Ethane staggered conformation

For ethane, staggered conformations occur at dihedral angles of 60°, 180°, and 300°. Eclipsed conformations occur at dihedral angles of 0°, 120°, and 240°. 

FIGURE 3 2 Some commonly used draw mgs of the staggered conformation of ethane... 

Of the two conformations of ethane the staggered is 12 kJImol (2 9 heal mol) more stable than the eclipsed The staggered conformation is the most stable conformation the eclipsed is the least stable conformation Two main explanations have been offered for the difference in stability between the two conformations One explanation holds that repulsions between bonds on adjacent atoms destabilize the eclipsed conformation The other suggests that better electron delocalization stabilizes the staggered conformation The latter of these two explanations is now believed to be the correct one... 

Rotation around carbon-carbon bonds is one of the fastest processes m chemistry Among the ways that we can describe the rate of a process is by its half life which is the length of time it takes for one half of the molecules to react It takes less than 10 s for half of the molecules m a sample of ethane to go from one staggered conformation to another at 25°C... 

Section 3 1 The most stable conformation of ethane is the staggered conformation It IS approximately 12 kJ/mol (3 kcal/mol) more stable than the eclipsed, which IS the least stable conformation... 

Staggered conformation of ethane (most stable conformation)... 

One of the frmdamental structural facets of organic chemistry, which has been explained most satisfactorily in MO terms, is the existence of a small barrier to rotation about single bonds. In ethane, for example, it is known that the staggered conformation is about 3kcal/mol more stable than the ecl sed conformation so that the eclipsed conformation represents a transition state for transformation of one staggered conformation into another by rotation. 

Ethane is the simplest hydrocar bon that can have distinct confonnations. Two, the staggered conformation and the eclipsed conformation, deserve special mention and are illustrated with molecular models in Figure 3.1. 

Among the various ways in which the staggered and eclipsed forms are portrayed, wedge-and-dash, sawhorse, and Newman projection drawings are especially useful. These are shown for the staggered conformation of ethane in Figure 3.2 and for the eclipsed conformation in Figure 3.3. 

Section 3.1 The most stable conformation of ethane is the staggered conformation. 

The difference in energy between the staggered and eclipsed forms is due almost entirely to the torsional strain in the eclipsed conformation. At any instant, almost all the molecules of ethane reside in the staggered conformation. 

It has been observed that ethane prefers a staggered conformation. Unfortunately, experiments do not tell us why this preference exists. 

Despite what we ve just said, we actually don t observe perfectly free rotation in ethane. Experiments show that there is a small (12 kj/mol 2.9 kcal/mol) barrier to rotation and that some conformers are more stable than others. The lowest-energy, most stable conformer is the one in which all six C-H bonds are as far away from one another as possible—staggered when viewed end-on in a Newman projection. The highest-energy, least stable conformer is the one in which the six C-H bonds are as close as possible—eclipsed in a Newman projection. At any given instant, about 99% of ethane molecules have an approximately staggered conformation... 

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