Conformational analysis of ethane

In the staggered conformation, each C—H bond of one carbon bisects an H—C—H angle of the other carbon. 

Newman projections were devised by Professor Melvin S. Newman of Ohio State University. 

The staggered and eclipsed conformations interconvert by rotation around the C—C bond, and do so very rapidly. WeTl see just how rapidly later in this section. 

Eclipsed bonds are characterized by a torsion angle of 0°. When the torsion angle is approximately 60°, we say that the spatial relationship is gauche and when it is 180° 

Structures A, B, and C represent different conformations of hydrogen peroxide. Conformations are different spatial arrangements of a molecule that are generated by rotation about single bonds. Although we can t tell from simply looking at these structures, we now know from experimental studies that all are in rapid equilibrium and that C is the most stable conformation. 

Conformational analysis is the study of how conformational factors affect the structure of a molecule and its properties. In this chapter we ll examine the conformations of various alkanes and cycloalkanes, focusing most of our attention on three of them ethane, butane, and cyclohexane. You will see that even simple organic molecules can exist in many conformations. Conformational analysis will help us to visualize organic molecules in 3D and to better understand their structure and properties. 

In the staggered conformation, each C—H bond of one carbon bisects an H—C—H angle of the other carbon. In the eclipsed conformation, each C—H bond of one carbon is aligned with a C—H bond of the other carbon. 

The staggered and eclipsed conformations of ethane shown as ball-and-spoke models (left) and as spacefilling models (right). 

Chapter 3 Alkanes and Cycloalkanes Conformations and cis-trans Stereoisomers 

FIGURE 3.3 Some commonly used representations of the eclipsed conformation of ethane. 

PROBLEM 3.1 Identify the alkanes corresponding to each of the drawings shown. 

SAMPLE SOLUTION (a) The Newman projection of this alkane resembles that of ethane except one of the hydrogens has been replaced by a methyl group. The drawing is a Newman projection of propane, CH3CH2CH3. 

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Buemi, G., Conformational analysis of ethane-1,2-dithiol and mercaptoethanol. An ab initio SCF HF/3-21G and 6-31G study, Phosphorus, Sulfur Silicon, 84, 239-247 (1993). 

An energy diagram showing the conformational analysis of ethane. 

Now let us consider a conformational analysis of ethane. Clearly, infinitesimally small changes in the dihedral angle between C—H bonds at each end of ethane could lead to an infinite number of conformations, including, of course, the staggered and eclipsed conformations. These different conformations are not aU of equal stability, however, and it is known that the staggered conformation of ethane is the most stable conformation (i.e., it is the conformation of lowest potential energy). The fundamental reason for this has recently come to light. 

All staggered conformations are degenerate, and the same is true for aU eclipsed conformations. As such, the energy diagram has a shape that is similar to the energy diagram for the conformational analysis of ethane ... 

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