n-pentane conformation: Easy Explanation

Conformation denotes the particular structure of a molecule and n-pentane conformation is quite complex. Conformation relates to the different spatial arrangement of atoms or groups obtained by rotating a single bond between carbon and carbon. Different conformations of a given molecule differ in their potential energies due to the angle of torsion or dihedral angle.

Conformation of n-pentane

In n-pentane, several conformations are achieved by rotating either the C(2,3) or C(3,4) bond. This represents a rotation around two CH₂-CH₂ bonds. There are nine different conformations that can be obtained by rotating a single bond between this carbon and carbon; the trans conformation is marked by (a), and the gauche conformation by (g). The gauche conformation is further specified whether the torsional angle is near +60^o or -60^o which are designated as (g⁺) and (g^–), respectively.

The nine staggered conformations of n-pentane are given below:

In the conformation (a, a) in the above figure, the single bond between carbon number 2(C₂) and carbon number 3 (C₃) when rotated 60^o in an anticlockwise direction, the conformation (g^–, a) is obtained. Similarly, by rotating a single bond between C₃ and C₄ to 60^o in a clockwise direction or near +60^o, the conformation (a, g⁺) is obtained. In the same way. by the rotation of the carbon-carbon single bond between C₂ and C₃ or C₃ and C₄ or both, the above conformations are obtained.

The conformations above and below (a, a) in the above figure are identical. The molecule (g⁺, g^–) when rotated 180^o becomes identical with (g^–, g⁺). The compounds (a, g^–) and (g^–, a) are identical. One of these compounds can be rotated 180^o to have the identical compound of the other. The same is the case with conformers (g⁺, a) and (a, g⁺). However, the compounds remaining left and right of the vertical line are enantiomers. That is, (g⁺, a) and (a, g^–) are enantiomers. Enantiomers are different compounds but contain equal energies. Again, (g⁺, g⁺) and (g^–, g^–) are enantiomers. They are also of equal energies. If we see the identical basis, there are only six distinct conformations:

1. (a, a)
2. (g⁺, g^– and g^–, g⁺)
3. (a, g^– and g^–, a)
4. (g⁺, a and a, g⁺)
5. (g^–, g^–)
6. (g⁺, g⁺)

On the basis of enthalpy, n-pentane possesses only 4 confirmations:

1. (a, a)
2. (g, g)±
3. (ag± or g±a)
4. (g^–, g^– or g⁺, g⁺)

The calculated conformation distribution at 298.16K in liquid pentane is aa (34.6%), 54.6% (a, g), 10.8% (g, g), and less than 0.4% for (gg)±

n-pentane conformation video