Reactivity ratio activation energy

Constant for determining reactivity ratios Activation energy (J/mol)... 

The same arguments can be applied to other energetically facile interconversions of two potential reactants. For example, many organic molecules undergo rapid proton shifts (tautomerism), and the chemical reactivity of the two isomers may be quite different It is not valid, however, to deduce the ratio of two tautomers on the basis of subsequent reactions that have activation energies greater than that of the tautomerism. Just as in the case of conformational isomerism, the ratio of products formed in subsequent reactions will not be controlled by the position of the facile equilibrium. 

It may be unsafe to carry this discussion further until more data are available. Knowledge of the activation parameters would be especially desirable in several respects. Reactivity orders involving different reagents or substrates may be markedly dependent on temperature. Thus, in Table IV both 2- and 4-chloroquinolines appear to be about equally reactive toward sodium methoxide at 86,5°. However, the activation energies differ by 3 kcal/mole (see Section VII), and the relative rates are reversed below and above that temperature. Clearly, such relative rates affect the rs-/ ro- ratios. 

Relative reactivity wiU vary with the temperature chosen for comparison unless the temperature coefficients are identical. For example, the rate ratio of ethoxy-dechlorination of 4-chloro- vs. 2-chloro-pyridine is 2.9 at the experimental temperature (120°) but is 40 at the reference temperature (20°) used for comparing the calculated values. The ratio of the rate of reaction of 2-chloro-pyridine with ethoxide ion to that of its reaction with 2-chloronitro-benzene is 35 at 90° and 90 at 20°. The activation energy determines the temperature coefficient which is the slope of the line relating the reaction rate and teniperature. Comparisons of reactivity will of course vary with temperature if the activation energies are different and the lines are not parallel. The increase in the reaction rate with temperature will be greater the higher the activation energy. 

In the comparison of 285, 286, and 287 the lower energy of activation for the para azine-nitrogen compounds 285 and 287 was responsible for their more rapid reaction. Both ratios of rates are about the same, the reactivity being greater for the azine-nitrogen compound in each case. Another relationship of moieties in the ortho-position is derived from comparison of 290 and 288, the rate difference being entirely due to the relationship of the activation energies. This ratio is essentially the same as that for 283 and 284, which involved both the entropy and... 

Table XIV, line 3). The rates are equal (only at 20°) due to a large, compensating difference between the entropies of activation. In piperidino-dechlorination, 4-chloroquinoline (Table XI, line 3) has a higher and a lower rate (by about 200-fold at 20°) than 1-chloroisoquinoline (Table XIV, line 1). This reversal of reactivity and of the relationship of the activation energies is attributed to the factors in amination reactions mentioned above. The relative reactivity of the chloro groups in 2,4-dichloroquinoline with methanolic methoxide is given as a 2 1 rate ratio of 4- to 2-displacement. 

The effect of temperature on the monomer reactivity ratio is fairly small. In those few cases examined with sufficient accuracy,the ratio nearly always changes toward unity as the temperature increases —a clear indication that a difference in activation energy is responsible, in part at least, for the difference in rate of the competing reactions. In fact, the difference in energy of activation seems to be the dominant factor in these reactions differences in entropy of activation usually are small, which suggests that steric effects ordinarily are of minor importance only. 

The effect of temperature on r is not large, since activation energies for radical propagation are relatively small and, more significantly, fall in a narrow range such that En Eu is less than 10 kJ mol-1 for most pairs of monomers. However, temperature does have an effect, since E 2 — E is not zero. An increase in temperature results in a less selective copolymerization as the two monomer reactivity ratios of a comonomer pair each tend toward unity with decreasing preference of either radical for either monomer. Temperature has the greatest... 

The basis of the scheme developed particularly by Alfrey and Price is the assumption that the activation energies of the propagation reactions, and hence the related rate constants and reactivity ratios, are governed primarily by resonance effects and by the interaction of the charges on the double bonds of the monomers with those in the active radicals. Accordingly, the rate constant of the reaction between a radical and a monomer is represented by ... 

Finally, Arrhenius treatments of the catalytic data were examined for the HTAD synthesized substitutional series, Bi(2-2x) 2x 030i2, and the binary bismuth molybdate series where Bi/Mo ratios were varied fi-om pure Mo oxide to pure Bi oxide. The noteworthy aspect of the oxidation results is that in the most reactive regime of x = 0-5% atom fi-action Fe, before separate phase Fe3Mo30j2 begins to dominate the catalyst composition in the iron series, the apparent activation energies were all in the range of 19-20 kcal/mol. Furthermore, the activation energies for the pure Bi-Mo series were between 19-20 kcal/mol while the activities were considerable different. Thus, the chief difference in the reactivities in both series is in the preexponential factor, i.e. the number of active sites. 

We would have thought the same thing after some studies of the competition between chloride, bromide, and iodide for some rhodium(III) complexes, because we found that the ratios of the rate constants at 50°C. were very nearly 1 1 1. However, when activation energies were measured, we found that these produce a much bigger discrimination. In fact, the activation energy for the addition of iodide to our reactive intermediates is no less than 6 kcal. greater than the activation energy for addition of chloride and bromide. 

Computational studies showed that the nature of the reactive species in the oxidation of trimethylamine, iodide ion, and dimethyl sulfide with lumiflavin is a C4 a-hydroperoxide complexed with water. The other two species, C4 a-hydroperoxide and C4 a-peroxide, yielded higher activation energies.237 Kinetic and spectroscopic studies on the effect of basic solvents, ethers, esters, and amides, on the oxidation of thianthrene-5-oxide with substituted peroxybenzoic acids indicated the involvement of the basic solvent in the transition state of the reactions. A solvent parameter, Xtc, based on the ratio of the trans to the cis form of thianthrene-5,10-dioxide, has been introduced.238... 

In order to estimate the kinetic parameters for the addition and condensation reactions, the procedure proposed in [11, 14] has been used, where the rate constant kc of each reaction at a fixed temperature of 80°C is computed by referring it to the rate constant k° at 80°C of a reference reaction, experimentally obtained. The ratio kc/k°, assumed to be temperature independent, can be computed by applying suitable correction coefficients, which take into account the different reactivity of the -ortho and -para positions of the phenol ring, the different reactivity due to the presence or absence of methylol groups and a frequency factor. In detail, the values in [11] for the resin RT84, obtained in the presence of an alkaline catalyst and with an initial molar ratio phenol/formaldehyde of 1 1.8, have been adopted. Once the rate constants at 80°C and the activation energies are known, it is possible to compute the preexponential factors ko of each reaction using the Arrhenius law (2.2). 

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