And acid strength

Click Coached Problems for a self-study module on molecular structure and acid strength. 

Bifunctional catalysis in nucleophilic aromatic substitution was first observed by Bitter and Zollinger34, who studied the reaction of cyanuric chloride with aniline in benzene. This reaction was not accelerated by phenols or y-pyridone but was catalyzed by triethylamine and pyridine and by bifunctional catalysts such as a-pyridone and carboxylic acids. The carboxylic acids did not function as purely electrophilic reagents, since there was no relationship between catalytic efficiency and acid strength, acetic acid being more effective than chloracetic acid, which in turn was a more efficient catalyst than trichloroacetic acid. For catalysis by the carboxylic acids Bitter and Zollinger proposed the transition state depicted by H. 

We can easily remember the relationship between Ka and acid strength if we recall that ... 

H->0 < H->S < H->Se < H-,Te. Because bond strengths decrease and acid strength increases down the group, it appears that bond strength dominates the trend in acid strength in these two sets of binary acids. 

TABLE 10.6 Correlations of Molecular Structure and Acid Strength... 

MolSurf parameters [33] are descriptors derived from quantum mechanical calculations. These descriptors are computed at a surface of constant electron density, with which a very fine description of the properties of a molecule at the Van der Waals surface can be obtained. They describe various electrostatic properties such as hydrogen-bonding strengths and polarizability, as well as Lewis base and acid strengths. MolSurf parameters are computed using the following protocol. 

XL30). Mossbauer spectroscopy (KFKIj was applied to follow the state of Fe species in the zeolites. Carbon monoxide and ammonia adsorption (monitored with FTIR) (EQUINOX 55) was used to determine the nature, concentration and acid strength of the active sites in the Fe-TON zeolites. 

The motor octane number jc7 was a function of the external isobutane-to-olefin ratio jc8 and the acid strength by weight percent x6 (for the same reactor temperatures and acid strengths as for the alkylate yield x4)... 

Extensive studies of the acidity and basicity of zeolites by adsorption calorimetry have been carried out over the past decades, and many reviews have been published [62,64,103,118,120,121,145,146,153,154]. For a given zeolite, different factors can modify its acidity and acid strength the size and strength of the probe molecule, the adsorption temperature, the morphology and crystallinity, the synthesis mode, the effect of pretreatment, the effect of the proton exchange level, the Si/Al ratio and dealumination, the isomorphous substitution, chemical modifications, aging, and coke deposits. 

---