Cyclohexane 1: introduction

Cyclohexane is an alicyclic hydrocarbon with chemical formula C₆H₁₂. It is an alkane but its formula does not fit the C_nH_2n+2 general formula because it is not a straight-chain molecule and so it contains only CH₂ groups with no CH₃ groups at the ends. It is a liquid under ambient conditions, and commonly used as a nonpolar solvent.

In the structure to the left the carbon atoms are coloured grey and the hydrogen atoms white. Unlike benzene the molecule is not planar and it only contains σ bonds. With four σ bonds around each carbon they are arranged tetrahedrally, giving a bond angle of 109°. However, the internal angle in a hexagon is 120° so a flat hexagon would introduce bond strain into the molecule. The molecular shape is therefore puckered. The shape that the molecule forms when it puckers is called its conformation. It is possible to calculate using the theory of symmetry that the number of possible ring conformations is n-3, where n is the number of atoms in the ring. The three ring conformations of carbon in six-membered rings are "chair", "boat" and "twisted boat". The most stable of the three is the chair conformation, which is seen in the structure on the left. By rotating the structure on the left you should be able to appreciate why it's called the chair conformation. Note how a similar carbon unit is visible in diamond, except the carbons do not carry a hydrogen atom but are fused (see page 8 of diamond).

Looking at the molecule to the left it is less obvious compared to benzene that the six carbons in the ring are in equivalent positions. In the chair conformation there is no six-fold rotation axis like there is in benzene; instead there is just a three-fold rotation axis. However, carbon atoms are equivalent with their neighbours. If the molecule is rotated 60° around its principal axis and then reflected in a plane perpendicular to that axis then each carbon atom is moved on to its neighbour's position, showing that neighbouring atoms are equivalent, and that therefore, considering a sequence of such operations, that all the carbon atoms in the ring are equivalent. This combination of a rotation and a reflection is known as an "improper rotation". Curiously this property is not found for the two hydrogens on a given carbon atom, so those hydrogens are not technically equivalent. However, under ambient conditions the molecule can flip position (the chair effectively turning inside out) and this serves to make the two types of hydrogen appear the same and behave the same.

Go to page 2 to look at the unit cell within the bulk crystalline structure of cyclohexane in the solid state.

Page 2