Torsional strain, strained hydrocarbons

The relative reactivities toward diimide cover a range of -10, from 1,2-dimethylcyclohexene to norbornene (ref. 6). Electron attractive substituents increase the reactivity of the double bond towards diimide although the data to place compounds such as maleic acid or acrylonitrile on the scale for Garbisch s hydrocarbons is lacking (ref. 21b). Garbisch et al. found that the main factors that contribute to the observed reactivities in diimide reductions of unsaturated hydrocarbons, eqn. (3), are torsional strain, bond angle... 

Cyclohexane, the most important cyclic hydrocarbon, is most stable in the chair conformation, which has very little angle and torsion strain. Hydrogens occupy equatorial and axial positions. 

Cyclopropane possesses both significant angle and torsion strain. Strain in saturated cyclic hydrocarbons is determined from the heat of combustion, and comparisons are made per CH2 group in a molecule. For cyclopropane the molar enthalpy change of combustion is 2090 kJ mol-1, i.e. 696.5 kJ mol-1 per CH2 group. The corresponding figure for cyclohexane is 3948 kJ mol-1, i.e. 658 kJ mol-1 per CH2. If cyclohexane is taken to be strain free (which is a reasonable approximation), one can conclude that the strain in cyclopropane is (696.5 - 658) = 38.5 kJ mol-1 per CH2. It should be noted that these experimentally deter-... 

This chapter deals exclusively with the structural properties of ring compounds. Two kinds of destabilizing effects on rings are discussed. Torsional strain, the destabilizing effect of eclipsed carbon-hydrogen bonds, was mentioned in Chapter 2 when the acyclic hydrocarbons were discussed. The planar forms of ring... 

Vq can be taken as being equal to the ethane barrier (2.S-2.9 kcal/mol). The potential energy diagram for rotation about the C-C bond of ethane is given in Fig. 3.1. The ethane barrier may be taken as a standard rotational barrier for acyclic hydrocarbons when analyzing the contribution of torsional strain to the total steric strain. The stereoelectronic origin of the ethane barrier was discussed in Chapter 1. Any steric interactions which are present in more highly substituted systems will make an additional contribution to the barrier." ... 

The classical treatment for the quantitative determination of the steric effects operative in molecules was developed by Westheimer. Steric effects were considered as the sum of various independent strain producing mechanisms (bond strain, angle strain, torsional strain, non-bonded interaction strain). Westheimer s assumptions proved to be the fundamental basis for the BIGSTRN program as well as for all subsequent molecular mechanics treatments of neutral hydrocarbons and carbocations. Reactivities ranging over 10 ° could be correlated by the strain differences between cation and the neutral precursor. Gleicher and Schleyer s work was a historical breakthrough in the development of molecular mechanics and provided the basis for the predictions of rate constants of solvolysis reactions. For the first time chemical reactions could reliably be predicted by the means of computational chemistry. 

However, energies are just as crucia 1, and these are sometimes harder to come by. For hydrocarbons and simpler organics, there is a large database of heats of formation, and, hence, strain energies, and these are valuable in parameterization. Other energies include rotation barriers and conformational differences. A competent force field should reproduce the butane torsional profile of Figure 2.6, and should obtain the A values for many cyclohexane substituents. Due to the similarity to real molecular vibrational modes, IR vibrations should be a valuable source for a force field, but in practice few modern force fields use them in their parameterization. 

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