Varying degree of protonation

Thus in the above case the elimination product is found to contain 82 % of (7). Unexpected alkenes may arise, however, from rearrangement of the initial carbocationic intermediate before loss of proton. El elimination reactions have been shown as involving a dissociated carbocation they may in fact often involve ion pairs, of varying degrees of intimacy depending on the nature of the solvent (cf. SN1, p. 90). 

M 1s 1 versus 0.037 M 1s 1 at 30C), but the measured rate constant varies somewhat from experiment to experiment. Considering the slowness of this proton transfer, the rate constant variation may reflect varying degrees of adventitious catalysis. 

When a proposed intermediate is so unstable that it cannot exist, i.e. it would have a lifetime less than that of a bond vibration ( 10 13 s), the reaction must proceed by a concerted mechanism (see reference [11], p. 5). The proton-transfer steps and other covalent bondforming and bond-breaking processes are concerted but with varying degrees of coupling between their motions. However, it is still not clear whether a concerted mechanism can occur when the intermediates, which would be formed in an alternative stepwise mechanism, have significant lifetimes. This is an important question for reactions catalysed by enzymes because the nature of the intermediate itself will control whether the enzyme- and non-enzyme-catalysed mechanisms are forced to be similar if the sole criterion for a concerted mechanism is the stability of the intermediate. 

Catalyst coking studies were carried out on the protonated and modified ZSM-5 zeolites. Approximately 0.8 g of zeolite was heated to 823 K followed by exposure to 2-butene at 100 ml/min for a specific time to obtain samples with varying degrees of coking, as shown in Table I. To achieve a similar wt % coke, the modified Na, H-ZSM-5 zeolite required a 33% longer exposure time to 2-butene. 

Regardless of classification, polysaccharides express varying degrees of ethanol tolerance neutral species are mostly unresponsive to electrolytes, including weak acid, and protonated species precipitate with low solvent retention from strong acid and Ca2+ solutions. Exceptions to many general rules abound for example, polyacids are generally the most stable species,... 

Effect of H and Al content. MOR and MFI zeolites have been primarily studied. Sodium forms exhibit very low activity in comparison with H-forms (76). The catalytic activity is a strong function of the degree of proton exchange, i.e. with the acidity of the catalyst (76). One should therefore expect that SCR activity be dependent on the Al content. Andersson et al. (77) found the activity proportional to Al content on a series of acid-leached H-MOR samples with Si/Al ratio from 16.5.3 to 9.9. Eng et al. (78) do not observe any correlation in their study on H-MFI and H-MOR. But they found that a minimum exists for Al content below which the SCR activity is negligible (Si/Al = 10 and 35 for MOR and MFI respectively). More recently, Stevenson et al. (75) reported that H-MFI catalysts of varying Si/Al ratios from 12 to 350 exhibited the highest activity at the lowest Si/Al ratio. 

Polyaniline exists in various forms which differ in the oxidation state and degree of protonation of the main chain. The average oxidation state of the polymer main chain can be varied chemically or electrochemically. The structures of several common oxidation states are shown in Fig. 64. The degree of protonation can be varied by treatment with acid or base. In addition to the structure obtained from head-to-tail coupling at the para-position, other structural units can be obtained. These units can be obtained by substitutions at other positions of the phenyl group as well as head-to-head and tail-to-tail coupling (Fig. 65). The polymerization conditions determine the structural characteristics, oxidation state, and... 

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