Basic Concepts
Conformational analysis deals with the shapes that molecules can adopt by rotations about single bonds. These structures are called conformational isomers or conformers. The following isomers cannot interconvert between each other without breaking bonds. Constitutional isomers have the same formula, but different connectivity. Stereoisomers have the same formula, the same connectivity, but different arrangement in space. Conformational isomers are interconvertable isomers of the same molecule that result from rotations of single bonds. They interconvert constantly and are not separable. Conformations of alkanes can best be viewed using either Newman projections or sawhorse representations. A Newman Projection is an end-on view of 2 carbons attached by a sigma bond. The front carbon is shown as the intersection of 3 bonds and the rear carbon is shown as a circle. The substituents are attached accordingly.
Conformations of Ethane
Ethane can adopt either staggered or eclipsed conformations. Staggered is the lowest energy conformer, with the hydrogens as far away from each other. Eclipsed is the highest energy conformer and the least stable. It is 3 kcal higher in energy than the staggered form due to repulsion between electrons in C-H bonds.
Conformations of Butane
As butane rotates about its sigma bond, four distinct conformational possibilities exist. Staggered, or anti, where the methyl groups are farthest apart; eclipsed methyl-H interactions; gauche, where the methyl groups are adjacent; and eclipsed, or syn, where the methyl groups are eclipsed.
Cycloalkanes
The interior angles of cycloalkanes deviate from the tetrahedral so strain develops. Cyclopropane has a bond angle of 60o; Cyclobutane has a bond angle of 90o; Cyclopentane has a bond angle of 108o; Cyclohexane has a bond angle of 109.5o.
Conformations of Cyclohexane
Cyclohexane=s Newman Projection illustrates the staggered, low energy conformation that the chair form results. In addition to the chair, cyclohexane can also adopt the boat form by flipping one hydrogen up. This form is higher in energy due to steric and eclipsing strain. By twisting the boat, cyclohexane adopts yet another form, the twist-boat, a result of relieving some of the boat=s strain. Cyclohexane=s carbons have two substitution positions, axial and equatorial. The axial is vertical and equatorial is parallel to the adjacent C-C bond in the chair.
