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|
For an animation of conformational motion in hexane .
The simple alkane ethane provides a good introduction to conformational analysis. Here there is only one carbon-carbon bond, and the rotational structures (rotamers) that it may assume fall between two extremes, staggered and eclipsed. In the following description of these conformers, several structural notations are used. The first views the ethane molecule from the side, with the carbon-carbon bond being horizontal to the viewer. The hydrogens are then located in the surrounding space by wedge (in front of the plane) and hatched (behind the plane) bonds. If this structure is rotated so that carbon #1 is canted down and brought closer to the viewer, the "sawhorse" projection is presented. Finally, if the viewer looks down the carbon-carbon bond with carbon #1 in front of #2, the Newman projection is seen.
Bond Repulsions in Ethane
To see an eclipsed conformer of ethane orient itself as a Newman projection, and then interconvert with the staggered conformer and intermediate conformers .
As a result of bond-electron repulsions, illustrated on the right above, the eclipsed conformation is less stable than the staggered conformation by roughly 3 kcal / mol (eclipsing strain). The most severe repulsions in the eclipsed conformation are depicted by the red arrows. There are six other less strong repulsions that are not shown. In the staggered conformation there are six equal bond repulsions, four of which are shown by the blue arrows, and these are all substantially less severe than the three strongest eclipsed repulsions. Consequently, the potential energy associated with the various conformations of ethane varies with the dihedral angle of the bonds, as shown below. Although the conformers of ethane are in rapid equilibrium with each other, the 3 kcal/mol energy difference leads to a substantial preponderance of staggered conformers (> 99.9%) at any given time.
Although steric and/or bond electron repulsion remain the most popular explanation for the hindered rotation of ethane, molecular orbital interactions have also been proposed as a significant factor. For a discussion of this feature .
Potential Energy Profile for Ethane Conformers
The above animation illustrates the relationship between ethane's potential energy and its dihedral angle
The hydrocarbon butane has a larger and more complex set of conformations associated with its constitution than does ethane. Of particular interest and importance are the conformations produced by rotation about the central carbon-carbon bond. Among these we shall focus on two staggered conformers (A & C) and two eclipsed conformers (B & D), shown below in several stereo-representations. As in the case of ethane, the staggered conformers are more stable than the eclipsed conformers by 2.8 to 4.5 kcal/mol. Since the staggered conformers represent the chief components of a butane sample they have been given the identifying prefix designations anti for A and gauche for C.
|Four Conformers of Butane|
The following diagram illustrates the change in potential energy that occurs with rotation about the C2–C3 bond. The model on the right is shown in conformation D, and by clicking on any of the colored data points on the potential energy curve, it will change to the conformer corresponding to that point. The full rotation will be displayed by turning the animation on. This model may be manipulated by click-dragging the mouse for viewing from any perspective.
|Potential Energy Profile for Butane Conformers|
(i) Most conformational interconversions in simple molecules occur rapidly at room temperature. Consequently, isolation of pure conformers is usually not possible.
(ii) Specific conformers require special nomenclature terms such as staggered, eclipsed, gauche and anti when they are designated.
(iii) Specific conformers may also be designated by dihedral angles. In the butane conformers shown above, the dihedral angles formed by the two methyl groups about the central double bond are: A 180º, B 120º, C 60º & D 0º.
(iv) Staggered conformations about carbon-carbon single bonds are more stable (have a lower potential energy) than the corresponding eclipsed conformations. The higher energy of eclipsed bonds is known as eclipsing strain.
(v) In butane the gauche-conformer is less stable than the anti-conformer by about 0.9 kcal/mol. This is due to a crowding of the two methyl groups in the gauche structure, and is called steric strain or steric hindrance.
(vi) Butane conformers B and C have non-identical mirror image structures in which the clockwise dihedral angles are 300º & 240º respectively. These pairs are energetically the same, and have not been distinguished in the potential energy diagram shown here.