Explanation contrastive

The former explanation contrasts however, especially in the benzilate system, with the very early transition state of the decarboxylation process and thus positive charge stabilization by the aromatic system (factor ii) rather than the stability of the radical formed by decarboxylation (factor i) may play a greater role. On the other hand the very similar values of A cc measured for the phenylacetoxyl radicals may reflect a relatively later transition state and a self-compensating stabilization by ring substituents of both reactant and product along the series. 

This explanation contrasts with the results of the hydrolysis of methyl gluco- and galacto-pyranosides, in which the energies and entropies of activation are larger for the a-D than for the /3-d anomer. The higher rate... 

Theoretical models of the film viscosity lead to values about 10 times smaller than those often observed [113, 114]. It may be that the experimental phenomenology is not that supposed in derivations such as those of Eqs. rV-20 and IV-22. Alternatively, it may be that virtually all of the measured surface viscosity is developed in the substrate through its interactions with the film (note Fig. IV-3). Recent hydrodynamic calculations of shape transitions in lipid domains by Stone and McConnell indicate that the transition rate depends only on the subphase viscosity [115]. Brownian motion of lipid monolayer domains also follow a fluid mechanical model wherein the mobility is independent of film viscosity but depends on the viscosity of the subphase [116]. This contrasts with the supposition that there is little coupling between the monolayer and the subphase [117] complete explanation of the film viscosity remains unresolved. 

The Brown-Winstein nonclassical ion controversy can be summed up as differing explanations of the same experimental facts (which were obtained repeatedly and have not been questioned) of the observed significantly higher rate of the hydrolysis of the 1-exo over the 2-endo-norbornyl esters. As suggested by Winstein, the reason for this is participation of the Ci-Q single bond leading to delocalization in the bridged nonclassical ion. In contrast. Brown maintained that the... 

The rate of saponification of ethyl 2-thenoate, in contrast to ethyl 3-thenoate, was found to be considerably slower than predicted from the pKa of the acid, showing that the reactivities of thiophenes do not parallel those of benzene. The first explanation, that this was produced by a steric effect of the ring sulfur similar to the case in or /lo-substituted benzenes and in ethyl 1-naphthoate, could not be upheld when the same effect was found in ethyl 2-furoate. It was later ascribed to a stereospecific acid strengthening factor, involving the proper relation of the carboxylic hydrogen and the heteroatom, as the rate of saponification of 2-thienylacrylic acid was in agreement with that predicted from the acid constants. ... 

The explanation of this pattern is that the well-aerated areas in the tidal zone become strongly cathodic while the metal just below water becomes anodic. This distribution is in striking contrast to the results quoted by Ambler and Bain . 

In contrast with the thermal [4 + 2] Diels-Alder reaction, the 2 + 2 cycloaddition of two alkenes to yield a cvclobutane can only be observed photo-chemically. The explanation follows from orbital-symmetry arguments. Looking at the ground-state HOMO of one alkene and the LUMO of the second alkene, it s apparent that a thermal 2 + 2 cycloaddition must take place by an antarafacial pathway (Figure 30.10a). Geometric constraints make the antarafacial transition state difficult, however, and so concerted thermal [2 + 2j cycloadditionsare not observed. 

We now know that Hammett s explanation is correct in all its aspects. This result is especially noteworthy because Hammett arrived at his conclusions not through extensive experimentation in his laboratory, but by the consistent application of the newer theories of organic chemistry to kinetic results already published by others. This is not the only example of such anticipation of views (now generally accepted) to be found in Hammett s book, and it is worth remembering that Hammett expressly postulates the diazonium ion as the reactive form of the diazo compound in coupling, in contrast to the then current opinion that the diazohydroxide was the effective species. 

The reversibility of aromatic diazotization in methanol may indicate that the intermediate corresponding to the diazohydroxide (3.9 in Scheme 3-36), i. e., the (Z)-or (is)-diazomethyl ether (Ar — N2 — OCH3), may be the cause of the reversibility. In contrast to the diazohydroxide this compound cannot be stabilized by deprotonation. It can be protonated and then dissociates into a diazonium ion and a methanol molecule. This reaction is relatively slow (Masoud and Ishak, 1988) and therefore the reverse reaction of the diazomethyl ether to the amine may be competitive. Similarly the reversibility of heteroaromatic amine diazotizations with a ring nitrogen in the a-position may be due to the stabilization of the intermediate (Z)-diazohydroxide, hydrogen-bonded to that ring nitrogen (Butler, 1975). However, this explanation is not yet supported by experimental data. 

Two additional results were noted during this series of experiments. It was found that plugging of the reactor occurred when the conversion reached about 60%. No satisfactory explanation or cause for the plugging was determined. It was also noted that, regardless of the rate of polymerization, no further reaction occurred after a period of about 60 to 75 minutes. This is in contrast to reaction times of up to three hours for the same recipe used in a batch reactor. 

This, in turn, reduced the number of polymer particles (the loci of reaction) and hence the reaction rate fell. However, this explanation is at variance with the results reported in Figure 12 where the molecular weight (weight-average) clearly increases with increasing Reynolds number. It seems more likely that the turbulent flow results could be explained by a decrease in the effective initiator concentration. This low concentration would also explain why there is no further reaction after a period of about one hour as contrasted with the batch reactions where the reaction is still proceeding after two to three hours. The current absence of corroborating evidence makes this explanation very tentative. 

In contrast to enzyme- and base-catalyzed DKRs, there are only a few reports of enzyme- and acid-catalyzed DKRs. A plausible explanation is that deactivation of the enzyme can occur under acidic conditions. Also, decomposition of the substrate has... 

When asked to describe and to explain the properties of crystals, all the students immediately nsed ideas of particles, even though they had not been taught these ideas at school before. In contrast to the historical researchers, who set their focus on the shape of crystals, the students mainly described and explained properties snch as the hardness or the colour of crystals. The concept most used by students was the density of package of the particles. According to the students explanations, the lighter and the less stable a material was, the smaller was its density,. 

In contrast to the above case, addition of HCl to 1,1-dimethylallene at —78°C gives at least two thirds and possibly exclusively l-chloro-3-methyl-2-butene, 33, although these results are complicated by rearrangement of the allene to isoprene and the addition of HCl to the isoprene (65). No satisfactory explanation was offered (65) and none is readily available within the carbonium framework to account for the unusual orientation in this addition. Certainly the tertiary carbonium ion, 34, should be more stable than the primary carbonium ion, 35, since neither is stabilized by the adjacent perpendicular n center. This result is all the more surprising since tetramethylallene, 36, behaves as expected... 

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