Vaporization enthalpies, CHLP protocol

In attempting a systematic analysis of related organolithium species, it would be advantageous to consider all of them in at least one common phase. Our preference is for the gas phase where intermolecular interactions are usually absent, although this may not hold true for organolithium species. Unfortunately, most of the desired phase change enthalpies for organolithiums are not found in the experimental literature. However, where the species of interest is a liquid, we can employ the published CHLP protocol for the estimation of the enthalpy of vaporization. ... 

Where enthalpies of vaporization are not otherwise available, the CHLP protocol is used to estimate these quantities according to equation 1. The quantities Wc and q refer respectively to the number of quaternary (non-hydrogen-bearing sp -hybrid) carbon... 

There are enthalpies of formation for two phases of cumyl perbenzoate. However, the associated sublimation enthalpy that interconnects these phases, 43 kJ mol , is much too small. In that the enthalpy of sublimation must exceed the enthalpy of vaporization, and the latter is at least 75 kJmol using the CHLP protocol (from the number of carbons alone), the enthalpy of formation data loses credibility. Furthermore, lacking the enthalpy of formation of the corresponding cumyl benzoate (in any phase) disallows comparison. This would seem to be an altogether normal species until it is recognized that the archetype arylcarboxylate ester, methyl benzoate, has provided complications for the calorimetrist . 

A simple method for estimating enthalpies of vaporization is the CHLP protocol . The quantities and req in equation 1 refer respectively to the number of quaternary carbon atoms and non-quaternary carbon atoms in the compound, and is a value that is characteristic of the functional group bonded to the hydrocarbon parent. 

J. S. Chickos, D. G. Hesse, J. F. Liebman and S. Y. Panshin, J. Org. Chem., 53, 3435 (1988). In the absence of a literature value suggested for a desired enthalpy of vaporization for a substituted hydrocarbon, this CHLP protocol will be used implicitly to estimate the desired quantity. 

J. S. Chickos, D. G. Hesse, J. F. Liebman and S. Y. Panshin, J. Org. Chem., 53, 3435 (1988). In the absence of a literature value suggested for a desired enthalpy of vaporization for a substituted hydrocarbon, this CHLP protocol will be used implicitly to estimate the desired quantity. We acknowledge that methylcyclopropane is no less a substituted cyclopropane as is methyl cyclopropanecarboxylate. However, for the former species we use the hydrocarbon equation 3 with Pic = 4, and for the latter species we use the substituted compound equation 4 with Z = 10.5 (for esters) and fic= 5 (remember the carbonyl carbon in calculating the number of nonquaternary carbons). 

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