Editing Cyclopentane (section)

==Conformations==

In a [[regular pentagon]], the angles at the vertices are all 108°, slightly less than the bond angle in perfectly tetrahedrally bonded carbon, which is about 109.47°. However, cyclopentane is not planar in its normal [[Conformational isomerism|conformations]]. It puckers in order to increase the distances between the hydrogen atoms (something which does not happen in the planar [[cyclopentadienyl anion]] {{chem2|C5H5-}} because it doesn't have as many hydrogen atoms). This means that the average C-C-C angle is less than 108°. There are two conformations that give local minima of the energy, the "envelope" and the "half-chair". The envelope has mirror symmetry ([[Point groups in three dimensions|C{{sub|s}}]]), while the half chair has two-fold rotational symmetry ([[Point groups in three dimensions|C{{sub|2}}]]). In both cases the symmetry implies that there are two pairs of equal C-C-C angles and one C-C-C angle that has no pair. In fact for cyclopentane, unlike for [[cyclohexane]] (C<sub>6</sub>H<sub>12</sub>, see [[cyclohexane conformation]]) and higher [[cycloalkane]]s, it is not possible geometrically for all the angles and bond lengths to be equal except if it is in the form of a flat regular pentagon.

<gallery>
File:Cyclopentane envelope conformation.svg|Envelope
File:Cyclopentane3D.png|3D envelope
File:Cyclopentane halfchair.svg|Half-chair
</gallery>