Gauche conformation, of butane

When the methyl group is axial each path mimics the gauche conformation of butane... 

It may be observed that the gauche conformation of butane (L) or any other similar molecule is chiral. The lack of optical activity in such compounds arises from the fact that L and its mirror image are always present in equal amounts and interconvert too rapidly for separation. 

Figure 5-10 Sawhorse and Newman conventions for showing the staggered conformations of butane. Only one gauche form is shown. Cyclohexane is shown to emphasize the resemblance of its stable conformation to the gauche conformation of butane.

This is the common definition of conformers. The IUPAC definition also requires that a conformer correspond to a distinct potential energy minimum, such as the anti and gauche conformations of butane. 

It is calculated that in aqueous solution, as simple ideas of hydrophobicity would predict, there is proportionately more of the gauche conformation of butane. However, it is simplistic even when discussing a butane molecule in neat butane, and far less so in water, to talk in terms of enthalpy and to ignore entropy. 

The gauche conformation of butane already shows conformational distortions in that the C—C—C—C dihedral angle is opened to about 70°, while the methyl group hydrogens are calculated to be skewed coherently by about 4.6°. The rotational barrier to anti-gauche interconversion derived spectroscopically is 3.63 kcal mol and as to the methyl eclipsing-methyl barrier, there has been considerable speculation described recently by Allinger and coworkers", who conclude it to be 4.89 kcal mol... 

The steric energy of the minimized gauche conformation of butane is found by this calculation to be more than 0.6 kcal/mol lower than the initial conformation. Note that the C—C—C—C dihedral angle is no longer 60° but is now 65° and that a torsional strain of 0.366 kcal/mol is associated with the larger dihedral angle. Note also that the van der Waals strain decreases from 2.42 kcal/mol in the 60 conformer to 1.75kcal/mol in the 65 conformer, and most of the other strain parameters are also less in the 65° conformer. This example indicates two important points ... 

Use Newman projection formulas for ethylene glycol (1,2-ethanediol) and butane to explain why the gauche conformer of ethylene glycol is expected to contribute more to its ensemble of conformers than would the gauche conformer of butane to its respective set of conformers. 

Gauche confonnation (Section 4.9) A gauche conformation of butane, for example, has the methyl groups at an angle of 60° to each other ... 

One of two equivalent energy-minimized gauche conformations of butane. 

Consider the rotation about the C(2)—C(3) bond in butane discussed in Section 3.2 and its potential energy diagram shown in Figure 3.7. As calculated by molecular mechanics, E, n for the anti and gauche conformations of butane are -21.2 and... 

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