Structure, acid-base strength Lewis

When the surface of fully dehydrated MgO is contacted with ethylene oxide at RT (260), the first surface product formed is the a adduct between the ethylene oxide molecules and the most coordinatively unsaturated Mg2+ ions of the surface, presumably localized on edges, steps, and corners (hereafter Mg ) (Fig. 9 IR bands labeled A) (Scheme 6). This first step is not unexpected since ethylene oxide, with its medium base strength, acts as a surface probe for the most reactive Lewis acid sites (three- and fourfold coordinated Mg2+ ions). This precursor species transforms at RT into a truly chemisorbed species, leading to the formation of a structure plausibly suggested to be the cyclic structure represented below (bands B in Fig. 9) because of the nucleophilic attack of the basic coordinatively unsaturated oxygen ion (O ) in adjacent position on the CH2 group (Scheme 7). 

The sttuctures of organoaluminum halides R AlXs are dominated by dimeric structures with AI2X2 four-membered rings for X = Cl, Br, and I. Unlike the aluminum alkyls, these hahde bridges are electron-precise and result from intermolecular Lewis acid-base complexation. These dimers do dissociate and readily react with Lewis bases, but the enthalpies of dissociation are higher than those for the aluminum alkyls. In many cases, the strength of the association is sufficient that dimers are observed in the gas phase. 

However, donor-acceptor interactions are affected not only by the Lewis acid and base strengths, but also by other, steric and electron structural, factors. Thus, even in systems where either solely the donor or the acceptor property of the solvent is manifested, solvents with different space requirements may interact to different extents because of the steric properties of the reference solute and a reference acceptor with a tendency for dative 7c-bonding (back-coordination) will interact more strongly with jr-acceptor solvent molecules (e.g., acetonitrile) than would be expected from their basicity. The solvent donicity investigations by Burger et al [Bu 71, 74] with transition metal complex reference acceptor model systems have clearly shown the great extent to which such secondary effects may distort the solvent scale. 

It is important to emphasize that spectroscopic evidence shows that water transforms the Lewis acid sites of sulfated zirconia into Bronsted acid sites [80]. At the same time, water promotes isomerization reactions over sulfated zirconia for a moderate extent of catalyst dehydration. Similarities were reported between the effect of rehydration on the isomerization activity of sulfated zirconia [81] and on that of other oxide catalysts [49] that are consistent with the role of surface donor sites in hydrocarbon isomerization reactions. However, when spectroscopic methods using basic probes were used to compare sulfated zirconia and zeolites in terms of the strength of their acid sites, the results were inconsistent with all catalytic data. These findings illustrate the danger of comparing the acidity of catalyst systems that differ in structure and composition, such as zeolites and sulfated zirconia in these systems the "catalytic" and the "physicochemical" scales for the strength of acid-base interaction may contain significantly different parameters. 

Depending on the strength of the Lewis acid and on the structure of the R substituent, the actual acylating agent may be the acid-base complex 1 or the acyl cation 2. The active reagents 1 or 2 or even a mixture of both undergo the electrophilic aromatic substitution affording the final ketone products [1]. 

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