Conformation of Propane and n-Butane

Consideration of the Newman projection of propane (9) suggests that the energy profile associated with rotation around the CH3-CH2 bond contains both three identical maxima and three identical minima per revolution. In eclipsed conformations there is now a non-bonded repulsion between a methyl and a hydrogen accordingly, the torsional barrier, at 14.2 kJ mol1, is a little higher than in ethane. 

The rotational barrier in propane has three-fold periodicity. 

Butane has two gauche conformations in which the dihedral angle between the CH3 groups is 60°, and an anti conformation, which is the most stable, with a corresponding dihedral angle of 180°. 

With the aid of models, verify that the eis, gauche and anti conformations have the relative energies indicated by the above data. 

The conformations of longer, unbranched hydrocarbons become more complicated the greater the number of carbons. For example, pentane has nine staggered conformations. 

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