In this chapter we shall learn the rotation about a single bond of ethane and the effect of rotation on the shape and energy of ethane molecule. The study on the relation between the rotation of single bonds and molecular structure and molecular energy is called conformational analysis. With the aid of Newman projection formula, conformational analysis of propane, butane and polysubstituted ethanes will be attempted. Naming of conformers will also be attempted by expanding the sequence rule.
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tortion angle-energy diagram
conformer (conformational isomer)
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conformational analysis: a study of the change of molecular structure (accompanied by the change of energy and properties) caused by a rotation of single bond.
the tortion angle-potential energy curve: a plot of potential energy(abscissa) versus twist angle (horizontal). indispensable for conformational analysis of simple molecule.
eclipsed form 3: conformation of ethane and analogs with É” = 0o, 120o and 240o
staggered form 4: conformation of ethane and analogs with É” = 60o, 180o and 300o
go to 2.2 Conformational analysis of ethane
gauche form; conformation of 1,2-disubstituted ethanes with É” = 60 o.
anti(trans) form: conformation of 1,2-disubstituted ethanes with É” = 180 o.
go to 2.3 Conformational Analysis of Butane
The relation between the value of ÉĘG and composition of conformers A and B are tabulated as below. Table ÉĘG and relative amount. K(298K) amount of the more stable isomer(%) ÉĘG(kJ mol-1)
ÉĘG = -2.303 RT log K
|amount of the more stable isomer(%)||67||75||80||83||91||95||99||99.9||99.99|
Temperature and Free Energy Difference vs. Molar Ratio (JAVA)
go to 2.4 Relative abundance of conformers.
|tortion angle q||name|
Nomenclature of Conformers (JAVA)
go to 2.5 Stereochemical nomenclature of ethane derivatives