along a line extending through a carbon–carbon bond axis. The classic example for demonstrating
conformational isomerism in a straight chain is butane, which is shown in Figure 2.3.
Figure 2.3. Newman Projection of Butane
Depiction of different atoms’ positions from the point of view of the C-2 to C-3 bond axis.Straight-Chain Conformations
For butane, the most stable conformation occurs when the two methyl groups (containing C-1 and
C-4) are oriented 180° away from each other. In this position, there is minimal steric repulsion
between the atoms’ electron clouds because they are as far apart as they can possibly be. Thus, the
atoms are “happiest” and in their lowest-energy state. Because there is no overlap of atoms along
the line of sight (besides C-2 and C-3), the molecule is said to be in a staggered conformation.
Specifically, it is called the anti conformation because the two largest groups are antiperiplanar (in
the same plane, but on opposite sides) to each other. This is the most energetically favorable type of
staggered conformation. The other type of staggered conformation, called gauche, occurs when the
two largest groups are 60° apart.
MNEMONIC
It’s gauche (unsophisticated or awkward) for one methyl group to stand too close to another
group. Groups are eclipsed when they are completely in line with one another—just like a
solar or lunar eclipse.To convert from the anti to the gauche conformation, the molecule must pass through an eclipsed
conformation in which the two methyl groups are 120° apart and overlap with the hydrogen atoms
on the adjacent carbon. When the two methyl groups directly overlap each other with 0° separation,