Benson increments

This is painfully unlike the case of enols, wherein a tentative set of Benson increments has been developed [F. Turecek and Z. Havlas, J. Org. Chem., 51, 4066 (1986)]. This is not particularly surprising—the collection of thermochemically characterized classical and thus unburied enols is much more coherent than that of enamines [see J. P. Guthrie, in The Chemistry of Enols (Ed. Z. Rappoport), Wiley, Chichester, 1990]. The reader is addressed, however, to S. W. Slayden and J. F. Liebman, in Supplement E The Chemistry of Hydroxyl, Ether and Peroxide Groups, Vol. 2 (Ed. S. Patai), Wiley, Chichester, 1993, for a brief excursion into the complications that arise in the study of more general enols. 

Interestingly, the product is head-to-head , i.e. it has one new C—C and Sb—Sb bond, as opposed to two new C—Sb bonds. Assume we were convinced of the more or less invariance of the Sb—Sb bond enthalpy and that we had a reliable value for the —Sb bond enthalpy (say from a more trustworthy measurement of the enthalpy of formation of triphenylstibine ). With judicious use of Benson increments and strain corrections, one could thus estimate the enthalpy of formation of the dimer, and accordingly derive a trustworthy value for the monomer. We would then have a handle on the aromaticity of antimonin. The enthalpy of reaction of bismin, CjHjBi, to form the analogous dimer has likewise been derived to be — 50kJmoCL However, this does not particularly help us since elemental bismuth is metallic, i.e. it is illogical to consider that Bi (s) is held together by covalent Bi—Bi bonds. 

See many of the other volumes in this series for a discussion of relevant Benson increments or studies by S. W. Benson himself. A good summary, but now some 15 years old, is his volume Thermochemical Kinetics Methods for the Estimation of Thermochemical Data and Rate Parameters, 2nd ed., Wiley, New York, 1976. In particular, see Benson s review of the thermochemistry of sulphur compounds, Chem. Rev., 78, 23 (1978). 

Figure 7-1 shows the groups that are obtained for alkanes, and the corresponding notation of these groups as introduced by Benson [Ij. Table 7-2 contains the group contributions to important thermochemical properties of alkanes. Results obtained with these increments and more extensive tables can be obtained from Refs. [1] and [2]. 

Enthalpy of Formation The ideal gas standard enthalpy (heat) of formation (AHJoqs) of chemical compound is the increment of enthalpy associated with the reaction of forming that compound in the ideal gas state from the constituent elements in their standard states, defined as the existing phase at a temperature of 298.15 K and one atmosphere (101.3 kPa). Sources for data are Refs. 15, 23, 24, 104, 115, and 116. The most accurate, but again complicated, estimation method is that of Benson et al. " A compromise between complexity and accuracy is based on the additive atomic group-contribution scheme of Joback his original units of kcal/mol have been converted to kj/mol by the conversion 1 kcal/mol = 4.1868 kJ/moL... 

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. 

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. ... 

We wish to emphasize that there are so few thermochemical data for classical (as opposed to buried ) enamines that few generalizing principles exist to interrelate these species with any other class of molecules. As such, paralleling our review8 of another class of substituted doubly bonded systems (the enones) we plead insufficient data and will omit Benson-like group increment analysis9,10. Instead we will use enthalpies of formation of related species11 to provide comparison12 with the entries in Table 1. 

The C(F)2(CF2)2 group increment allows evaluation of the strain energy in octa-fluorocyclobutane. There are three published experimental studies providing AHf (g) data for this compound The first value (taken from Pedley and Rylance s thermochemical archives ) is — 368.7 kcal mol" and is based upon reaction with sodium while the O Neal and Benson value ( — 367.8 kcal mol" if one employs the archival value for C2F4) is based upon the experimental equilibrium with tetrafluoroethylene. The third value, based upon combustion measurements, is - 365.2 kcal mol" if one employs archival values for the products. The strain energy is thus between 14.5 and 18 kcal mol" some 8.5-12 kcal mol" lower than in the parent hydrocarbon. Why is this value low One explanation may follow recent work by Wiberg . He concludes that a large part of the strain in cyclobutane is due to repulsion between non-bonded carbons and... 

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. ... 

Strain energies were estimated using Benson s standard group increments. 

Table 11 lists the AH s for the C H2n-6 benzenoid isomers. The values for C8-C12 were obtained utilizing Benson s group increments (25a) and agree well with experimental values. Empirical force field calculations can be used to obtain a strain energy for aromatic molecules but this is not necessary when group increment values are available. The best aromatic for each C was selected based on group increment estimatimi of AH from a list of candidates generated... 

These examples only hint at the analysis of heats of formation of organic compounds that is possible. Benson and co-workers summarized the methods and data for calculations for the major functional groups in organic chemistry. ° In addition, the data allow calculation of heat capacities and entropies of these compounds in the same marmer in which heats of formation are determined. Heats of formation are valuable reference points in discussing the stabilities of various isomers or products of reactions, whether they are calculated by bond increments or group increments or are derived as part of a theoretical calculation. 

If one looks at a molecule such as heptane, for example, one can add all of the appropriate increments and calculate the heat of formation with acceptable accuracy by the method previously described. But there are a few things that are really not proper about that kind of calculation. Heptane in the gas phase at 25°C (where heats of formation are defined) is actually a complicated mixture (a Boltzmann distribution) of a great many conformations, most of which have different enthalpies and entropies. Additionally, each of these conformations is also a Boltzmann distribution over the possible translational, vibrational, and rotational states. The Benson method works adequately for many cases like this because these statistical mechanical terms can be lumped into the increments and averaged out, and they are not explicitly considered. By adjusting the values of the parameters in Eq. (11.1) or (11.2), much of the resulting error of neglecting the statistical mechanics can be canceled out, or at least minimized, in simple cases. But we would like for this scheme to work for more complex cases too. As the system becomes more complicated, errors tend to cancel out less well. So let us go back and approach this problem in a more proper way. 

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