Ethane, bond angles conformations

Peptides built from y-amino acids with L-amino acid-derived chirahty centers form a right-handed (P)-2.6i4 hehx of ca. 5 A pitch with both ethane bonds in a -y)-synclinal conformation (mean values for dihedral angles 9 and 82 of central residues 2-5 in compounds 141 are 72.5 and 64.3°, respectively Fig. 2.36 A and B Tab. 2.8). 

If this carbon holds in different atoms, the bond angles are somewhat (a little) changed and the tetrahedron ceases to be regular. But the real foundation for conformational study was laid in 1935 when it was observed that there was discrepancy between the entropy of ethane as found from the heat capacity measurements and as calculated from spectral data. From this the physical chemists concluded that there must be hindrance to rotation about the carbon bond in ethane. Later it was found that there was tortional barrier to free rotation to the extent of about 2.8 K cals per mole. 

Ball-and-stick models of organic substances provide a convenient means of studying the structures of various organic compounds. Balls with holes represent the atoms, and sticks represent the covalent bonds. Figure 1-3 shows two conformations of ethane in which the dark balls represent carbon and the light balls represent hydrogen. All bond angles are the normal 109.5°. 

Ethane is the simplest hydrocarbon that can have distinct conformations. Two, the staggered conformation and the eclipsed conformation, deserve special attention and are illustrated in Figure 3.1. The C—H bonds in the staggered conformation are arranged so that each one bisects the angle made by a pair of C—H bonds on the adjacent carbon. In the eclipsed conformation each C—H bond is ahgned with a C—H bond on the adjacent carbon. The staggered and eclipsed conformations interconvert by rotation around the carbon-carbon bond. Different conformations of the same molecule are sometimes called conformers or rotamers. 

Of the two conformations of ethane, the staggered is more stable than the eclipsed. The measured difference in potential energy between them is 12 kJ/mol (2.9 kcal/mol). A simple explanation has echoes of VSEPR (Section 1.10). The staggered conformation allows the electron pairs in the C—H bonds of one carbon to be farther away from the electron pairs in the C—H bonds of the other than the eclipsed conformation allows. Electron-pair repulsions on adjacent carbons govern the relative stability of staggered and eclipsed conformations in much the same way that electron-pair repulsions influence the bond angles at a central atom. 

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