4- fluorophenol hydrogen-bond enthalpies

Table 4.28 Comparison between values of hydrogen-bond enthalpies (kj moD ) of 4-fluorophenol In CCI4 obtained by the van t Hoff equation over different temperature intervals.

Table 4.29 Hydrogen-bond enthalpies (kj mol j of 4-fluorophenol in CCI4. Agreement between values determined by the van t Hoff equation (vH) and calorimetric values (cal). 

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... 

A new procedure is implemented for an accurate and fast determination of the hydrogen-bond enthalpy. In this method, a single solution of cyclopropylamine and 4-fluorophenol is studied by measuring its absorbance change as a function of temperature. 

In the course of this study, the authors determined /Lvalues for dibenzyl, methyl phenyl, methyl p-nitrophenyl, di-p-tolyl, di-isopropyl and tetramethylene sulphoxides and for diethyl, dipropyl and dibutyl sulphites. The /Lscales are applied to the various reactions or the spectral measurements. The /Lscales have been divided into either family-dependent (FD) types, which means two or more compounds can share the same /Lscale, family-independent (FI) types. Consequently, a variety of /Lscales are now available for various families of the bases, including 29 aldehydes and ketones, 17 carboxylic amides and ureas, 14 carboxylic acids esters, 4 acyl halides, 5 nitriles, 10 ethers, 16 phosphine oxides, 12 sulphinyl compounds, 15 pyridines and pyrimidines, 16 sp3 hybridized amines and 10 alcohols. The enthalpies of formation of the hydrogen bond of 4-fluorophenol with both sulphoxides and phosphine oxides and related derivatives fit the empirical equation 18, where the standard deviation is y = 0.983. Several averaged scales are shown in Table 1588. 

FIGURE 15. Solvatochromic hydrogen-bond shifts for 4-nitrophenol (longest wavelength ii n transition) in HBA solvents plotted against the enthalpy of hydrogen-bond formation of 4-fluorophenol in pure HBA solvents ... 

The earlier one was made by Arnett and co-workers [10,11], using two different methods. The first is referred to as the high-dilution calorimetric method . In this case, a small quantity of base is injected into a dilute CCU solution of 4-fluorophenol. The observed heat of reaction is corrected for the heat due to the solution of the base in pure CCU. It is related to the enthalpy of hydrogen-bond formation (AH°) by the expression... 

In this alternative method, referred to as the pure-base calorimetric method , the base is used as the solvent, so complete complexing of the 4-fluorophenol and base can usually be assured. The two heat terms involved are (i) the heat due to the hydrogen-bond formation and (ii) the heat produced by other interactions. The latter contribution is obtained by using 4-fluoroanisole, which resembles 4-fluorophenol in all respects except in its ability to hydrogen bond. The enthalpy is determined through Equation 4.16 by measuring heats of solution, AHs, for 4-fluorophenol and 4-fluoroanisole in the pure base and the reference inert solvent CCI4. 

Table 4.27 Determination of the enthalpy of hydrogen bonding of trimethyl phosphate (initial mole fraction 6.5 x to 4-fluorophenol (initial mole fraction 4 x W ) in CCI4.

Table 4.30 Enthalpies (kj mol j and entropies (J K mol j of hydrogen bonding between 4-fluorophenol and Lewis bases in CCI4 (C2CI4,... 

Figure 4.8 Limited linear enthalpy relationship for formation of hydrogen-bonded complexes of cyanoacetylene and 4-fluorophenol. The upper line represents n bases (benzene, mesity-lene, hexamethylbenzene), the middle line sulfur (tetrahydrothiophene), Nsp (acetonitrile) and O (ethers, CO, SO, PO) bases, and the lower line pyridine and tertiary amine bases.

Table 4.33 Family-dependent relationships between enthalpies and Cibbs energies for the hydrogen bonding of 4-fluorophenol to Lewis bases, in CCI4 at 25 °C.

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