Hydrogen-bond affinity scales

The same distinction must be made for hydrogen-bond affinity scales. When the heats of reaction of, for example, 4-fluorophenol with a series of bases are measured in a ternary system (4-FC6H40H/B/solvent), a solute scale is obtained, whereas measurements on the binary system 4-FC6H4OH/B, corrected by the heats of solution of a similar but non-HBD probe such as 4-FC6H40Me (the so-called pure base calorimetric method) [10], furnish a solvent scale. The degree of equivalence of solute and solvent scales will be considered later. 

Hydrogen-Bond Affinity Scales Early Studies... 

In spite of a large number of determinations of hydrogen-bond enthalpies, particularly in the period 1960-1980, there were only two attempts to build a scale of hydrogen-bond affinity. 

The applicabiUty of the 4-fluorophenol affinity scale to solvents other than the definition solvent ecu is difficult to test rigorously because of a lack of data. However, the data for 3-CF3C6H4OH and 3-FC6H4OH [120-122] can be used to test the application of Equation 4.26 to the effects of solvents, because these phenols have structures and hydrogen-bond donor properties very close to those of 4-FC6H4OH. Table 4.32 lists the values of hydrogen-bond affinities of various bases in six solvents cyclohexane, carbon tetrachloride, benzene, 1,2-dichlorobenzene, dichloromethane and 1,2-dichloroethane. It also gives the values of the solvent polarity parameter Ej [103], which increases in the above order from cyclohexane to 1,2-dichloroethane. 

In summary, the correlation of hydrogen-bond affinity and basicity scales exhibits a scatter caused by family-dependent relationships and steric effects. For unhindered bases, the scatter can be analysed into a series of nearly parallel lines for (i) amines, (ii) pyridines, (iii) polar bases and (iv) jt bases. 

Basicity in the gas phase is measured by the proton affinity (PA) of the electron donor and in solution by the pAj,. A solution basicity scale for aldehydes and ketones based on hydrogen bond acceptor ability has also been established [186]. Nucleophilicity could be measured in a similar manner, in the gas phase by the affinity for a particular Lewis acid (e.g., BF3) and in solution by the equilibrium constant for the complexation reaction. In Table 8.1 are collected the available data for a number of oxygen systems. It is clear from the data in Table 8.1 that the basicities of ethers and carbonyl compounds, as measured by PA and p , are similar. However, the nucleophilicity, as measured by the BF3 affinity, of ethers is greater than that of carbonyl compounds, the latter values being depressed by steric interactions. 

A new acidity scale has been developed based on calorimetric measurement of A-methylimidazole and A-methylpyrrole in bulk solvents.A revised version of this method was shown to give better results in some cases.Another scale of solvent acidities was developed based on the hydrogen-bond donor acidities in aqueous DMSO. It is noted that bond energies, acidities, and electron affinities are related in a thermodynamic cycle, and Kass and Fattahi have shown that by measuring two of these quantities the third can be found. " ... 

C. Laurence and J.-F. Gal, Lewis Basicity and Affinity Scales Data and Measurement, John WUey Sons, New York, 2010, is an outstanding reference with respect to both tabulated data and discussion of methodology. G. GUh and P. GUh, The Nature of the Hydrogen Bond, Oxford University Press, New York, 2009, provides an in-depth discussion of this topic. For an excellent review of superbases Superbases for Organic Synthesis Guanidines, Amidines, and Phosphazenes and Related Organocatalysts, Ishikawa, T., eds., Wdey, New York, 2009. 

There is the possibility of hydrogen bonding and multiple interactions of the adsorbed base with the zeolite. The case of adsorbed water is an interesting situation where the distinction between hydrogen bonded and protonated species is difficult to make. The initial formation of an hydrogen bonded complex may be followed by proton transfer from the acidic site to the adsorbate . As indicated earlier, the coupling of the acidic proton of the zeolite with its environment in the framework as well as the electronic structure of the adsorbate base ( proton affinity ) makes it rather difficult to establish a unique acidity scale based simply on (0(O-H) stretching frequencies. 

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