Apparent activation entropies

The mechanism of the cycloaddition of phenyl azide to norbornene has been shown to involve a concerted mechanism with a charge imbalance in the transition state (199). In a similar manner the cycloaddition of phenyl azide to enamines apparently proceeds by a concerted mechanism (194, 194a). This is shown by a rather large negative entropy of activation (—36 entropy units for l-(N-morpholino)cyclopentene in benzene solvent at 25°C), indicative of a highly ordered transition state. Varying solvents from those of small dielectric constants to those of large dielectric constants has... 

The solid product, BaO, was apparently amorphous and porous. Decomposition rate measurements were made between the phase transformation at 1422 K and 1550 K (the salt melts at 1620 K). The enthalpy and entropy of activation at 1500 K (575 13 kJ mole-1 and 200 8 J K"1 mole-1) are very similar to the standard enthalpy and entropy of decomposition at the same temperature (588 7 kJ and 257 5 J K-1, respectively, referred to 1 mole of BaS04). The simplest mechanistic explanation of the observations is that all steps in the reaction are in equilibrium except for desorption of the gaseous products, S02 and 02. Desorption occurs over an area equivalent to about 1.4% of the total exposed crystal surface. Other possible models are discussed. 

Usually, activation energies for dissociation are much higher than k T and situations like this, where the apparent activation energy becomes negative for the rate-limiting process, are rare. Nevertheless, the present case illustrates nicely that entropy changes may play an important role and that it is not always the activation energy that dominates the process. 

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... 

The only example of this system (573) was prepared by the oxidation of the dihydrazine (572) (78JA5978). The thermal stability of (573) was markedly lower than that of its six-membered analogue, due apparently to its less rigid structure, and surprisingly its decomposition to give (574) had a negative entropy of activation. This indicates either a concerted process or one in which the optimum geometry for decomposition differs from the equilibrium conformation. 

The conclusion from these experiments is that the associated ions [RXY]-are stereochemically related to the SN2 transition state. Furthermore, little variation occurs in the stability of the associated ions for CH3C1, CH3Br and CH3I, and steric hindrance is apparently of small consequence in the enthalpic and entropic contributions of these equilibria. These data can be compared directly to the solution values for enthalpy and entropy of activation to show that solvation effects on both parameters are indeed responsible for the large variations in rates observed in solution. 

Values for the average and the apparent AS were obtained for four different values of x and are given in Table IV. At the beginning of the reaction the apparent entropy of activation was negative, but as the reaction continued, it increased to zero and then became positive. An explanation for this... 

The value of —19.5 e.u. for the apparent entropy of activation obtained at the beginning of the extraction includes a term for the limited number of sites where dissolution could occur (Table IV and Figure 11). It is considered probable that the dissolution of material from the pores (process Rr) occurring simultaneously can account for part of the large negative value. Chariot (I)... 

The data obtained were treated exactly as those obtained in former experiments, and a series of average enthalpies of activation and apparent entropies of activation was obtained for different x s (Table V, Figures 14 and 15). 

It was found that removing only 12% of coal during the first stage of extraction increased the initial apparent entropy of activation from a negative to a positive value. It further shows that this value increases as x increases. This is expected if the surface area available for reaction increases during the course of the reaction. 

Coal is a complicated material, and more than one simple chemical process takes place during solvent extraction. The activation energy necessary for dissolving coal increases with the extent of the process up to a point at a certain temperature insufficient energy is available for more extraction. When the activation energy necessary for extraction becomes more than maximum energy supplied, additional coal will not dissolve. The rate constants, average heats of activation, and apparent activation entropies predict the dissolution... 

Values for the enthalpy and entropy of activation for the reaction catalyzed by 1F7 were determined from the temperature dependence of kcat. Apparently, the observed rate acceleration is due entirely to a lowering of the enthalpic barrier (15 kcal/mol versus 21 kcal/mol for the uncatalyzed reaction (24)), consistent with the notion that induced strain might be an important component of catalysis. The entropy of activation for the antibody-promoted reaction (-22 eu) is actually less favorable than for the spontaneous reaction (-13 eu) (23). This fact may reflect the need for some conformational change in the antibody binding pocket during catalysis. However, possible solvent effects make the interpretation of AS difficult. 

The activation parameters for the acid-catalyzed hydrolysis of long chain alkyl sulfates compared to those for non-micellar ethyl sulfate calculated from potentiometric data indicate that the rate acceleration accompanying micellization is primarily a consequence of a decrease in the enthalpy of activation rather than an increase in the entropy (Kurz, 1962). However, the activation energies for the acid-catalyzed hydrolysis of sodium dodecyl sulfate calculated from spectrophotometric data have been reported to be identical (Table 8) for micellar and non-micellar solutions, but the entropy of activation for the hydrolysis of the micellar sulfate was found to be 6 9 e.u. greater than that for the non-micellar system (Motsavage and Kostenbauder, 1963). This apparent discrepancy may be due to the choice of the non-micellar state as the basis of comparison, i.e. ethyl sulfate and non-micellar dodecyl sulfate, to temperature dependent errors in the values of the acid catalyzed rate constant determined potentiometrically, or to deviations in the rate constants from the Arrhenius equation. 

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