituent Z will decrease AG or increase ka. This is because there is a disproportionately strong stabilization of the transition state compared to that of the product anion due to the closer proximity of Z to the charge at the transition state than in the anion. As discussed earlier, this also leads to an exalted BrlAnsted aCH value and is the reason why aCH > Pb for the deprotonation of carbon acids such as 11-13 and others (Table 2). 

When NMA+ reacts with phenyl-substituted N-phenyldihydronicotin-amides, X-PhNAH, also in anhydrous acetonitrile (Powell and Bruice, 1983b), rate and equilibrium data yield a Bronsted plot with a slope of 0.51, consistent with a centrally located transition state. The primary k.i.e. s h2/ d2, increase from 3.98 for X = />-methoxy to 4.77 for X = m-trifluoro-methyl at 50° and may indicate a trend to a more symmetrical transition state. Marcus treatment of the substituent dependence of the k.i.e. s yields an intrinsic barrier AG = 22.2 kJ mol - L. The temperature dependence of the k.i.e. for reduction by X-PhNAH with X = / -methyl gives [A ] = 7.68 kJ mol-1, but AJA = 4.3 is unusually large. A tunnelling correction of ca. 2 was estimated so that the semi-classical k.i.e. was in the range 2 to 3. 

The contribution of polar structures reduces the barrier and also the intrinsic barrier. This results for non thermoneutral reaction in a reduction of isotope effect. This has been a controversial subject for several years it is extensively covered by Russell29. The variation with substituents in the low isotope effects for the reaction of aryl radical with arene thiols were explained using such an effect. We may possibly further account for the lower intrinsic barrier for the R-H-Cl system (3.8 Kcal) than for the R-H—S system (5 Kcal) in terms of the greater electronegativity of chlorine. 

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