Determination of Relative Reactivities

Determination of Relative Reactivity by the Method of Competitive Reactions... 

The determination of relative reactivities of different substrates is a crucial requisite for both Hammett and KIE studies. Usually, this is done either by measuring... 

A more quantitative approach to the influence of the thiazole ring on the reactivity of a lateral functional chain was made in a recent study by Noyce and Fike (383), already discussed in Section 10.4. The first-order rates of solvolysis for three isomeric 1-thiazolylethyl chlorides were determined in 80% ethanol. The order of relative reactivity observed. 

Sulfonate Esters. Sucrose sulfonates are valuable intermediates for the synthesis of epoxides and derivatives containing halogens, nitrogen, and sulfur. In addition, the sulfonation reaction has been used to determine the relative reactivity of the hydroxyl groups in sucrose. The general order of reactivity in sucrose toward the esterification reaction is OH-6 OH-6 > OH-1 > HO-2. 

Because it s much easier to measure the acidity of a substituted benzoic acid than it is to determine the relative reactivity of an aromatic ring toward electrophilic substitution, the correlation between the two effects is useful for predicting reactivity. If we want to know the effect of a certain substituent on electrophilic reactivity, we can simply find the acidity of the corresponding benzoic acid. Worked Example 20.1 gives an example. 

Frequency factors for addition of small radicals to monomers are higher by more than an order of magnitude than those for propagation (Table 4.12). Activation energies are typically lower. However, trends in the data are very similar suggesting that the same factors are important in determining the relative reactivities for both small radicals and propagating species. The same appears to be true with respect to reactivities in copolymerization (Section 73.1.2)/88... 

Evenson551 used aqueous trifluoroacetic acid to determine some relative reactivities in dedeuteration as follows benzcyclobutene < o-xylene < indane < tetralin > benzsuberone > benzcyclo-octene. The relative reactivities of the... 

The method of determining reactivities was such that rate coefficients were not obtained, but a range of relative reactivities were (Table 261 )670,684,742. The... 

Elegant methods for determining the relative reactivity of transient intermediates have been developed.6 Consider the reactions of benzyne with water and hydroxide ions ... 

In summary, the order of reactivity for the most commonly used ruthenium-based metathesis catalysts was found to be 56d>56c>9=7. This order of reactivity is based on IR thermography [39], determination of relative rate constants for the test reaction 58—>59 (Eq. 8) [40], and determination of turnover numbers for the self metathesis of methyl-10-undecenoate [43]. 

We select repeat units at random, and if they are unreduced V units a check of whether or not the units adjacent to the selected unit are E or V is made. Having determined the triad structure (both comonomer and stereosequence) of the repeat unit selected for reduction, we divide the relative reactivity of this E-V triad by the sum of relative reactivities for all V centered E-V triads as listed in Table III to obtain the probability of reduction. A random number between 0.0 and 1.0 is generated, and if it is smaller than the probability of reduction of the selected E-V triad, we remove the chlorine from the central V unit which becomes an E unit. 

Only limited success was achieved in determining the relative reactivity of primary, secondary, and tertiary carbon-hydrogen bonds to sulphonyl nitrenes 8>. Insertion of p-toluenesulphonyl nitrene into 2-methylbutane gave a mixture of products which could not be completely resolved. The ratio of (primary) (secondary + tertiary) = [38 + 39 40 + 41] was 1.53, compared to a ratio of 5.6 for carbethoxynitrene58>, indicating the lowered selectivity of the sulphonyl nitrene relative to the carbethoxynitrene, as might be expected from the possible resonance stabilization of the latter species. 

Electrophilic substitution in furan, thiophene, selenophene and pyrrole has, up to 1970, been comprehensively reviewed by Marino.66 Italian workers have determined the relative reactivities of selenophene and thiophene as well67 relative rates are given in Table I. Including furan, the order of reactivity is furan > selenophene > thiophene. 

Isodesmic reactions 2 and 3 were calculated as a measure of the relative ease of formation of the aza-epoxides, in order to obtain some insight as to the effect of nitrogen atom on metabolic activation of aza-PAHs. It was presumed that formation of the epoxides is not as relevant as carbocation formation in determining the relative reactivities of Phe, BhQ and BfQ, because almost no energy differences were observed for the isodesmic reactions involving these compounds. 

This comparison of reactivity can be completed by analyzing cooxidation curves further, as described in the next section this resulting in directly determining the relative reactivity toward abstraction by some free radicals. 

Figure 7.4. Interactions which determine the relative reactivities of carboxyalkyl (left) and acyloxy radicals.

Copolymers are readily prepared by conducting polymerizations of a mixture of monomers. However, to obtain a product having any reasonable, structural homogeneity, it is necessary to take the reaction mechanism into account, and to perform the experiment under conditions consistent with classical, copolymerization theory. With properly controlled experiments, it is possible to determine the relative reactivities of the monomers, and the range of compositions and mer sequence-length distributions in any copolymer produced.81,82... 

The hydrogenation of dienes to monoenes introduces several problems of selectivity. Regioselective saturation of one of the double bonds is governed basically by the same effects that determine the relative reactivities of monoalkenes in a binary mixture that is, a terminal double bond is reduced preferentially to other, more substituted double bonds. During the reduction of a diene, a new competition also emerges since the newly formed monoene and the unreacted diene compete for... 

These results, obtained with chiral substrates, agree with the general sense of enantioselective hydrogenation of prochiral 3-oxo carboxylic esters. Obviously, the chirality of the BINAP ligand controls the facial selectivity at the carbonyl function, whereas cyclic constraints determine the relative reactivities of the enantiomeric substrates. Sterically restricted transition states that lead to the major stereoisomers are shown in Scheme 66. Overall, one of four possible diastereomeric transition states is selected to afford high stereoselectivity by dynamic kinetic resolution that involves in situ racemization of the substrates. 

Exercise 22-30 a. In the nitration of para-cymene by ethanoyl nitrate in ethanoic anhydride, the observed product composition at 0° is 41 % 5 and 6, 41 % 3, 8% 4, and 10% of 4-nitromethylbenzene. Use these results to determine the relative reactivities of the para-cymene ring carbons towards N02 . Give your answer relative to C3 as unity (C3 is the carbon next to the isopropyl group). Determine the relative reactivities based on the data obtained in Equation 22-1. How does neglect of ipso substitution affect calculation of relative reactivities of the ring carbons b. Write a mechanism for the solvolytic conversion of 5 and 6 to 3. 

By examining the rates of intramolecular nucleophilic cyclization of 13 at different pH values, Kirby has determined the relative reactivity of RCH=CH2, RCH=CHCOO and RCH=CHCOOH towards nucleophilic substitution14. The ratios 1 4000 8 x 107 are in broad agreement with the relative rates, 3 x 104 1, for nucleophilic attack of ammonia on fumaric acid in the neutral and dianionic forms at 135 °C. The relative rates for nucleophilic attack of hydroxide on fumaric acid in the neutral and dianionic forms at 135 °C changed to 3 x 107 1 because of the adverse electrostatic interaction between the hydroxide ion and the dianion15. 

Competition experiments have been carried out to determine the relative reactivities of 23 alkyl chlorides toward allyltrimethylsilane in the presence of ZnCh. The kre scale has been found to span 11 orders of magnitude from the least reactive 1-adamantyl chloride to the most reactive bis(p-methoxyphenyl)methyl chloride384. By contrast, analogous acetals RCH(OMe)2 exhibited very little differences in reactivity385. 

These results point to an interesting interplay of energy and entropy terms in determining the relative reactivities. Further, as Dunbar and Hammett (1950) note, they support the proposal that decreased reactivities accompanying a- or /3-methylation are primarily due to increased energy barriers, presumably caused by steric effects. 

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