Organic Chemistry: Conformations
Conformations of Higher Alkanes
Propane, $C_3 H_8 $, is the next member of the alkane family. Conceptually, propane can be viewed as ethane with one methyl substituent. Instead of analyzing both C-C bonds at the same time, it is more convenient to look at a single C-C and generalize the behavior of the remaining methyl group. The Newman projections show that propane has a set of eclipsed and staggered conformations similar to ethane, but with a torsional strain of 3.3 kcal/mol. Each eclipsed conformation now consists of two eclipsed C-H bonds and one C-H eclipsed with a $C-CH_3 $ bond. We can infer that the latter eclipsing interaction has an energetic "cost" of 1.3 kcal/mol. Bear in mind that these energies do not measure electronic repulsion in any absolute sense, but merely provide figures relative to the more stable staggered state.
Butane, $C_4 H_10 $, has many conformations since its dihedral angles could vary across three C-C bonds. We focus on the central $C_2 C_3 $ bond and treat the end carbons generally as methyl groups. Conformationally, butane is more complex than ethane or propane. For ethane or propane, the three eclipsed forms are identical in energy, as are the three staggered forms. These structures are degenerate. Such degeneracy is broken in butane, which has two different eclipsed conformations and two different staggered conformations. These conformations differ by the relative positions of the two methyl substituents.
In the most stable conformation, the two methyl groups lie as far apart from each other as possible with a dihedral angle of 180 degrees. This particular staggered conformation is called anti. The other staggered conformation has a Me-Me dihedral angle of 60 degrees and is called gauche. The gauche form is less stable than the anti form by 0.9 kcal/mol due to steric hindrance between the two methyl groups. Such an interaction is often referred to as a gauche-butane interaction because butane is the first alkane discovered to exhibit such an effect.
Pentane and Higher Alkanes
Pentane and higher alkanes have conformational preferences similar to ethane and butane. Each dihedral angle tries to adopt a staggered conformation and each internal C-C bond attempts to take on an anti conformation to minimize the potential energy of the molecule. The most stable conformation of any unbranched alkane follows these rules to take on zigzag shapes:
Let's analyze the staggered conformations of pentane in more detail, considering conformations about the $C_2 C_3 $ and $C_3 C_4 $ bonds. shows a few possible permutations. The most stable conformation is anti at both bonds, whereas less stable conformations contain gauche interactions. One gauche-gauche conformer is particularly unfavorable because methyl groups are aligned with parallel bonds in close proximity. This conformation is called syn. This type of steric hindrance across five atoms is called a syn-pentane interaction. Syn-pentane interactions have an energetic cost of about 3.6 kcal/mol relative to the anti-anti conformation and are therefore disfavored.