Conformational analysis ethane

In this chapter we 11 examine the conformations of various alkanes and cyclo alkanes focusing most of our attention on three of them ethane butane and cyclo hexane A detailed study of even these three will take us a long way toward understanding the mam ideas of conformational analysis... 

A similar analysis of data obtained from molecules with asymmetric end groups is more complicated. Apart from the problems connected with the separability of the torsional motion from the framework vibration, experience shows that several more terms have to be included in the Fourier series to describe the torsional potentials properly. On the other hand, the electron-diffraction data from asymmetric molecules usually contain more information about the potential function than data from the higher symmetric cases. In conformity with the results obtained for symmetric ethanes the asymmetric substituted ethanes, as a rule, exist as mixtures of two or more conformers in the gas phase. Some physical data for asymmetric molecules are given in Table 4. The electron-diffraction conformational analysis gives rather accurate information about the positions of the minima in the potential curve. Moreover, the relative abundance of the coexisting conformers may also be derived. If the ratio between the concentrations of two conformers is equal to K, one may write... 

Conformation and Conformational Analysis Conformation of Ethane Conformation of Propane Conformation of Butane Eclipsed and Staggered Eorms Ring Strains in Cycloalkanes Principles of Conformation 165... 

Let US first examine current practice in nomenclature of the phenom-menon of internal rotation. Standard textbooks on conformational analysis on the one hand and those on spectroscopy on the other hand deal differently with the basic definitions. Internal rotation is measured by one or more torsion angles (dihedral angles, azimuthal angles). In the simple cases of e.g. a 1,2-disubstituted ethane the potential energy associated with the internal rotation may be written in a formal sense as a truncated Fourier expansion ... 

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). 

Arguments derived from conformational analysis were used73 to account for the fact that the ion 44 is generated in substantially higher abundance when threo- 1,2-diphenyl-l,2-bis(trimethylsiloxy)ethane (101) serves as precursor in comparison with the erythro-isomer (102). In the former both the cyclization step 103 - 104 and the elimination of trans-stilbene oxide (105) are favoured on stereochemical grounds (reaction 47). In the erythro-precursor 102, however, the destabilizing interaction of the two phenyl groups retards the formation of 44 (reaction 48). 

The electrons in a C—H bond will repel the electrons in another C—H bond if the bonds get too close to each other. The staggered conformation, therefore, is the most stable conformation of ethane because the C—H bonds are as far away from each other as possible. The eclipsed conformation is the least stable conformation because in no other conformation are the C—H bonds as close to one another. The extra energy of the eclipsed conformation is called torsional strain. Torsional strain is the name given to the repulsion felt by the bonding electrons of one substituent as they pass close to the bonding electrons of another substituent. The investigation of the various conformations of a compound and their relative stabilities is called conformational analysis. 

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