Conformation minimization

Compare energies for both diaxial and diequatorial chair conformers of trans-2-fluoromethyIcyclohexane (X = Me). Which conformer is preferred Examine a space-filling model of each conformer. Which group is largest methyl, fluorine, or hydrogen Which is smallest Does the preferred conformer minimize steric repulsion Explain. 

The consequences with respect to the corresponding thietane dioxides are straightforward in the trans-isomer, 187a, one phenyl group (i.e. R1) is necessarily axial, whereas in the isomer 187b both substituents are equatorial (equation 76). Clearly these preferred conformations minimize the potential repulsive interaction between 1,3-diaxial substituents66. 

Table III. Gentiobiose Conformations Crystal Conformation Minimized in Four Potential Energy Functions...

In toluene (LXX), like in a number of analogs, the preferred conformation shows eclipsing between a hydrogen atom and the w-system, as represented in diagram LXX. This conformation minimizes a- and w-electron loss to the 7r-system (hyperconjugation) and therefore allows maximal one-electron interaction [80]. Toluene is characterized by an extremely low rotation barrier (a few cal/mol) due to the fact that this barrier is sixfold, and the latter is always found to be extremely small. Indeed, an equivalent conformation is found after every 60° rotation, and no real relief of conformational strain is obtained after a rotation of only 30°. 

The only unsaturated heterocycles for which a structure has been calculated by the method of molecular mechanics are silacyclopent-2-ene and silacyclopent-3-ene where all conformers minimized to the planar form (330). The far-IR spectra are consistent with a planar conformation (198, 199). 

These special stereochemical properties arise from the steric influence of the diortho iodines upon the diphenyl ether conformation. Minimal steric interaction between the 3,5-iodines and the 2, 6 -hydrogens is maintained when one ring is coplanar with, and the other perpendicular to, the plane of the two C-0 ether bonds. This gives rise to two skewed conformations (Figure 2) which can be described by the torsion angles < > (C5-C4-041-C11) and (J)1 (C4-041-C1,-C6 ) of 0°/90° and 90°/0° for /cj>1, respectively. Only the skewed conformer = 90°/0° has been ob-... 

Figure 15.6 Depicting cycloalkanes. Cycloalkanes are usually drawn as regular polygons. Each side is a C—C bond, and each corner represents a C atom with its required number of H atoms. The expanded formulas show each bond in the molecule. The ball-and-stick and space-filling models show that, except for cyclopropane, the rings are not planar. These conformations minimize electron repulsions between adjacent H atoms. Cyclohexane (D) is shown in its more stable chair conformation.

In most systems, however, more complicated situations are encountered. Due to different strains of rings in cyclic oligomers, their distribution is different from that predicted by the JS theory and those oligomers that may assume conformation minimizing the strain are preferentially formed. This is the case of cationic polymerization of oxiranes. In the... 

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