Benson group

The chemistry of carbenes in solution hits been extensively studied over the past few decades.1-5 Although our understanding of their chemistry is often derived from product analyses, mechanistic details are often dependent on thermodynamic and kinetic data. Kinetic data can often be obtained either directly or indirectly from time-resolved spectroscopic methods however, thermochemical data is much less readily obtained. Reaction enthalpies are most commonly estimated from calculations, Benson group additivities,6 or other indirect methods. 

The Benson group contribution method, and more recent methodologies, allow the computation of heat of hydrogenation reactions, even for large molecules (note that Benson method gives the reaction enthalpy assuming each species to be a perfect gas ). Software and database (e.g., NIST) are also available. 

By contrast, we do not use Benson group increments , a generally powerful thermochemical technique summarized in the volume by S. W. Benson himself, Thermochemical Kinetics, 2nd edition, Wiley, New York, 1976, and used in many thermochemical chapters throughout the Patai series. For the classes of compounds discussed in the current chapter, we believe the necessary number of parameters (included to reflect electrostatic interactions, proximity effects, steric repulsions and ring corrections) is excessive. 

A major difference with desired reactions is that the stoichiometry is often unknown, that is, the decomposition products are unknown. The reason is that decomposition reactions are often affected by the triggering conditions and thus often run along different reaction paths. This is a major difference compared to a total combustion, for example. The consequence is that the decomposition enthalpy cannot be predicted using standard enthalpies of formation AHjj taken from, for example, tables or estimated by group increment methods, such as Benson groups [3, 4] ... 

A well-known tool for the estimation of reactivity hazards of organic material is called CHETAH [5]. The method is based on pattern recognition techniques, based on experimental data, in order to infer the decomposition products that maximize the decomposition energy, and then performs thermochemical calculations based on the Benson group increments mentioned above. Thus, the calculations are valid for the gas phase, but this may be a drawback, since in fine chemistry most reactions are performed in the condensed phase. Corrections must be made, but in general they remain small and do not significantly affect the results. 

Simple Substitution On occasion, one may substitute one Benson group for another one of similar nature. For example, replacement of a doubly bonded carbon, Cd, in the group environment, for an aromatic carbon generally leads to only small errors ( 2 kcal/mol). Note that the central/main part of the group is the central atom and all univalent attachments. The environment of the group consists of all polyvalent attachments. Substitutions in the environment are allowable but the environmental changes should be for as similar an... 

This example shows how to determine the correct group value for AHf and S for C-(H)(Br)2(C) by considering known values in the sequence C-(H)3(C) to C-(Br)4. This example represents changes in the main/central group. Note that the entropy values for Benson groups such as C-(Br)4 have the symmetry contribution removed. The user must add in any symmetry contribution after the molecule is built with the complete set of Benson groups. For consistency in any interpolation scheme, one must remove the symmetry contribution from the entropy for the whole molecule. In this case, owing to the tetrahedral symmetry, an amount R In 12 (where R is the gas constant) was subtracted from the literature entropy value for C-(Br)4. 

A price of our analysis is that the enthalpy of formation of some of the CH3XYZCH3 species will have to be estimated. In principle, one could use Benson group increments [28] in lieu of these estimates but then, many of these increments would also have to be estimated because many compounds containing the groups are unknown, inadequately precedented, or even limited to but one species, the one of interest. Benson said in a related context to one of the authors (JFL) some decades ago If the universe was kind, your approach would agree with mine, and both would agree with the universe. ... 

The data necessary for thermodynamic estimates are available from experimental as well as computational methods. In many systems AGh can be approximated by experimentally accessible AGJ. The approximation is valid (to within 0.05-0.15 eV) if the radical coupling has no barrier (is diffusion limited) and the thermolysis is carried out under conditions selected to minimize the cage recombination [79]. The homolytic bond strengths can also be obtained in many cases from the Benson group-additivity tables [80] or semiempirical quantum or molecular mechanics calculations [81]. With appropriate entropy corrections [75f], relatively accurate AGh values can be obtained in that way. 

Table 10.1 Benson Group Contributions to Ideal-Gas Properties for Hydrocarbon Groups (copied w/permission from—see reference 8)...

VS c to c = 0. (Scheme 2) (c) from the direct measurement of the difference between the volumes of reactants and products employing dilatometry. To a first approximation the molar volume of neat liquid compounds (Tm = M/d) and, hence, the reaction volumes can be calculated with additive group increments which were derived empirically by Exner for many groups such as CH3, CH2, or CH from the molar volumes, Tm. easily determined from the known densities for many different types of compounds. This method is comparable to that of the calculation of enthalpies of formation by the use of Franklinor Benson group increments. In all cases where the volume of reaction could be determined by at least two independent methods, the data were in good agreement. ... 

Estimated using Benson group inciements[29] and estimated 4.0 kcal/mol strain. 

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