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Conformational Stereoisomers

Structural formulas show the manner in which the atoms of a molecule are bonded together (its constitution), but do not generally describe the three-dimensional shape of a molecule, unless special bonding notations (e.g. wedge and hatched lines) are used. The importance of such three-dimensional descriptive formulas became clear in discussing configurational stereoisomerism, where the relative orientation of atoms in space is fixed by a molecule's bonding constitution (e.g. double-bonds and rings). Here too it was noted that nomenclature prefixes must be used when naming specific stereoisomers. In this section we shall extend our three-dimensional view of molecular structure to include compounds that normally assume an array of equilibrating three-dimensional spatial orientations, which together characterize the same isolable compound. We call these different spatial orientations of the atoms of a molecule that result from rotations or twisting about single bonds conformations.

In the case of hexane, we have an unbranched chain of six carbons which is often written as a linear formula: CH3CH2CH2CH2CH2CH3. We know this is not strictly true, since the carbon atoms all have a tetrahedral configuration. The actual shape of the extended chain is therefore zig-zag in nature. However, there is facile rotation about the carbon-carbon bonds, and the six-carbon chain easily coils up to assume a rather different shape. Many conformations of hexane are possible and two are illustrated below.

 

Extended Chain Coiled Chain
hexane1.gif hexane2.gif

For an animation of conformational motion in hexane   .

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