Correlations Br0nsted

Br0nsted s correlation was developed some years before Hammett s. Here we present a formulation that shows how both LFERs are related for general acid catalysts. For the sake of simplicity, we ignore the statistical factors of p and q. The Hammett equation applied to the values of bh is... 

Two-Dimensional correlation analysis to study Br0nsted acid sites in zeolites... 

A recent study of the reactions of 2,4-dinitrochlorobenzene and of picryl chloride with a series of nucleophiles that are presented in Table 6 shows that a plot (not shown) of log k against the pK values of all the nucleophiles is badly scattered77. Differences of up to 108 are observed for bases with similar pKa values. Part of this scatter is due to deviations that result because different families of nucleophiles (with different nucleophilic atoms) give rise to different Br0nsted correlation lines. Thus, for the reactions of picryl chloride good correlations are observed for a family of oxyanions (ft = 0.38, plot not shown), primary and secondary amines (Figure 4, ft = 0.52) and quinuclidines (Figure 4, P = 0.66). 

Rare-earth exchanged [Ce ", La ", Sm"" and RE (RE = La/Ce/Pr/Nd)] Na-Y zeolites, K-10 montmorillonite clay and amorphous silica-alumina have also been employed as solid acid catalysts for the vapour-phase Beckmann rearrangement of salicylaldoxime 245 to benzoxazole 248 (equation 74) and of cinnamaldoxime to isoquinoline . Under appropriate reaction conditions on zeolites, salicyl aldoxime 245 undergoes E-Z isomerization followed by Beckmann rearrangement and leads to the formation of benzoxazole 248 as the major product. Fragmentation product 247 and primary amide 246 are formed as minor compounds. When catalysts with both Br0nsted and Lewis acidity were used, a correlation between the amount of Br0nsted acid sites and benzoxazole 248 yields was observed. 

In acylation with acidic ammonium and cesium salts of phosphotungstic acid, direct correlation was found between activity and the number of accessable Br0nsted acid sites.69,70 Moreover, cesium salts were found to be more active, than HY and H-Beta zeolites.70... 

Rates of model reactions are more commonly used to determine relative rather than absolute surface acidities and a variety of acid-catalyzed reactions have been used for this purpose (1-3). Xylene isomerization is a particularly well-substantiated model reaction, thanks to work by Ward and Hansford (43). They demonstrated that the conversion of o-xylene to p- and /n-xylenes over a series of synthetic silica-alumina catalysts increases as the alumina content is increased from 1 to 7%. The number of strong Brdnsted acids in each member of the catalyst series was measured by means of infrared spectroscopy. Since conversion of o-xylene was found to be a straight-line function of the number of Br0nsted acids (see Fig. 9), rate of xylene isomerization appears to be a valid index of the amount of surface acidity for this catalyst series. This correlation also indicates that the acid strengths of these silica-alumina preparations are roughly equivalent. 

Several studies have shown that a pAa match provides only moderate energy (-5 kcal/mol) in solution.(8,12-15) Ab initio calculations also confirmed this conclusion.(16) A linear correlation between AGHB and Ap a was proposed in both experimental and computational studies where the dimensionless Br0nsted slope j3 is... 

As in the case of Br0nsted correlations, one can expect different quantitative behaviour in plots of this kind when the type of substrate and the base used is varied. It can readily be shown that superposition of all currently available data leads to large scatter in... 

Ai (140) measured the acid site concentration by the adsorption of ammonia. No correlation was found between the P/V ratio, the acidity, and the catalytic activity. This result has been attributed to the use of ammonia as a probe molecule that cannot distinguish between Lewis and Br0nsted acidity. Comaglia et al. (137) measured the acid sites using pyridine and acetonitrile as probes. However, the pyridine results showed no correlation between the Lewis to Br0nsted acid site ratio or the Lewis acid site concentration and the activity and selectivity of the catalyst for MA formation. 

Periodic Table Correlations. When comparing Br0nsted acids and bases that differ in the position of an element in the periodic table ... 

Marcelin and coworkers have shown that both Al and Si NMR techniques correlate well with other methods in terms of determining total number of Br0nsted sites in zeolites. ... 

Pines, Fleming and coworkers have utilized a free-energy correlation between the excited-state equilibrium constant of the photoacid and the proton dissociation rate . Such correlations are extensions of similar correlations existing between the equilibrium constant and reactivity of ground-state acids (the Br0nsted relation ). 

Figure 3. Br0nsted-type correlations with the pK of the leaving group of the ratio of the second-order rate constants for reactions of phosphorylated pyridines with anionic nucleophiles and water (Sue = succinate, TFE = trifluoroethanol) (79).

Figure 19.5 More O Ferrall-Jencks diagram to illustrate how cross-sections of the potential energy surface for gas-phase hydration of formaldehyde yield a family of barriers from which a Br0nsted correlation may be generated (cf. Ref [38]).

Figure 19.6 Curved Br0nsted correlation for gas-phase proton transfer between mimicked pyridine donors and acceptors AMI results from Ref [39].

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