Chapter 3: Conformations of Alkanes and Cycloalkanes

Higher Alkanes

The same basic ideas of conformational analysis developed previously for simple alkanes also apply, but some new situations are encountered.

Butane: CH₃-CH₂-CH₂-CH₃

In butane it is the rotation about the C2-C3 bond that is of most interest since the relative position of the two methyl groups is important.  This can be seen most easily by rotating the molecule to view it down the C2-C3 bond. Try rotating the 3D model in the animation so that you are looking directly along the C-C bond to see the interconversion of the two conformations. The more important conformations are shown below:
ANTI a staggered conformation with the Me groups at 180o with respect to each other. This is the most stable conformation since the Me groups are as far apart as possible.
GAUCHE a staggered conformation with the Me groups at 60o with respect to each other.
SYN an eclipsed conformation with the Me groups at 0o with respect to each other. This is the least stable conformation due to the steric strain caused by the proximity of the Me groups and the torsional strain of the eclipsed bonds.
GAUCHE a staggered conformation with the Me groups at 60o with respect to each other
note: the two gauche conformations are not quite the same, try to convince yourself of this.  How are they related to each other ?	ANSWER

The animation below shows how the potential energy of the butane molecule varies for a full rotation about the central C-C bond.
Use the controls to show the energies of the important conformations.

© Dr. Ian Hunt, Department of Chemistry