Arrhenius parameters compounds

Further problems arise if measurements of the rate of nitration have been made at temperatures other than 25 °C under these circumstances two procedures are feasible. The first is discussed in 8.2.2 below. In the second the rate profile for the compound imder investigation is corrected to 25 °C by use of the Arrhenius parameters, and then further corrected for protonation to give the calculated value of logio/i fb. at 25 °C, and thus the calculated rate profile for the free base at 25 °C. The obvious disadvantage is the inaccuracy which arises from the Arrhenius extrapolation, and the fact that, as mentioned above, it is not always known which acidity functions are appropriate. 

The case of i-methyl-4-quinolone is puzzling. The large proportion of the 3-nitro isomer formed in the nitration (table 10.3 cf. 4-hydroxyquinoline) might be a result of nitration via the free base but this is not substantiated by the acidity dependence of the rate of nitration or by the Arrhenius parameters. From r-methyl-4-quinolone the total yield of nitro-compounds was not high (table ro.3). 

Some workers in this field have used Eyring s equation, relating first-order reaction rates to the activation energy d(7, whereas others have used the Arrhenius parameter E. The re.sults obtained are quite consistent with each other (ef. ref. 33) in all the substituted compounds listed above, AG is about 14 keal/mole (for the 4,7-dibromo compound an E value of 6 + 2 keal/mole has been reported, but this appears to be erroneous ). A correlation of E values with size of substituents in the 4- and 7-positions has been suggested. A/S values (derived from the Arrhenius preexponential factor) are... 

The conclusion that the nitration of quinoline in sulphuric acid takes place via the conjugate acid has been confirmed by Moodie et al.50, who measured the rates of nitration of a wide range of heterocyclic compounds in nitric acid-sulphuric acid mixtures at a range of temperatures. A summary of the second-order rate coefficients and Arrhenius parameters is given in Table 4. From an analysis of the shapes of the plots of log k2 versus sulphuric acid acidity (or some function of this), it was concluded that all of the compounds starred in Table 4... 

Arrhenius parameters for nitration of 4-aikylphenyltrimethyiammonium ions in nitric acid-sulphuric acid mixtures (Table 12). It was argued that the observed Baker-Nathan order of alkyl substituent effect was, in fact, the result of a steric effect superimposed upon an inductive order. However, a number of assumptions were involved in this deduction, and these render the conclusion less reliable than one would like it would be useful to have the thermodynamic parameters for nitration of the methyl substituted compound in particular, in order to compare with the data for the /-butyl compound, though experimental difficulties may preclude this. It would not be surprising if a true Baker-Nathan order were observed because it is observed for all other electrophilic substitutions in this medium1. 

Contrary to the results obtained with carbon tetrachloride solvent and entirely in accord with the postulate that the effect arises from the steric hindrance to solvation, the rates of cleavage of ArSnR3 compounds in methanol decrease on increasing the size of the group R. This is shown by the rate coefficients in Table 265, though it is difficult to draw any conclusion from the Arrhenius parameters... 

The Arrhenius parameters and the thermochemical sum of the phenyl-carbon and phenyl-halogen bond dissociation energies are shown in Table 8. The extent of the diphenyl mercury decomposition was determined from the weight of mercury produced. It is the present author s opinion that in calculating the Arrhenius parameters for this compound Carter et al.81 gave too great a statistical... 

The values of these Arrhenius parameters contrast dramatically with those obtained for the bicyclo[2,2,0]hexane isomerization. In this compound there is no weak bridgehead bond, and hence the reaction path is more closely akin to that for cyclobutane itself. The similarity of the A factors for this reaction and that for other simple cyclobutanes supports this contention. If this is so, then the lowering of the energy of activation in this bicyclic compound by some 7 kcal mole from that observed in the alkylcyclobutanes is to be attributed to extra strain energy in this molecule. 

Rate constants and Arrhenius parameters for the reaction of Et3Si radicals with various carbonyl compounds are available. Some data are collected in Table 5.2 [49]. The ease of addition of EtsSi radicals was found to decrease in the order 1,4-benzoquinone > cyclic diaryl ketones, benzaldehyde, benzil, perfluoro propionic anhydride > benzophenone alkyl aryl ketone, alkyl aldehyde > oxalate > benzoate, trifluoroacetate, anhydride > cyclic dialkyl ketone > acyclic dialkyl ketone > formate > acetate [49,50]. This order of reactivity was rationalized in terms of bond energy differences, stabilization of the radical formed, polar effects, and steric factors. Thus, a phenyl or acyl group adjacent to the carbonyl will stabilize the radical adduct whereas a perfluoroalkyl or acyloxy group next to the carbonyl moiety will enhance the contribution given by the canonical structure with a charge separation to the transition state (Equation 5.24). 

An example of the application of this test to a compound that nitrates as its free base is provided by pyridine 1-oxide. Under an identical set of conditions, nitration of this N-oxide had a half life of 20 min, whilst 1-methoxypyridinium gave no nitro compound in 144 h. Two further criteria have been used to provide confirmatory evidence, namely comparison of the rate of nitration for the reactive species with the encounter controlled rate, and by determination of the Arrhenius parameters. 

In conclusion, if one is to work in the field of radical oxidation of hydrocarbons or organic compounds, he must remember that there are strong constraints on the mechanisms which we may write. Arrhenius parameters are no longer mysterious numbers but can be related with... 

The first set of substrates studied were compounds of the type Me3SiCH2C6H4X Eaborn and Parker23 obtained a value of 0.48 x 10 6 l.mole-1.sec-1 for k2 for the parent compound (X = H) at 49.7 °C in the solvent 39 wt.% water-methanol. The Arrhenius parameters were Ea = 29.5 kcal.mole-1 and A = 4.5x10 3 l.mole-1.sec 1. A number of substrates with substituents in the benzyl group were examined, and it was shown that electron-withdrawing substituents markedly aided reaction23. 

For compounds like TCA, for which both high temperature and room temperature hydrolysis rates have been measured and contaminant plumes have been observed over a considerable length of time, extrapolation using the Arrhenius parameters appears valid. In addition, apparent augmentation of reactivity in real environmental situations often can be rationalized in terms of additional reaction pathways, so that it is not necessary to invoke the non-validity of the extrapolation process. 

The accurate determination of rate constants for the reactions of 19F atoms is often hampered by the presence of reactive F2 and by the occurrence of side reactions. The measurement of the absolute concentration of F atoms is sometimes a further problem. The use of thermal-ized 18F atoms is not subject to these handicaps, and reliable and accurate results for abstraction and addition reactions are obtained. The studies of the reactions of 18F atoms with organometallic compounds are unique, inasmuch as such experiments have not been performed with 19F atoms. In the case of addition reactions, the fate of the excited intermediate radical can be studied by pressure-dependent measurements. The non-RRKM behavior of tetraallyltin and -germanium compounds is very interesting inasmuch as not many other examples are known. The next phase in the 18F experiment should be the determination of Arrhenius parameters for selected reactions, i.e., those occurring in the earth s atmosphere, since it is expected that the results will be more precise than those obtained with 19F atoms. 

Before the first muonic radicals were observed in 1978 by Roduner et al. (122), rate constants had been measured for the addition of Mu to unsaturated compounds in aqueous solution (Table XI). If k Jku is larger than 3, tunneling may be important, since otherwise differences in the vibrational zero-point energy in the transition state can decrease this ratio (128). In the case of maleic acid, the Arrhenius parameters were determined as A = (2.3 0.2) x 1013 AT-1 sec-1 and E = 18.8 1.7 kJ mol-1. Rate constants were also measured for addition to the CN triple bond (127). 

Under conditions of maximal inhibition the decomposition behaviour often closely resembles that which is characteristic of a unimolecular reaction. For example Agius and MaccolP have shown that thermal decomposition of bromopropane alone has an order of 1.5, but when maximally inhibited by propene the decomposition becomes first-order and the Arrhenius parameters are A = 7.94 x 10 sec E - 50.7 kcal.mole It has been argued that, in such cases, decomposition in the absence of inhibitor proceeds by simultaneous radical-chain and unimolecular processes and that under conditions of maximal inhibition, the radical-chain process is suppressed and the residual unimolecular process is predominant. Wojciechowski and Laidler have argued against this proposition, maintaining that the maximally inhibited reaction may still represent a residual chain process, and that the lack of inhibitor action for compounds in Class 1 cannot be taken as an indication that the mechanism is molecular. (See also the comments of Benson and Bose .)... 

BROMINE COMPOUNDS, UNIMOLECULAR DBHYDROBROMINATIONS ARRHENIUS PARAMETERS... 

Support for the c/j-nature of the elimination reaction has come from the work of Barton et on the pyrolysis of menthyl chloride, and the results of this study have recently been confirmed by Bamkole > who also examined the pyrolysis of neomenthyl chloride. The product ratio of menthene-3 to methene-2 is 3 1 in the case of menthyl chloride and 1 6 in the case of neomenthyl chloride, thus demonstrating a preference for m-elimination in each case. These two decompositions do, however, have some unusual characteristics the Arrhenius parameters are considerably lower than those reported for other secondary chlorides, and the rate of elimination of hydrogen chloride from each compound is appreciably faster than from cyclohexyl chloride . (The relative rate of pyrolysis of menthyl chloride and cyclohexyl chloride at 300 °C is about fifty.)... 

CHLORINE AND BROMINE COMPOUNDS. ARRHENIUS PARAMETERS FROM TOLUENE VAPOUR TECHNIQUE FOR REACTION RXR+X... 

The toluene vapour-flow method has been used for estimating the rate of homol-ytic rupture of the carbon-iodine bond in some organic iodides. Yang and Con-way have reported the following Arrhenius parameters for the case of ethyl iodide A = 4.5 x 10 see , E = 50.0 kcal.mole Shilov and Sabirova have reported the values A = 2.l4x 10 " sec E = 51.6 kcal.mole for the same compound. For allyl iodide, Shilov and Sabirova have reported A = 4.2x 10 sec , E = 66.5 kcal.mole . Errors in the use of this technique for iodides have been pointed out by Klemm and Bernstein and by Boyd et al. 

In a system where acetone is photolyzed in the presence of a hydrogen containing compound (RH), a competition exists between the acetone and the RH molecules for the methyl radicals. The Arrhenius parameters of several hydrogen transfer reactions of the methyl radical have been determined in such systems ... 

Early kinetic experiments on the thermal decomposition of nitro compounds established that for the simplest derivative, nitromethane, the process was first order, but that the reaction was chemically complex owing to further reactions between the products and nitromethane. Cottrell et re-examined the nitromethane pyrolysis and reported values of = 53.2 kcal.mole" and log A = 13 for the Arrhenius parameters of the homogeneous decomposition a radical mechanism was proposed, initiated by C-N cleavage... 

Absolute values of kj were obtained from the known parameters Ajp and Ejp (see Table 1.8). Table 1.7 summarizes the recommended Arrhenius parameters for the temperature range 600-800 K for a number of compounds RH used as additives in the C3H6 -I- O2 system. For alkenes, bearing in mind that only allylic C—H bonds undergo reaction (1) under the conditions used, Aip = Ai (per C—H bond) is assumed to obtain the activation energies. 

Kinetic data for H + CH4 and H + QHg have been critically analyzed [9, 50] and Tsang has reviewed H + CsHg [56] and H + isobutane [57]. Kerr and Moss [49] list uncritically Arrhenius parameters for a wide range of alkanes and related compounds. 

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