Entropy of activation, for

An alternative approach is to assume, in the light of the experimental evidence just mentioned, that the reactions of cations and neutral molecules have similar values of (or, equivalently, of log ( /l mol and to try to calculate the difference which would arise from the fact that the observed entropy of activation for a minority free base includes a contribution from the acidic dissociation of the conjugate acid in the medium in question (see (5) above). Consider the two following reaction schemes one (primed symbols) represents nitration via the free base, the other the normal nitration of a non-basic majority species (unprimed symbols) ... 

Wide variation in enthalpy and entropy of activation for different reaction systems is possible, as illustrated by the following two reactions. 

Kinetic studies on 2-, 3-, and 4-chloro-l-methylpyridinium salts showed a 30 10 ratio of the reaction rates at 50° with 4-nitro-phenoxide ion in methanol. The activation energy for reaction at the 4-position is one kilocalorie lower ( 8-fold higher rate) than for reaction at the 2-position. The reversal in rates relative to the corresponding halopyridines is the result of a much higher entropy of activation for the 2-chloro compound. The 3-chloro compound has a favorable entropy of activation also, but the energy of activation is about 13 kcal higher than that of the isomers (cf. Table II and Section III, A, 2). 

Tautomerism of 2-substituted hexahydropyrimidines has been studied (980PP53), and free energies, enthalpies, and entropies of activation for this ring-chain tautomeric equilibrium have been measured [97JCS(P2)169]. 

Hcuts et a .,64 while not disputing that penultimate units might influence the activation energies, proposed on the basis of theoretical calculations that penultimate unit effects of the magnitude seen in Ihe S-AN and other systems (i.e. 2-5 fold) can also be explained by variations in the entropy of activation for the process. They also proposed that this effect would mainly influence rate rather than specificity. 

On the other hand, the catalytic effect of water as a base is stronger at the 2-position. This result can be explained if one assumes that the proton is transferred by a water molecule which solvates the O- group in the reagent 3-sulfo-l-naphth-oxide dianion. As can be seen in 12.148, the base is already in the optimum position when the stage of the o-complex is reached. This explanation is supported by a comparison of the entropies of activation for reaction at the 2- and 4-positions. 

The data in Table 191 show a large difference in entropy of activation for reaction of the 4-methoxy compound in the three acids. The difference in the data for perchloric and sulphuric acids was again thought to arise from a significant amount of reaction in the latter occurring via bisulphate anion. This anion was proposed as being insufficiently electrophilic to react except by coordination with the side chain, as in scheme (2S0)... 

With application of reasonable values for trapping parameters and AS2, it was possible to bracket the enthalpy and entropy of activation for isomerization of cyclobutadiene. Hence, A/Zj was estimated to fall between 1.6 and lOkcal/mol, where the upper limit was consistent with theoretical predictions for square-planar cyclobutadiene. Most surprising, though, was the conclusion that AS for automeriza-tion must lie between -17 and -32cal/(molK), based on the AS values normally observed for Diels-Alder reactions as a model for AS2. ... 

In another study (see Atwood, 1985, p. 168), a series of oxad reactions was carried out at 30°C with methyl iodide and trans-[IrCl(CO)(P(p-C6H4Y)3)2]. The identities of the groups in the para position of the phosphine ligands, Y, are shown in Figure 22.4, which is a graph of the values for the enthalpy and entropy of activation for reactions. 

Of reactions (5)-(7) only reaction (7) has been directly investigated. The entropy of activation for this reaction, —3.0 e.u., probably indicates that the mechanism for the isomerization of sec.-butyl is closely related to that of iso-propyl. 

Intrinsic entropy of activation for homogeneous self-exchange. Values for V and... 

In order to account for the large positive entropy of activation for the reaction, it is necessary to assume that there is virtually free rotation of the incipient ethylene molecules in the complex. The second possible transition complex involves the complete rupture of one carbon-carbon bond to give the tetramethylene biradical, and the reaction path may be envisaged as shown below ... 

Is it possible that low site densities are obtained in some desorption reactions because an incorrect assumption is made about the entropy of activation For Step 5 we have assumed that AS = 0. Were we to modify this step to obtain a larger log L, we would have to postulate that the adsorbed molecule loses more entropy as the activated complex forms (4.6 e.u. per unit change in log L) than it does in Step 5 as we have described it. Such a sequence of events is not impossible for a surface reaction. But if the adsorbed molecule is immobile, it is difficult to imagine such a species losing... 

From Azides and x-Acylphosphorus ylids Addition of azides to a-acylphosphorus ylids takes place at room temperature in dichloromethane or at 80°C in benzene, giving triazolines from which a phosphine oxide is spontaneously eliminated. " The ylids exist almost exclusively in the cis-enolate configuration, and a mechanism involving concerted 1,3-dipolar addition has been proposed (Scheme 12) on the basis that there is a low entropy of activation for the reaction, and that the reaction rate is insensitive to changes in solvent polarity. ... 

The thermal behavior of la-lg observed by DSC (Fig. 1) confirms the presence of mesophases and is typical of low molecular weight thermotropic LC materials (M). The lower T , for lb and Id are consistent with the higher entropy of activation for crystallization of odd-n spacers, demonstrated in several main chain LC polymers (23). The apparent absence of nematic-smectic transitions in the DSC... 

Discuss why 3-methyloxetane is more reactive than oxetane. Is a negative entropy of activation for propagation consistent with the proposed propagation mechanism ... 

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