Rate data, relative

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... 

These relative rate data per position are experimentally determined and are known as partial rate factors They offer a convenient way to express substituent effects m elec trophilic aromatic substitution reactions... 

The numbers computed usiag this approach are only as good as the failure rate data for the specific equipment. Frequendy, failure rate data are difficult to acquire. For this case, the numbers computed only have relative value, that is, they are useful for determining which configuration shows iacreased reUabiUty. 

The reactivity sequence furan > tellurophene > selenophene > thiophene is thus the same for all three reactions and is in the reverse order of the aromaticities of the ring systems assessed by a number of different criteria. The relative rate for the trifluoroacetylation of pyrrole is 5.3 x lo . It is interesting to note that AT-methylpyrrole is approximately twice as reactive to trifluoroacetylation as pyrrole itself. The enhanced reactivity of pyrrole compared with the other monocyclic systems is also demonstrated by the relative rates of bromination of the 2-methoxycarbonyl derivatives, which gave the reactivity sequence pyrrole>furan > selenophene > thiophene, and by the rate data on the reaction of the iron tricarbonyl-complexed carbocation [C6H7Fe(CO)3] (35) with a further selection of heteroaromatic substrates (Scheme 5). The comparative rates of reaction from this substitution were 2-methylindole == AT-methylindole>indole > pyrrole > furan > thiophene (73CC540). 

Rate data are also available for the solvolysis of l-(2-heteroaryl)ethyl acetates in aqueous ethanol. Side-chain reactions such as this, in which a delocalizable positive charge is developed in the transition state, are frequently regarded as analogous to electrophilic aromatic substitution reactions. In solvolysis the relative order of reactivity is tellurienyl> furyl > selenienyl > thienyl whereas in electrophilic substitutions the reactivity sequence is furan > tellurophene > selenophene > thiophene. This discrepancy has been explained in terms of different charge distributions in the transition states of these two classes of reaction (77AHC(21)119>. 

Where the resistance of the precoat bed is significant in comparison to the resistance of the deposited solids, the thickness of the precoat bed effec tively controls the filtration rate. In some instances, the resistance of the deposited solids is veiy large with respec t to even a thick precoat bed. In this case, variations in thickness through the life of the precoat bed have relatively little effec t on filtration rate. This type of information readily becomes apparent when the filtration rate data are correlated. 

Cleavage occurs with acid. The following tables give relative rate data that are useful for comparing other, more commonly employed, derivatives of phenylalanine (Phe). 

The reaction is second-order overall, with the rate given by A [R2C=0][NaBH4]. The interpretation of the rate data is complicated slightly by the fact that the alkoxyborohy-drides produced by the first addition can also fimction as reducing agents, but this has little apparent effect on the relative reactivity of the carbonyl compoimds. Table 8.3 presents some of the rate data obtained from these studies. 

Reductions by NaBKt are characterized by low enthalpies of activation (8-13kcal/mol) and large negative entropies of activation (—28 to —40eu). Aldehydes are substantially more reactive than ketones, as can be seen by comparison of the rate data for benzaldehyde and acetophenone. This relative reactivity is characteristic of nearly all carbonyl addition reactions. The reduced reactivity of ketones is attributed primarily to steric effects. Not only does the additional substituent increase the steric restrictions to approach of the nucleophile, but it also causes larger steric interaction in the tetrahedral product as the hybridization changes from trigonal to tetrahedral. 

There are relatively few kinetic data on the Friedel-Crafts reaction. Alkylation of benzene or toluene with methyl bromide or ethyl bromide with gallium bromide as catalyst is first-order in each reactant and in catalyst. With aluminum bromide as catalyst, the rate of reaction changes with time, apparently because of heterogeneity of the reaction mixture. The initial rate data fit the kinetic expression ... 

Absolute rate data for Friedel-Crafts reactions are difficult to obtain. The reaction is complicated by sensitivity to moisture and heterogeneity. For this reason, most of the structure-reactivity trends have been developed using competitive methods, rather than by direct measurements. Relative rates are established by allowing the electrophile to compete for an excess of the two reagents. The product ratio establishes the relative reactivity. These studies reveal low substrate and position selectivity. 

All free energies are in kilocalories per mole on the mole fraction scale, relative to DMF. See Table 8-5 for rate data. 

In this subsection the relative rate data are surveyed to give a general picture of aza-activation in its various aspects. 

Figures 62.8, 62.9, 62.10 show the data for generator fan failure plotted on exponential, normal and log normal hazard paper respectively. The exponential plot is a reasonably straight line which indicates that the failure rate is relatively constant over the range of the data. It should be noted that the reason the probability scale on the exponential hazard plot is crossed out is because that is not the proper way to plot data. (This will be discussed later.) The normal plot is curved concave upward which...

The development of methods for the kinetic measurement of heterogeneous catalytic reactions has enabled workers to obtain rate data of a great number of reactions [for a review, see (1, )]. The use of a statistical treatment of kinetic data and of computers [cf. (3-7) ] renders it possible to estimate objectively the suitability of kinetic models as well as to determine relatively accurate values of the constants of rate equations. Nevertheless, even these improvements allow the interpretation of kinetic results from the point of view of reaction mechanisms only within certain limits ... 

The above mentioned studies were in most cases performed with the aim of obtaining relative reactivities or relative adsorption coefficients from competitive data, sometimes also from the combination of these with the data obtained for single reactions. In our investigation of reesterification (97,98), however, a separate analysis of rate data on several reactions provided us with absolute values of rate constants and adsorption coefficients (Table VI). This enabled us to compare the relative reactivities evaluated by means of separately obtained constants with the relative reactivities measured by the method of competitive reactions. The latter were obtained both from integral data by means of the known relation... 

There is an excellent, if non critical, compilation of absolute and relative rate data for reactions of oxygen-centered radicals covering the literature through 1982389 and for 1982-1992.39 1 Selected data from these and other sources are summarized in Table 3.7 and Table 3.8. The reactions of oxygen-centered radicals and their use in organic synthesis has been recently reviewed by Hartung el uIS )]... 

The vast majority of the kinetic detail is presented in tabular form. Amassing of data in this way has revealed a number of errors, to which attention is drawn, and also demonstrated the need for the expression of the rate data in common units. Accordingly, all units of rate coefficients in this section have been converted to mole.l-1.sec-1 for zeroth-order coefficients (k0), sec-1 for first-order coefficients (kt), l.mole-1.sec-1 for second-order coefficients (k2), l2.mole-2.sec-1 for third-order coefficients (fc3), etc., and consequently no further reference to units is made. Likewise, energies and enthalpies of activation are all in kcal. mole-1, and entropies of activation are in cal.deg-1mole-1. Where these latter parameters have been obtained over a temperature range which precludes the accuracy favoured by the authors, attention has been drawn to this and also to a few papers, mainly early ones, in which the units of the rate coefficients (and even the reaction orders) cannot be ascertained. In cases where a number of measurements have been made under the same conditions by the same workers, the average values of the observed rate coefficients are quoted. In many reactions much of the kinetic data has been obtained under competitive conditions such that rate coefficients are not available in these cases the relative reactivities (usually relative to benzene) are quoted. 

As expected initial examination of the hydrogenation of this substrate revealed its relatively low activity compared to dehydroamino acids that provide 3-aryl-a-amino acids. By carrying out the hydrogenation at an elevated temperature, however, the inherent low activity could be overcome. A screen of the Dowpharma catalyst collection at S/C 100 revealed that several rhodium catalysts provided good conversion and enantioselectivity while low activity and selectivity was observed with several ruthenium and iridium catalysts. Examination of rate data identified [(l )-PhanePhos Rh (COD)]Bp4 as the most active catalyst with a rate approximately... 

The reactivity of different alkenes toward mercuration spans a considerable range and is governed by a combination of steric and electronic factors.24 Terminal double bonds are more reactive than internal ones. Disubstituted terminal alkenes, however, are more reactive than monosubstituted cases, as would be expected for electrophilic attack. (See Part A, Table 5.6 for comparative rate data.) The differences in relative reactivities are large enough that selectivity can be achieved with certain dienes. 

So far as the overall substitution reaction (— 107) is concerned, marked differences from electrophilic and nucleophilic attack become apparent as soon as the behaviour of substituted benzene derivatives (C6HjY) is considered. Thus homolytic attack on C6H5Y is found to be faster than on C6H6, no matter whether Y is electron-attracting or -withdrawing, as shown by the relative rate data for attack by Ph ... 

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