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** Entropy factors causing an increase **

** Entropy of activation, and calculation A factors **

** Entropy, as a factor in the formation **

** Entropy, as a factor in the formation hemiacetals and acetals **

The entropy factor should also be considered since cyclization results in a more ordered structure. The C5 cyclization of n-hexane involves an entropy decrease of about 15-17 entropy units (e.u.). The corresponding values for cyclohexane and benzene formation are about 25 and 38-45 e.u., respectively. These values are comparable with calculated values of adsorption entropy (29). Thus, adsorption of a molecule to be cyclized may supply a considerable part of the entropy change in other words, adsorption should take place in a geometry favorable for cyclization. This is one of the main roles of the catalyst. [Pg.277]

Where ortho effects and special entropy factors control the relation of the reaction rates, it seems more appropriate to evaluate relative activation from the energies of activation. [Pg.308]

It is more common to find that AH° and AS° have the same sign (Table 17.2, III and IV). When this happens, the enthalpy and entropy factors oppose each other. AG° changes sign as temperature increases, and the direction of spontaneity reverses. At low temperatures, AH° predominates, and the exothermic reaction, which may be either the forward or the reverse reaction, occurs. As the temperature rises, the quantity TAS° increases in magnitude and eventually exceeds AH°. At high temperatures, the reaction that leads to an increase in entropy occurs. In most cases, 25°C is a low temperature, at least at a pressure of 1 atm. This explains why exothermic reactions are usually spontaneous at room temperature and atmospheric pressure. [Pg.464]

Since there are two different connections possible for n-octane, 1,6 or 3, 8, which could lead eventually to ethylbenzene, there is a statistical entropy factor involved here which is not part of the o-xylene route. Therefore, if both closures were equally possible from an enthalpy perspective, one would predict a 2 1 ethylbenzene to o-xylene ratio. The formation of the m- and p-xylene requires prior isomerization of n-octane to 2- and 3-methylheptane, respectively. [Pg.297]

As a result of thermodynamic analysis it is shown that protein bonding to carboxylic CP exhibiting a local internal chain structure is determined by the entropy factor, whereas, if the arrangement of flexible chain parts on the protein globule is possible, the energetic component predominates. [Pg.30]

According to Hercules 5> a measure of the relationship between direct excitation of the first excited singlet state by radical-ion recombination and triplet-triplet annihilation is the entropy factor FAS, estimated to be on average 0.2 eV. The enthalpy of the radical cation-radical anion recombination can be measured as the difference between the redox potentials 1/2 Ar—Ar

In a reaction in which the number of product molecules is equal to the number of reactant molecules, (e.g., A + B —> C + D), entropy effects are usually small, but if the number of molecules is increased (e.g., A —> B + C), there is a large gain in entropy because more arrangements in space are possible when more molecules are present. Reactions in which a molecule is cleaved into two or more parts are therefore thermodynamically favored by the entropy factor. Conversely, reactions in which the number of product molecules is less than the number of reactant molecules show entropy decreases, and in such cases there must be a sizable decrease in enthalpy to overcome the unfavorable entropy change. [Pg.278]

The equilibrium constants in Table 22 suggest that the larger the peri substituent, the less favored is the sc form. Since the phenyl group takes a conformation in which it does not directly interact with the peri CH group, this result may originate in the ease or difficulty of solvation in addition to an entropy factor due to limitations of certain conformations. [Pg.60]

The general equation for L is Eq. (10), and the expressions to be used for C in that equation are listed in Table 1 in terms of partition functions. But the entropies of both the activated complex and the reactants, and therefore AS, can also be expressed in terms of partition functions. Therefore, C can be expressed in terms of the entropies of the activated complex and the reactants. As we shall see, it is possible to eliminate partition functions entirely. Also, in all but one case. Step 11, the entropy factor in C can be determined if one knows only the entropy of activation in those cases the entropy of a reactant or the activated complex is not needed. [Pg.118]

Scheme 13 may look unfavorable on the face of it, but in fact the second two reactions are thermally allowed 10- and 14-electron electrocyclic reactions, respectively. The aromatic character of the transition states for these reactions is the major reason why the benzidine rearrangement is so fast in the first place.261 The second bimolecular reaction is faster than the first rearrangement (bi-molecular kinetics were not observed) it is downhill energetically because the reaction products are all aromatic, and formation of three molecules from two overcomes the entropy factor involved in orienting the two species for reaction. [Pg.51]

See also in sourсe #XX -- [ Pg.277 ]

See also in sourсe #XX -- [ Pg.29 , Pg.450 ]

See also in sourсe #XX -- [ Pg.87 , Pg.116 ]

See also in sourсe #XX -- [ Pg.87 , Pg.115 ]

See also in sourсe #XX -- [ Pg.352 ]

See also in sourсe #XX -- [ Pg.2 ]

** Entropy factors causing an increase **

** Entropy of activation, and calculation A factors **

** Entropy, as a factor in the formation **

** Entropy, as a factor in the formation hemiacetals and acetals **

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