Hydrogen-Bond Affinity Scales Early Studies

4 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 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 

NMR and/or IR spectrometry. Forty values were obtained in this way, spanning from —8.0 1.7 kJ mol for diphenyl ether to -39.8 0.8 kJ mol for quinuclidine. Values of AH° determined by this dilution calorimetric method are very sensitive to errors in K, especially if Kc is low. In order to sidestep the determination of Kc, a different approach was also used. 

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. 

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