Butane anti conformation

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 7 illustrates the potential energy relationships among the various confer matrons of butane The staggered conformations are more stable than the eclipsed At any instant almost all the molecules exist m staggered conformations and more are present m the anti conformation than m the gauche The point of maximum potential... 

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 can relate the conformational preference for an equatorial methyl group m methylcyclohexane to the conformation of a noncyclic hydrocarbon we discussed ear her butane The red bonds m the following structural formulas trace paths through four carbons beginning at an equatorial methyl group The zigzag arrangement described by each path mimics the anti conformation of butane... 

Section 3 2 The two staggered conformations of butane are not equivalent The anti conformation is more stable than the gauche... 

Anti conformation of butane Gauche conformation of butane... 

Example The reaction coordinate for rotation about the central carbon-carbon bond in -butane has several stationary points. A, C, E, and G are minima and B, D, and Fare maxima. Only the structures at the minima represent stable species and of these, the anti conformation is more stable than the gauche. 

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... 

A Fourier series is an example of an orthogonal polynomial, meaning that the individual terms which it comprises are independent of each other. It should be possible, therefore, to dissect a complex rotational energy profile into a series of N-fold components, and interpret each of these components independent of all others. For example, the onefold term (the difference between syn and anti conformers) in /7-butane probably reflects the crowding of methyl groups. 

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