Group increments corrections

These problems illustrate the danger of quantitatively extrapolating notions of conformational analysis based on very simple systems such as butane to more complicated structures. This caveat is true both in thinking about structures in general and in applying group increments correction factors. The correction factors are the best possible estimate, but we must be wary of their use. Another important example of this concerns -pentane and related structures, which we consider next. 

Strain Energies and Group Increment Correction Factors for Cycloalkanes, (CH2> (in kcal/mol)... 

After examination of Figure 2.17, what would you calculate as a proper group increment correction for the placement of two /-butyl groups cis in an alkene Assume no strain correction needed for the alkane you need to consider to answer the question. 

Heats of formation can be estimated with reasonable accuracy by additivity of group increments and corrections for ring effects. 

Schafer, L., C. Van Alsenoy, and J. N. Scarsdale. 1982. Estimates for Systematic Empirical Corrections of Consistent 4-21G Ab Initio Geometries and Their Correlations to Total Energy Group Increments. J. Mol. Struct. (Theochem) 86, 349-364. 

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. 

The acid-catalyzed ester hydrolysis provides a good target for MM treatments. DeTar first used hydrocarbon models in which an ester was approximated by an isoalkane (74) and the intermediate (75) by a neoalkane (76). He assumed that if the rate of reaction truly is not influenced by polar effects but is governed only by steric effects of R, as has been generally postulated, the rate must be proportional to the energy difference (AAH ) between 74 and 76. The AAH f is mainly determined by the van der Waals strain in these branched alkanes. Nonsteric group increment terms were carefully adjusted, and statistical mechanical corrections for conformer populations... 

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. 

The function Ym (like Yg) has the dimension (K kg moF1). The group increments for this function could be derived from the available literature data on crystalline melting points of polymers, totalling nearly 800. The quantity Ym (like Yg) does not show simple linear additivity due to intra- and inter-molecular interactions between structural groups. The available group contributions and their structural corrections are summarised in Table 6.8. We shall again discuss these data step by step. 

Calculated from group increments in Table 1.7 without correcting for gauche interactions. "Data from the previous column corrected for gauche interactions. See Table 1.7 and Figure 1.4. 

Now if we consider the heat of formation of 4-methyldiamantane (compound 7), we can see that we have simply added a bridghead adamantane-like methyl into a very similar environment in diamantane. Since the methyl group increments are additive in the adamantane case, we would expect that the number should carry over equally well to the 4-methyldiamantane example. And indeed this is found to be trae (Table 11.4). Accordingly, we can now be fairly well convinced that these adamantane/diamantane numbers mentioned are all correct to within the stated experimental errors. 

In this second empirical approach, which has also been used for C NMR spectra, predictions are based on tabulated chemical shifts for classes of structures, and corrected with additive contributions from neighboring functional groups or substructures. Several tables have been compiled for different types of protons. Increment rules can be found in nearly any textbook on NMR spectroscopy. 

Sets of empirical rules, often referred to as Woodward s rules or the Woodward-Fieser rules, enable the absorption maxima of dienes (Table 7.11) and enones and dienones (Table 7.12) to be predicted. To the respective base values (absorption wavelength of parent compound) are added the increments for the structural features or substituent groups present. When necessary, a solvent correction is also applied (Table 7.13). 

In order to choose the worst conditions, possible substances that are not listed in Part Three were taken, ie substances that were not used for self-ignition increment calculations by regression. Indeed, choosing as examples substances from the database, the results are almost too good to be used in a demonstration. These new substances come from the publication by Hilado quoted before. The table below gives the list of compounds, the estimated ATTest. the AIT found in the publication AITexp, the list of group numbers taken into account for the calculation as well as cyclic corrections and corrections of potential positions. There is no ketone in the list since they were mentiored in the text. 

The liquid enthalpy of formation difference between 1-hexyl and 1-heptyl hydroperoxides is almost twice that of a normal enthalpy of formation methylene increment of about 25 kJmol . But which of these two, if either, is correct For hydrocarbon snb-stituents bonded to electronegative functional groups, the secondary isomers are more stable than the n-isomer. Accordingly, either the 1- or 4-heptyl hydroperoxide, or both, have an inaccurate enthalpy of formation because the primary isomer is reported to have the more negative enthalpy of formation. All of the enthalpies of formation for the Cg and C7 hydroperoxides cited in Reference 2 come from a single source. There is a reported value for the gas phase enthalpy of formation of fert-butyl hydroperoxide that is 11 kJ mol less negative than the value in Reference 2. 

For cyclohexadienol 10 for example the value of AGf(aq) may be estimated from the corresponding value for cyclohexadiene —39.5 (AGOH for EtOH) — 1.2 (the increment <5AGOH for cyclohexen-3-ol), plus a correction of 1.0 kcal moF1 for the unfavorable inductive interaction between the OH group and the remote double bond of the dienol. In so far as AGr(aq) for benzene is known, pAjqo for addition of water to benzene [Equation (16)] can now be estimated as 22.2. 

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