Ethane rotational isomerism

We shall first consider rotational isomerism in ethane. In particular, we shall compare the staggered and eclipsed conformations shown below. 

High-pressure FT-IR spectroscopy has been used to clarify (1) the rotational isomerism of molecules, (2) characteristics of water and the water-head group, and (3) RSO3 Na4- interactions in reverse micellar aggregates in supercritical ethane. This work demonstrates interesting pressure, temperature, and salt effects on an enzyme-catalyzed esterification and/or maintenance of a one-phase microemulsion in supercritical fluids from practical and theoretical points of view (Ikushima, 1997). 

Rotational isomerism relative to a single bond is illustrated by ethane and 1,2-dichloroethane, both depicted in Figure 3-4. First, take the ethane molecule, H3C-CH3. During a complete rotation of one methyl group around the C-C bond relative to the other methyl group,... 

Gas-phase study G. Schultz, I. Hargittai, Electron diffraction investigation of ethane-1,2-dithiol. Acta Chim. Hung. 1973, 75, 381-388 solid-state study M. Hayashi, Y. Shiro, T. Oshima, and H. Murata, The Vibrational Assignment, Rotational Isomerism and Force Constants of 1,2-Ethanedithiol. Bull. Chem. Soc. Japan 1965, 38, 1734-1740. 

Simple hydrocarbons represent rather well-behaved extensions of the conformational principles illustrated previously in the analysis of rotational equilibria in ethane and n-butane. The staggered conformations correspond to potential energy minima, the eclipsed conformations to potential energy maxima. Of the staggered conformations, anti forms are more stable than gauche. The magnitudes of the barriers to rotation of many small organic molecules have been measured. Some representative examples are listed in Table 3.3. The experimental techniques used to study rotational isomerism include microwave spectroscopy, electron diffraction, ultrasonic absorption, and infrared spectroscopy. ... 

Unimolecular reactions that have been used to investigate the solvation properties of supercritical fluids include tautomeric reactions (67-71), rotational isomerization reactions (72-78), and radical reactions (79-83). O Shea et al. used the tautomeric equilibrium of 4-(phenylazo)-l-naphthol to examine the solvent strength in supercritical ethane, CO2, and fluoroform and to determine its dependence on density (67). The equilibrium is strongly shifted to the azo tautomer in supercritical ethane and the hydrazone tautomer in supercritical chloroform and... 

Conformational isomerism can be presented with the simplest example, ethane (C2Hg), which can exist as an infinite number of conformers by the rotation of the C-C ct bond. Ethane has two sp -hybridized carbon atoms, and the tetrahedral angle about each is 109.5°. The most significant conformers of ethane are the staggered and eclipsed conformers. The staggered conformation is the most stable as it has the lowest energy. 

Conformational isomerism in propane Propane is a three-carbon- (sp -hybridized) atom-containing linear alkane. All are tetrahedrally arranged. When a hydrogen atom of ethane is replaced by a methyl (CH3) group, we have propane. There is rotation about two C-C cr bonds. 

The two areas of orbital overlap in the n bond (above and below the plane of the molecule) cause ethene to be a rigid molecule there is no rotation around the carbon—carbon bond. This is in contrast to ethane, and other non-cyclic alkanes, and leads to the existence of cis-trans (geometric) isomerism in alkenes (Chapter 20). 

The simplest molecule to illustrate conformational isomerism is ethane. If we rotate one methyl group around the carbon-carbon single bond axis, keeping the other fixed, and imagine looking along the axis of the bond, there will be times when the hydrogen atoms on the front carbon atom obscure ( eclipse ) the atoms on the far carbon, and times when they do not. 

Figure 2-10 Potential-energy diagram of the rotationai isomerism in ethane. Because the eclipsed conformations have the highest energy, they correspond to peaks in the diagram. These maxima may be viewed as transition states between the more stable staggered conformers. The activation energy (EJ corresponds to the barrier to rotation.

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