Experimental Determination of the Donor Number

Heats were measured by thermometry in an adiabatic calorimeter [1] by breaking a sealed glass ampoule containing the pure base, (i) first in the pure solvent and (ii) second in a solution of excess SbCls. The first measurement gives the enthalpy of solution of the base in the solvent, AH 23), which is generally small and may be positive or negative. It 

Typically, the reaction mixtures in the calorimeter were 1,2-dichloroethane solutions containing 0.01-0.03 mol 1 of B and usually about 0.5 mol 1 of SbCls. In cases where complexation was not complete in the concentration range used, it was necessary to determine the equilibrium constant, K. Both the enthalpy of complexation and the equilibrium constant could be calculated simultaneously from the calorimetric data [6]. K values could also be determined independently by a spectrometric method [14]. 

Over 100 SbCls affinity values are collected in Table 2.2. The sample of Lewis bases contains mainly oxygen bases and, among these, mainly carbonyl compounds. Only nine nitrogen bases (seven nitriles, one pyridine, one amine) have been studied. The scale lacks second-row bases, carbon bases and halogen bases. We have not reported the values estimated indirectly from correlations between DN and solvent basicity-dependent properties, such as nuclear magnetic resonance chemical shifts of the Na nucleus [27] or of the chloroform proton [28], These values would be valid only if there was no separation into families of bases in the correlation chart, which is rarely found. As shown in this book, family-dependent correlations between basicity-dependent properties are generally the norm. 

When different values are given by different authors for the same base, we have selected the value that we consider the most reliable on the basis of critical discussions in the literature. For example, Gutmann s DN values for weak bases seem underestimated because, apparently, no correction was made to take into account the dissociation of the complex. Such is the case of nitrobenzene the DN value (converted into kJ moP ) of 18.4 is too low compared with the value of 34 kJ moP given by Olofsson [11], The latter value includes the equilibrium constant = 20 moP in determining the equilibrium concentration of the complex C6H5N02-SbCl5 [11], while Gutmann s value is calculated under the incorrect assumption that SbCls is totally complexed. 

The formation of these ions certainly contributes to the total heat quantity measured. Consequently, we have selected the lower values. In any case, the DN values of phosphoramides must be used with great caution. 

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