Relative rate comparisons

Swain, C. G., and C. B. Scott, Quantitative correlation of relative rates. Comparison of hydroxide ion with other nucleophilic reagents toward alkyl halides, esters, epoxides, and acyl halides , J. Am. Chem. Soc., 75, 141-147 (1953). 

Substituent effects in the allyl ester rearrangements are very similar to those observed in the ester reverse ene-type eliminations. This is apparent from the relative rate comparisons of Table 8. At the a- and y-carbons, reaction rates are observed to increase in the order CF3 < H < CH3. The rate accelerations by methyl substitution for hydrogen at the a-carbons are factors of 40 and 23, and at the y-carbon are factors of 55 and 23. These effects should be compared with the rate accelerations by methyl for hydrogen substitution at the a-carbon in the ester ene reactions, i.e., from Table 2, i-PrOAc/EtOAc = 18.7 and t-BuOAc/i-PrOAc = 53. One may conclude that the positive formal charge densities at the a- and... 

C.H. Rochester, Acidity Functions, Academic Press, New York, 1970. C.G. Swain and C.B. Scott, Quantitative Correlation of Relative Rates. Comparison of Hydroxide Ion with Other Nucleophilic Reagents toward Alkyl Halides, Esters, Epoxides and Acyl Halides, J. Am. Chem. Soc., 1953,75, 141. 

III) Relative Rate Comparisons between Corresponding Electrochemical and Homogeneous Reactions. Providing each reaction is outer sphere, and work terms can be neglected or cancel, the relative rates for electrochemical reduction (or oxida-... 

Homomorphs can also be useful for relative rate comparisons in certain aliphatic reactions. In a search for participation by the remote dimethyl-amino group in compounds such as (13), the corresponding compound with an isopropyl group, i.e., (14), has been used " as a model in order to maintain similar steric requirements (see Chapter 6). 

Quack M 1979 Quantitative comparison between detailed (state selected) relative rate data and averaged (thermal) absolute rate data for complex forming reactions J. Phys. Chem. 83 150-8... 

Unfortunately, insufficient data make it impossible to know whether the activity coefficients of all aromatic compounds vary slightly, or whether certain compounds, or groups of compounds, show unusual behaviour. However, it seems that slight variations in relative rates might arise from these differences, and that comparisons of reactivity are less sound in relatively concentrated solutions. 

The relative rate is derived from the kinetic data " by stepwise comparison with m-nicrotoluene, chlorobenzene and benzene. Kinetic data are available for the acidity range 8o-o-Q5-6 % sulphuric acid. See also ref. 43. 

Here, and with the chlorotoluenes, the precise values for the calculated figures depend on the values adopted for the partial rate factors in the mono-substituted compounds. These and the relative rates do depend slightly on conditions. As has been pointed out several times previously, comparisons with benzene for nitration in sulphuric acid have to be made with care. 

The greater steric hindrance to acetylation was also shown by a comparison of the rate of (103At2) of acetylation of toluene (0.763), ethylbenzene (0.660), i-propylbenzene (0.606) and f-butylbenzene (0.462) with those (determined by the competition method) for benzoylation both sets of data (Table 112) were obtained with dichloroethane as solvent at 25 °C, all reagent concentrations being 0.1 A/421. Relative rates of acylation other aromatics under the same conditions have also been obtained and are given in Table 113422. The different steric requirements for acetylation and benzoylation are further shown by the following respective relative rates for acylation of naphthalene derivatives in chloroform at 0 °C naphthalene (1 position) 1.00,1.00, (2 position) 0.31,0.04 2,3-dimethylnaphthalene (1 position) 1.59, 172, (5 position) 7.14, 38.2, (6 position) 3.68, 7.7422a. 

Comparisons of relative rate constants obtained with Mv s of the total polymer and M s of the HMWF for the same samples show similar trends negligible transfer and termination control of molecular weights for the f-BuCl/Et2AlCl/MeCl system in the —40° to —60 °C range and also for the f-BuBr/Et2AlCl/MeCl at —50 °C (Table 7). For the samples prepared with the f-BuCl/Et AlCl system Mayo plots based on Mv s show zero intercept while that based on Mn s of the HMWF shows a small but finite intercept, z., ktr/kp = 1.91 x 10-5 and 2.14 x 10-s at —50° and -60 °C. Similarly, for the samples prepared with the t-BuBr/Et2AlCl system the Mayo plot based on Mn s of HMWF shows zero intercept while the Mayo plot based on Mv s show a very small intercept, ie., ktr/kp = 5.0 x 10-s at —50 °C. The reasons for this small discrepancy are not known. 

The relative rates of cycloaddition of 88b-88e were measured in comparison with that of the parent 88a as a reference. The methoxy substituent has practically no effect on the reaction rate. However, it is apparent that electron-withdrawing substituents (88b, 88c and 88e) significantly accelerate the anti-addition, whereas in xyn-addition the acceleration is not as large the rate is comparable to that of the reference compound (88a). In the reactions of the tetrafluoro-substituted dienophile 88d, we found significant rate acceleration on both sides, though anti-side addition was still substantially favored. 

Figure 9. Relative rate of CO hydrogenation as a function of copper coverage on a Ru(OOOl) catalyst Reaction temperature 575K. Results for sulfur poisoning from Figure 7 have been replotted for comparison.

TABLE 2.2 Comparison of Relative Rates of Double Bond Migration on Platinum... 

Absolute rates for the addition of the methyl radical and the trifluoromethyl radical to dienes and a number of smaller alkenes have been collected by Tedder (Table l)3. Comparison of the rate data for the apolai4 methyl radical and the electrophilic trifluoromethyl radical clearly show the electron-rich nature of butadiene in comparison to ethylene or propene. This is also borne out by several studies, in which relative rates have been determined for the reaction of small alkyl radicals with alkenes. An extensive list of relative rates for the reaction of the trifluoromethyl radical has been measured by Pearson and Szwarc5,6. Relative rates have been obtained in these studies by competition with hydrogen... 

Streitwieser and Boerth studied the kinetic acidities of cycloalkenes with lithium cyclo-hexylamide (LiCHA) in cyclohexylamine for comparison with those of benzene and toluene66. The relative rates of deprotonation and the corresponding equilibrium pK values are tabulated in Table 12. These proton transfer transition states are stabilized by conjugation of the reacting C—H bond with the double bond. 

TABLE 5. Comparison of substituent effects on the relative rates of carbenium ion addition to carbon-carbon double bonds... 

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