Butane gauche conformation

The trans conformation corresponds to a torsion angle of 180°, the gauche(+) conformation to oni + 60° and the gauche -) conformation to -60°. These approximately correspond to the torsion anj of the three minimum energy conformations of butane. 

Figure 4-18 The Potential Energy for Rotation of n-Butane About its Central Bond Axis. The anti conformer in the center is slightly lower in energy than the two gauche conformers.

FIGURE 3 6 The gauche and anti conformations of butane shown as ball and spoke mod els left) and as Newman projections right) The gauche conformation is less stable than the anti because of the van der Waals strain between the methyl groups... 

Higher alkanes having unbranched carbon chains are like butane most stable m then-all anti conformations The energy difference between gauche and anti conformations is similar to that of butane and appreciable quantities of the gauche conformation are pres ent m liquid alkanes at 25°C In depicting the conformations of higher alkanes it is often more helpful to look at them from the side rather than end on as m a Newman projec tion Viewed from this perspective the most stable conformations of pentane and hexane... 

We have seen that alkanes are not locked into a single conformation Rotation around the central carbon-carbon bond m butane occurs rapidly mterconvertmg anti and gauche conformations Cyclohexane too is conformationally mobile Through a process known as ring inversion, chair-chair mterconversion, or more simply ring flipping, one chair conformation is converted to another chair... 

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

Gauche conformation (Section 3.7) The conformation of butane in which the two methyl groups lie 60° apart as viewed in a Newman projection. This conformation has 3.8 kj/mol steric strain. 

Gasoline, manufacture of, 99-100 octane number of, 100 Gatterman-Koch reaction. 596 Gauche conformation, 95 butane and, 95-96 steric strain in, 96... 

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. 

So butane molecule has a higher population of the anti-conformer and the population of the two gauche conformers is equal. 

A similar description applies to reaction of gauche w-butane leading to the more stable anti conformer. Again, the reaction coordinate may be thought of as a torsion about the central carbon-carbon bond, and... 

The more common case is, as depicted in the above diagram, where the energy of the products is lower than that of the reactants. This kind of reaction is said to be exothermic, and the difference in stabilities of reactant and product is simply the difference in their energies. For example, the reaction of gauche n-butane to anti n-butane is exothermic, and the difference in stabilities of the two conformers is simply the difference in their energies (0.9 kcal/mol or 3.8 kJ/mol). 

Chemists routinely manipulate physical models in an attempt to ascertain what actually occurs during a conformational change. A successful example of this is in showing first-time students of organic chemistry that interconversion between anti and gauche conformers of w-butane involves a simple rotation about the central carbon-carbon bond (see discussion in Chapter 1). Much less satisfactory is the attempt to show the interconversion of chair forms of cyclohexane. Here, computer animations provide a better alternative. 

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.

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