Conformations of Alkanes

Every carbon in an alkane is sp3 hybridised, with its four attached groups arranged tetrahedrally. Where an alkane has two or more carbons, free rotation about each C-C single bond lets the atoms/groups on either carbon sweep through an infinite number of readily interconvertible three-dimensional arrangements, called conformations. For ethane, the two extreme conformations obtained by rotating one methyl group relative to the other are the eclipsed conformation, in which the hydrogens on one carbon sit directly behind those on the other, giving maximum repulsion and making it the LEAST stable conformer, and the staggered conformation, in which the hydrogens on the two carbons are as far apart as possible, giving minimum repulsion and making it the MOST stable conformer; the infinite intermediate arrangements are called skew conformations, and the overall stability order is staggered > skew > eclipsed, with a potential-energy difference of about 12.5 kJ/mol between the staggered and eclipsed forms. These conformations are drawn using Newman projection formulae (a front atom shown as a dot with three bonds radiating out, a rear atom shown as a circle with three bonds emerging from behind it). For n-butane, viewed as ethane with one hydrogen on each carbon replaced by a methyl group, the eclipsed conformation (the two methyl groups at minimum distance, maximum repulsion) is again the least stable, and the anti or staggered form (the two methyl groups at maximum distance, minimum repulsion) is the most stable; a potential-energy-versus-dihedral-angle (phi) plot for n-butane shows a repeating pattern of eclipsed (local en …