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.
Figure %: The conformations of propane.
Butane
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.
Figure %: The anti and gauche conformations of butane.
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:
Figure %: The zigzag shapes of unbranched alkanes in their most stable
conformations.
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.
