Staggered conformations of butane

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

We have seen that Newman projections are a powerfnl way to show the different conformations of a molecule. We mentioned earlier that there are staggered conformations and eclipsed conformations. In fact, there are three staggered and three eclipsed conformations. Let s draw all three staggered conformations of butane. The best way to do this is to keep the back carbon atom motionless (so the fan in the back is not spinning), and let s slowly turn the groups in the front (only the front fan is spinning) ... 

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.

PROBLEM 2.9 Draw Newman projections for two different staggered conformations of butane (looking end-on at the bond between carbon-2 and carbon-3), and predict which of the two conformations is more stable. (If you have a model kit, build a model of butane to help you visualize the Newman projections.)... 

Two of the three staggered conformations of butane exhibit gauche interactions. 

The two staggered conformations of butane differ in energy by 3.8 kj mole . The anti conformation is more stable, and at 298 K, the ratio of anti to gauche conformation is about 2 1. The anti and gauche conformations interconvert about 10 times per second at room temperature. 

Figure 3.7 illustrates the potential energy relationships fflnong the various conformations of butane. The staggered conformations are more stable than the eclipsed. At any instant, almost all the molecules exist in staggered conformations, and more are present in the anti conformation than in the gauche. The point of maximum potential... 

Two groups, the methyl groups in this case, are said to be anti if the dihedral angle between them is I80c. The and conformation is the most stable conformation of butane because it is staggered and has the two large methyl groups as far apart as possible. 

Conformation and Conformational Analysis Conformation of Ethane Conformation of Propane Conformation of Butane Eclipsed and Staggered Eorms Ring Strains in Cycloalkanes Principles of Conformation 165... 

In Sec. 3.5, we saw that there are several different staggered conformations of /i-butane, each of which lies at the bottom of an energy valley—at an energy minimum—separated from the others by energy hills (see Fig. 3.4, p. 79). Different conformations corresponding to energy minima are called conformational isomers, or conformerS. Since conformational isomers differ from each other only in the way their atoms are oriented in space, they, too, are stereoisomers. Like stereoisomers of any kind, a pair of conformers can either be mirror images of each other or not. 

Consider the two conformations of butane shown in Figure 28-8. Again, staggered conformations are slightly more stable than eclipsed ones. At room temperature many conformations are present in a sample of butane. 

Figure 2.4 Newman projections of the eclipsed and staggered conformations of a n-butane molecule.

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