The 4-Fluorophenol Affinity Scale

In Section 4.3, we reported the equilibrium constants for the hydrogen bonding of 4-fluorophenol to Lewis bases in CCU at 25 °C. These values allow the calculation of the Gibbs energy of this reaction, AG°, by means of Equation 4.20  

AG° is determined by AH°, AS° and T, which are the enthalpy, entropy and absolute temperature of the hydrogen-bond formation, respectively. To understand the thermodynamics of hydrogen-bond basicity, one must know whether this basicity is controlled by enthalpy, by entropy or by both. If it is the last case, the respective contributions of the enthalpic and entropic terms need to be determined. 

Unlike Arnett and Catalan, Laurence el al. chose to measure the enthalpy of hydrogen bonding from the temperature dependence of the equilibrium constant. The variation of an equilibrium constant with temperature is given by Equation 4.22  

At a given temperature, the slope of the graph of InAi versus T multiplied by —R equals AH° at that temperature. If AH° is essentially constant over the temperature range plotted, the graph is a straight line. 

The FTIR spectra of such a single solution are recorded at five temperatures between —5 and +55 °C. The method requires the preliminary determination of the temperature dependenee of the absorption coefficient, b, of the i (OH) IR band. This quantity falls off linearly with increasing temperature according to Equation 4.24, in which the temperature t is in °C  

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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... 

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. 

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. 

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 ... 

This chapter is intended to provide detailed examples of the spectroscopic and thermodynamic determination of most Lewis basicity scales presented in the previous chapters, namely the BF3 affinity scale, 4-fluorophenol basicity and affinity scales, the methanol infrared (IR) shift scale, the 4-nitrophenol solvatochromic shift scale, diiodine basicity and affinity scales, the iodine cyanide IR shift scale, the diiodine blue shift scale and the lithium cation basicity scale. With these examples, it is hoped that professional chemists, and also students of physicochemical sciences, will be able to supplement the scales for the molecules in which they are interested. 

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