Hydrogen-bond affinity scales 4-fluorophenol

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. 

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. 

A number of scales proposed in the literature for measuring the hydrogen-bond basicity (affinity) of Lewis bases are summarized in Table 4.1. In this book, we will select four of them. The first is a thermodynamic scale of hydrogen-bond basicity built from the equilibrium constant of the reaction 4.2 of 4-fluorophenol with a series of Lewis bases in CCI4 at 25 °C ... 

The applicability of the 4-fluorophenol affinity scale to other hydrogen-bond donors is now... 

However, in the case of CH donors and pyrrole, a weak NH donor, the correlation is limited by base type. In Figure 4.8, the hydrogen-bond enthalpies of HC=CC=N are plotted versus the 4-fluorophenol affinity scale in CCI4. The results may be described by three roughly parallel lines the upper line for n bases, the middle line for sulfur, oxygen and Nsp bases and the lower line for Nsp and Nsp bases. This line separation amounts to about 5 kJ mol between the middle and lower lines. Thus, whereas the cyanoacetylene affinities... 

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