Cyclohexane strain energy

Make molecular models of the two chair conformations of cis 1 tert butyl 4 phenyl cyclohexane What is the strain energy calculated for each conformation by molecular mechanics Which has a greater preference for the equatorial onentation phenyl or tert butyD... 

If the heat of fonnation parameters are derived on the basis of fitting to a large variety of compounds, a specific set of parameters is obtained. A slightly different set of parameters may be obtained if only certain strainless molecules are included in the parameterization. Typically molecules like straight chain alkanes and cyclohexane are defined as strainless. Using these strainless heat of formation parameters, a strain energy may be calculated as illustrated in Figure 2.14. 

Figure 4.3 Cycloalkane strain energies, calculated by taking the difference between cycloalkane heat of combustion per CH2 and acyclic alkane heat of combustion per CH2, and multiplying by the number of CH2 units in a ring. Small and medium rings are strained, but cyclohexane rings are strain-free.

Diaxial interaction (Section 4.8) The strain energy caused by a steric interaction between axial groups three carbon atoms apart in chair cyclohexane. 

In marked contrast to the n-alkanes, the cycloalkanes exhibit thermodynamic properties where such regularities are no longer present. Heats of formation (AH ) for a substantial number of cycloalkanes are available from heats of combustion. With the exception of cyclohexane, AH°f is always more positive than the quantity — 4.926n. The difference between the two quantities leads to a quantitative assessment of the important notion of ring strain. The AH -values and strain energy data listed in Table 1 were taken from Skinner and Pilcher (1963). Other references give different but usually comparable... 

The spherical shape causes a pyramidalization of the C atoms and, therefore, a large amount of strain energy. Addition chemistry is driven by the strain reKef introduced by the formation of almost strain-free sp -C atoms. At a certain degree of addition this strain relief mechanism has to compete with new strain-introducing processes such as the increasing introduction of eclipsing interactions and the formation of planar cyclohexane substructures. 

The increased reactivity of cyclopropanes results from the presence of bent bonds which can interact with electrophiles, and can be more easily cleaved thermally than ordinary C C bonds. One indication of the consequences of the distortion is found in the strain energies (SE)37 that are calculated as the difference between the observed heat of formation and that estimated for a strain-free model. One might, for example, consider cyclohexane as strain-free, and then a model for cyclopropane would be half the heat of formation for cyclohexane. The available data for heats of formation of cyclopropane and cyclobutane derivatives are given in Table 2,38 The heat of formation of cyclohexane is — 29.4 kcal/mol, and the strain energy of cyclopropane is 12.7-0.5( — 29.4) or 27.5 kcal/mol. 

The closeness of the numerical values for the strain energies of cyclopropane and cyclobutane can be recast in numerous ways. Among the simplest are the differences of the enthalpies of formation of real cyclopropane and cyclobutane with that equated with 3/6 and 4/6 of that of cyclohexane. These two values are 115.2 and 110.7 kJmol"1. Equivalently, the strain energy of a species c-(CH2) is defined as the enthalpy of the gas-phase reaction 34 ... 

Axial strain energy (Section 6.7) The amount of destabilization caused by a group in the axial position in the chair conformation of cyclohexane. 

As mentioned before, the AHg, value for the polymerization of 6,8-dioxabicyclo [3.2.1] octane is less negative than that for l,3-dioxolanes. Bicyclo [3.2.1] octane, in which a five-membered ring is fused to a cyclohexane ring at two axial positions, has 51 kJ/mol strain energy. However, from the fact that 7-oxabicyclo [2.2.1] heptane has less strain energy than bicyclo [2.2.1] heptane can be deduced that the strain energy of 6,8-dioxabicydo [3.2.1] octane may also be less than that of bicyclo [3.2.1] octane. 

It must be emphasized that the ratio is independent of the value of the equilibrium constant for C5H11 + 02 C5Hn02. The relative amount of 1,2- to 1,4 isomer is, however, much larger than expected (Table 1.17) for alkyl radicals, k(l,4s)/k(l,6s) 0.07, compared with the value of [1,2 cyclohexane oxide]/[l,4 cyclohexane oxide] = 0.60 0.10. Walker and Gu-lati point out that although about 99.5% of the cyclohexane molecules adopt the chair form at 753 K, the O—O group at either an axial or an equatorial position in the chair form is not favourably structured for a low strain energy in the transition state, as shown by... 

The heats of formation are not convenient quantities for comparing compounds having different numbers of carbons. Here, the strain energies are much more useful. The simplest definition is the difference between the observed AH and that calculated using a strain-free model. In the case of cyclopropane, AHf = 12.73 kcal mol If cyclohexane is taken as strain-free, the expected AHf for cyclopropane is one-half that for cyclohexane, or —14.75 kcal mol The difference is 27.5 kcal mol and is the strain energy. The strain-free model is frequently derived from Franklin s group equivalents, and for common hydrocarbon groups they are in kcal mol ... 

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