2.2 Conformational analysis of ethane
2.2 Conformational analysis of ethane
Then how we can describe the relation between 1 and 2? They are stereoisomers each other since the spatial relation among eight atoms forming each ethane is different. The difference is, however, not the same one observed for 14 and 15 of Ch. 1. (comment: these are two enantiomers of CHFClBr) 14 and 15 are not interconvertible without breakage and reunion of some chemical bonds. On the other hand, 1 and 2 in this chapter are interconvertible by rotation about the bond without bond breakage. In such a case 1 and 2 are refereed to having a different conformation.
How can we distinguish conformations 1 and 2? Rotation about C-C bond will cause a change in the tortion angleφ and consequently the distance between HA and HB. If the distance between two hydrogen atoms(or between a hydrogen atom and an alkyl group, or between two alkyl groups) becomes short, the potential energy of the molecule will increase because the steric repulsion will increase. When the tortion angleφ = 0o, 120o and 240o , it becomes maximal; when the tortion angleφ = 60o, 180oand 300o, it becomes minimal. Thus, the plot of φ with the potential energy will give the tortion angle-potential energy curve given in Fig. 2.3 A study of the change of molecular structure and energy caused by a rotation of C-C (and other) single bond is referred to conformational analysis.
Fig. 2.3 The tortion angle-potential energy curve of ethane.
▶go to S2.1 conformational analysis
Potential energy of ethane is maximal when φ =0o, 120o and 240o, and minimal when φ = 60o, 180o and 300o. The structure for the former is referred to staggered, 4 and that for the latter to eclipsed. 3.
The potential energy difference between two forms is ca 12 kJ mol－1 (2.86 kcal mol－1) Conformations corresponding to the structure with maximal or minimal energy are referred to conformational isomer or simply conformer. In this book we use conformer since this term.
