Conformational analysis of butane

The 4-carbon alkane Butane is used to illustrate the ideas of conformational analysis around a typical C-C bond. All carbon atoms in the molecule are sp3 hybridized, with 4 single bonds in approximately tetrahedral geometry. The molecule may be drawn in many different ways; here we translate the "zig-zag" picture (below, left) into a "side-on" depiction (centre), and ultimately the Newman depiction (below, right).


When looking Newman depictions along the C2-C3 bond axis we see three valences coming from the central C atoms (C2 at the front, C3 at the back). In the anti staggered conformation the two large CH3 groups are as far away as possible from each other which contributes to this being the most stable conformation. In the gauche staggered conformation the CH3 groups are closer together thereby causing increased torsional strain. Further rotation around the C2-C3 bond causes the two CH3 groups to become eclipsed which is the point of highest energy during the rotation. 


Rotation through 360 degrees around the C-2 to C-3 bond of butane will then produce an infinite number of different conformations, the most significant of which are shown below. Anti will typically be the lowest energy, since large groups are faraway from each other, while eclipsed will usually be less stable since large groups are interacting. 


detailed ConformationAl analysis

Since molecules are continually in motion we have to consider the possibilities of different molecular shapes (conformations) and their relative energies. Having knowledge of the lowest and highest energy shapes helps us to understand the stereochemical and regiochemical outcomes of reactions later on.