Acidity correlation analysis

A few examples have been reported in which no steric parameter is involved in the correlation analysis of cyclodextrin catalysis. Straub and Bender 108) showed that the maximal catalytic rate constant, k2, for the (5-cyclodextrin-catalyzed decarboxylation of substituted phenylcyanoacetic acid anions (J) is correlated simply by the Hammett a parameter. 

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

D correlation analysis is a powerful tool applicable to the examination of data obtained from infrared spectroscopy. The correlation intensities, displayed in the form of 2D maps, allow us to correlate the shift induced by CO adsorption and acidity of sites in dealuminated zeolites. Results are in accordance with previous results, obtained using only IR measurements, proving the validity of this technique. New correlations allowed the assignment of very complex groups of bands, and 2D correlation revealed itself as a great help for understanding acidity in dealuminated zeolites. 2D correlation has allowed us to validate the model obtained by NMR. 

In Section IV.B we will discuss the ortho-effect of NO2 as manifested in the ionization of carboxylic and other acids and (in Section IV.C) in the reactions of substituted benzoic acids with diazodiphenylmethane (DDM). Only in the case of the latter system can really satisfactory correlation analysis be taken as the basis for discussion. For most of the other systems discussion will have to be qualitative or, at best, semi-quantitative. 

Some further light on the behaviour of the nitro group as an orf/ o-subsliUicnl in affecting the strength of benzoic acid can be obtained by correlation analysis employing the extended Hammett equation (Section n.B) in the form of equation 14 ... 

A rather striking feature of this equation is that the intercept 4.093 does not correspond closely to the observed pKa value of 4.205 for benzoic acid itself, i.e. this acid is not acting as the true parent of the series of ori/zo-substituted benzoic acids. Examination of the data and the results of the correlation analysis suggests that all the ori/zo-substituted acids receive an initial increment in acidity caused in some way by the replacement of o-H by any other substituent. The regression was therefore repeated without the benzoic acid point and equation 16 was the result ... 

Attempts at correlation analysis for X—C—H acidity have been made for a long time. Bowden and colleagues192 examined the problem in 1970 for the pKa values of 9-X-fluorenes and related series. For a limited selection of substituents X in 9-X-fluorenes, a correlation with Taft s a values for X was found89. A more general correlation, albeit with considerable scatter, was found with AM, a parameter based on LCAO-MO calculations. Several dinitro-substituted diphenylmethanes fitted the line quite well and 2-nitrofluorene fitted passably 9-cyanofluorene also fitted quite well, but malononitrile deviated strongly. 

There are extensive data for the acid-catalyzed protiodesilylation of XCgELrSiMes in methanol-aqueous perchloric acid or acetic acid-aqueous sulphuric acid at 50°C225. Correlation analysis of the partial rate factors (relative rate constants) by means of the Yukawa-Tsuno equation (Section n.B) finds p = —5.3 and r+ = 0.65. These values are consistent with a relatively low demand for stabilization of the transition state by electron delocalization, i.e. the transition state is early along the reaction coordinate, p-NO2 is highly deactivating with / = 14 x 10 but 0-NO2 is even more deactivating, with / = 6.8 x 10-5. This contrasts with the deactivation order discussed above for nitration and chlorination (Table 6), and may be explained in terms of the early transition state, well removed from the Wheland intermediate. 

The new treatment had its origins partly in ab initio molecular orbital calculations of substituent effects and partly in extensive studies of gas-phase proton transfer reactions from about 1980 (Section V.A). Various aspects of this work essentially drew attention to the importance of substituent polarizability. In 1986 Taft, Topsom and their colleagues252 developed a scale of directional substituent polarizability parameters , oa, by ab initio calculations of directional electrostatic polarization potentials at the 3-21G//3-31G level for a large set of CH3X molecules. The oa values were shown to be useful in the correlation analysis of gas-phase acidities of several series of substrates252, and such work has subsequently been extended by Taft and Topsom151. 

Vai der Waals Radii And Related Sterlc Parameters Van der Waals radii have long been considered a valid meas ure of atomic size Taft proposed the first valid set of sterlc parameters for correlation analysis defined from acid hydrolysis of esters caiarton derived equations for the calculation of Van der Waals radii, ry of symmetric top MZs groups( ) These values of the Van der Waals radii were used, together with that for H, to show that Es Is a linear function of i>y ... 

Examples of the application of correlation analysis to oxime and hydroxamic acid pK data sets are considered below. In the best of all possible worlds all data sets have a sufficient number of substituents and cover a wide enough range of substituent electronic demand, steric effect and intermolecular forces to provide a clear reliable description of the kind and magnitude of structural effects on the property of interest. In the real world this is often not the case. We will therefore try to show how the maximum amount of information can be extracted from small data sets. 

The application of correlation analysis to oximes and hydroxamic acid acidity has been taken as an Ulnstration of how it can be applied to a wide range of properties and reactivities. It does not need fnrther discnssion. In the Tables below data sets and resnlts of correlations that exemplify these applications to varions types of data are presented. 

The interaction between a solvated peptide and an RPC sorbent in a fully or partially aqueous solvent environment can be discussed in terms of the interplay of weak physical forces. Based on linear free energy considerations, the effects of these forces can, to a first approximation, be linearly summated. Consequently, knowledge of the amino acid sequence of a peptide permits, to a first approximation, the effective hydrophobicity of the peptide to be derived by correlation analysis methods using data derived from other techniques, or... 

Cesium fluoroxysulfate in acetonitrile medium at 35 C converts primary alcohols and alkyl and aryl aldehydes into acid fluorides in high yields.i" Hammett correlation analysis of the fluorination of various benzene-substituted aldehydes gives the reactivity constant31 q = —0.38. It has been shown that solvent polarity plays an extremely important role in the conversion of aldehydes into fluorides the conversion is almost quantitative in acetonitrile, but completely stopped in dichloromethane, hexane or tetrahydrofuran. The presence of ni-... 

This method was used in the examination of an Eskimo who died 1600 years ago. The body was discovered in a frozen state on St. Lawrence Island, Alaska in 1972 and remained frozen until it was brought to Fairbanks in 1973. Examination of the female individual revealed that she had a skull fracture, probably resulting from instant bunal caused by a landslide. Aspaitic acid laceiuization analysis of a tooth from the mummy yielded an age at death of 53 5 years, which correlated well with earlier estimates based upon morphological features. This method is an example of the need to preserve mummies (Alaskan. Egyptian, and Peruvian, among others) for application of new dating techniques as they develop. 

To improve the separation of the derivatives of fatty acids with the same effective carbon number, e.g., palmitoleic (C16 1), linoleic (18 2), andmyristic (C14 0), Baty et al. (33) reported the preparation of the anthrylmethyl esters derivatives of several fatty acids (with 9-hydroxy-methylanthracene and the catalyst 2-bromo-l-methylpyridinium iodide (BMPI)) with a view to analysis by HPLC and LC-MS (with gradient elution on a ZORBAK 5-/zm Cl8 column) (see Chemical Structure 1). The excess reagents were evaporated under nitrogen at 50°C, and the de-rivatized acids were taken up in 1 ml of mobile phase prior to chromatography. This method did not allow the resolution of the C16 1, 08 2, and C14 0 esters, although HPLC data obtained for the other acids correlated well with that obtained by capillary gas-liquid chromatography. 

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