Conformational analysis of butane

Distribution of energies. The number of molecules with energy greater than fact at temperature 7i is shown as the darker green-shaded area. At some higher temperature T2, the curve is flatter, and more molecules have energies in excess of act- 

Sketch a potential energy diagram for rotation around a carbon-carbon bond in propane. Identify each potential energy maximum and minimum with a structural formula that shows the conformation of propane at that point. Does your diagram more closely resemble that of ethane or of butane Would you expect the activation energy for bond rotation in propane to be more than or less than that of ethane Of butane  

Chapter 3 Alkanes and Cycloalkanes Conformations and cis-trans Stereoisomers 

The gauche and anti conformations of butane shown as ball-and-spoke models [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. 

Acetylcholine is a neurotransmitter in the central nervous system in humans. Sighting down the C-1 to C-2 bond, complete the Newman projection formulas for the anti and gauche conformations of acetylcholine. 

Potential energy diagram for rotation around the central carbon-carbon bond in butane. 

An energy diagram showing the conformational analysis of propane. 

The energy cost associated with a methyl group eclipsing a hydrogen atom amounts to 6 kJ/mol. 

19 For each of the following compounds, predict the energy barrier to rotation (looking down any one of the C—C bonds). Draw a Newman projection and then compare the staggered and eclipsed conformations. Remember that we 

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

Conformers like these cannot be isolated except at extremely low temperatures. 

You should build a molecular model of butane and examine Its various conformations as we discuss their relative potential energies. 

If we consider rotation about the C2—C3 bond of butane, we find that there are six important conformers, shown as I-VI below  

Although the barriers to rotation in a butane molecule are larger than those of an ethane molecule (Section 4.8), they are still far too small to permit isolation of the gauche and anti conformations at normal tanperatures. Only at extremely low temperatures would the molecules have insufficient energies to surmount these barriers. 

We saw earlier (Section 2.16C) that dispersion forces can be attractive. Here, however, we find that they can also be repulsive, leading to steric hindrance. Whether dispersion interactions lead to attraction or to repulsion depends on the distance that separates the two groups. As two nonpolar groups are brought closer and closer together, the first effect is one in which a momentarily unsymmetrical distribution of electrons in one group induces an opposite polarity in the other. 

---

Conformational analysis of butane is a bit more complex than the conformational analysis of either ethane or propane. Look carefully at the shape of the energy diagram for butane (Figure 4.11), and then we will analyze it step by step. 

There can be more than one staggered and one eclipsed conformation conformational analysis of butane... 

---