Competitive rate factors

Dewar and his co-workers, as mentioned above, investigated the reactivities of a number of polycyclic aromatic compounds because such compounds could provide data especially suitable for comparison with theoretical predictions ( 7.2.3). This work was extended to include some compounds related to biphenyl. The results were obtained by successively compounding pairs of results from competitive nitrations to obtain a scale of reactivities relative to that of benzene. Because the compounds studied were very reactive, the concentrations of nitric acid used were relatively small, being o-i8 mol 1 in the comparison of benzene with naphthalene, 5 x io mol 1 when naphthalene and anthanthrene were compared, and 3 x io mol 1 in the experiments with diphenylamine and carbazole. The observed partial rate factors are collected in table 5.3. Use of the competitive method in these experiments makes them of little value as sources of information about the mechanisms of the substitutions which occurred this shortcoming is important because in the experiments fuming nitric acid was used, rather than nitric acid free of nitrous acid, and with the most reactive compounds this leads to a... 

The first quantitative studies of the nitration of quinoline, isoquinoline, and cinnoline were made by Dewar and Maitlis, who measured isomer proportions and also, by competition, the relative rates of nitration of quinoline and isoquinoline (1 24-5). Subsequently, extensive kinetic studies were reported for all three of these heterocycles and their methyl quaternary derivatives (table 10.3). The usual criteria established that over the range 77-99 % sulphuric acid at 25 °C quinoline reacts as its cation (i), and the same is true for isoquinoline in 71-84% sulphuric acid at 25 °C and 67-73 % sulphuric acid at 80 °C ( 8.2 tables 8.1, 8.3). Cinnoline reacts as the 2-cinnolinium cation (nia) in 76-83% sulphuric acid at 80 °C (see table 8.1). All of these cations are strongly deactivated. Approximate partial rate factors of /j = 9-ox io and /g = i-o X io have been estimated for isoquinolinium. The unproto-nated nitrogen atom of the 2-cinnolinium (ina) and 2-methylcinno-linium (iiiA) cations causes them to react 287 and 200 more slowly than the related 2-isoquinolinium (iia) and 2-methylisoquinolinium (iii)... 

Displacement Development A complete prediction of displacement chromatography accounting for rate factors requires a numerical solution since the adsorption equilibrium is nonlinear and intrinsically competitive. When the column efficiency is high, however, useful predictious can be obtained with the local equilibrium theoiy (see Fixed Bed Transitions ). 

Finally, it is customary to compare the partial rate factors obtained under different conditions to indicate the reactivity of the electrophile. Unfortunately, the medium, nitric acid in sulphuric acid, in which the nitronium ion is most clearly established as the electrophile, is such a poor solvent for aromatics that meaningful competitive nitrations are impossible and kinetic studies are hampered by the difficulties noted above. However, since the isomer distribution is a function of the rate factors, inspection of these distributions (Table 15) shows a very... 

Olivier and Berger335, who measured the first-order rate coefficients for the aluminium chloride-catalysed reaction of 4-nitroben2yl chloride with excess aromatic (solvent) at 30 °C and obtained the rate coefficients (lO5/ ) PhCI, 1.40 PhH, 7.50 PhMe, 17.5. These results demonstrated the electrophilic nature of the reaction and also the unselective nature of the electrophile which has been confirmed many times since. That the electrophile in these reactions is not the simple and intuitively expected free carbonium ion was indicated by the observation by Calloway that the reactivity of alkyl halides was in the order RF > RC1 > RBr > RI, which is the reverse of that for acylation by acyl halides336. The low selectivity (and high steric hindrance) of the reaction was further demonstrated by Condon337 who measured the relative rates at 40 °C, by the competition method, of isopropylation of toluene and isopropylbenzene with propene catalyzed by boron trifluoride etherate (or aluminium chloride) these were as follows PhMe, 2.09 (1.10) PhEt, 1.73 (1.81) Ph-iPr, (1.69) Ph-tBu, 1.23 (1.40). The isomer distribution in the reactions337,338 yielded partial rate factors of 2.37 /mMe, 1.80 /pMe, 4.72 /, 0.35 / , 2.2 / Pr, 2.55337 339. 

Gallium bromide was used as the catalyst in a determination346 of the partial rate factors, by the competition method, for ethylation of some substituted benzenes in 1,2-dichloroethane at 25 °C. No direct rate measurements were made and the results, summarised in Table 80, show the low selectivity and high steric hindrance in the reaction. 

The reliability of this work has also been questioned by Brown and Nelson339, who could not get any reaction at all with benzene under the conditions quoted by Szmant and Dudek382. If the benzene value is in error it could account for the low selectivities that were observed. By using the competition technique with a deficiency of paraformaldehyde (but an excess of hydrogen chloride) they obtained a toluene benzene reactivity ratio of 112 and partial rate factors of / Me = 117, /mMc = 4.37, /pM = 430. 

Augood, D.R., Hey, D.H., Williams, G.H. (1953) Homolytic aromatic substitution. Part HI. Ratio of isomerides formed in the phenylation of chlorobenzene. Competitive experiments on the phenylation of p-dichloro benzene and 1,3,5-trichlorobenzene. Partial rate factors of chlorobenzene. J. Chem. Soc. (London), pp. 44—50. 

In competitive studies (73IJS295, 74AHC(16)123) using aprotic diazotization of aniline to generate phenyl radicals, it has been shown that the relative rates of phenylation for furan and thiophene compared to benzene are 11.5 and 2.6, respectively. The partial rate factors... 

The nitration of pyrrole has been investigated under a variety of experimental conditions.313 The reaction with nitric acid in acetic anhydride proceeds smoothly and both spectroscopic and kinetic results indicate that the acetyl nitrate is the effective electrophilic agent. The a. J3 ratio is about 4 for nitration in acetic anhydride over a wide temperature range. By using competitive procedures, the partial rate factors for a- and -positions of the pyrrole ring in the nitration were estimated by these authors to be 1.3 x 105 and 3 X 104, respectively. 

The free radical arylation of thiazole (391) has been performed either by the Gomberg-Bachmann (392) decomposition of aryldiazonium chlorides (119,393), by the thermal decomposition of benzoyl peroxide (394-397) or N-nitrosoacetanilide (398), or by the photolysis of benzoyl peroxide or iodobenzene (398). The three monophenylthiazoles are obtained in the practically constant proportions 2-phenyl, 60% 5-phenyl, 30% 4-phenyl, 10%, giving the order, 2>5>4, of decreasing reactivity of the three positions of thiazole toward phenyl radicals (398). Competition reactions with nitrobenzene (397) gave an estimation of the globed reactivity of thiazole relative to benzene of 0.75 with the partial rate factors f2 = 2.2, /s=1.9, /4 = 0.5. When the thermolysis of benzoyl peroxide is performed in acetic acid solution, the substrate in reaction is the conjugate acid of thiazole the global reactivity is enhanced to 1.25,... 

Equimolar mixtures of toluene and benzene were passed over beds of DHY at low temperatures (25-60°) in experiments where the two aromatics of different reactivity competed for the electrophilic deuterium (75). The distribution of deuterium between toluene and benzene (apparent ACeHsCHs/fcCeHe) and among the ring positions of the toluene-di samples was determined. A plot of log pt vs selectivity factor (St) for these data from the competitive experiments at 25-60° (Fig. 20, black circles) falls on the line obtained from a study of 47 electrophilic substitution reactions by H. C. Brown and associates (83). The partial rate factors pt and mt give the rate of substitution of the para position and one of the meta positions in toluene, relative to the rate of substitution of one of the six equivalent ring positions of benzene. Points a, b, c, d, and e fall quite close to the line, which represents a linear free energy relationship in both positional and substrate selectivity. 

In 1961, Olah, Kuhn, and Flood reported that the nitration reactions with nltronium salts such as nltronlum tetrafluoro-borate and hexafluorophosphate were quite anomalous. When benzene and toluene were reacted competitively with these nltronlum salts in sulfolane, analysis of the products revealed that toluene was only about 1.5-fold more reactive than benzene. Indeed, neither m-xylene nor mesltylene were Importantly more reactive than benzene under these conditions. The partial rate factors... 

The partial rate factor concept suggests that there is a unique rate constant for reaction at each position of a benzene derivative. Therefore, competitive reactions at different sites within one molecule may be described in the same way as are competitive reactions at sites in different molecules. In a comparison of the ortho and para sites of toluene with the meta position, it is useful to compare the energies of the tr-intermediates for ortho and para reactions to that for meta substitution. Lewis structures for the intermediates formed by reaction of an electrophile with toluene are shown in Figure 8.49. In the case of ortho and para substitution, one of the three resonance structures for the intermediate can be described as a 3 carbocation, while all the contributing resonance structures for the intermediate in meta substitution are 2° carbocat-ions. Based on the Hammond postulate, we expect a lower activation energy for formation of a more stable intermediate, so the rate constants for formation of the ortho and para substitution products should be greater than the rate constant for formation of the meta product. 

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In 1964, Corey and co-workers determined relative reactivities for competitive additions of photoexcited CH to alkenes by measuring the ratio of adducts derived from 1 1 alkene mixtures. The so-called relative rate factors, correcting for statistical factors, were 4.66 for 1,1-dimethxyethene, 1.57 for meth-oxyethene, 1.00 for cyclopentene (reference compound), 0.40 for isobutene, and 0.234 for allene acrylonitrile was much less reactive than any of the other alkenes listed. Based upon these data, Corey concluded that photoexcited CH was a moderately electrophilic species toward alkenes. 

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