Chapter 4 : Stereochemistry B.sc 1st Year Page : 10 Organic Chemistry
Conformational Analysis of Cyclohexane
From the potential energy diagram it is clear that chair conformation is the most stable form. One chair conformation can be easily converted into alternate chair conformation with full boat, twist boat and half chair conformations are intermediates as shown in figure 4.09 (Conformational Analysis of Cyclohexane)
The stability of chair conformation is due to its staggered form and is also free from angle and torsional an. It is about 29.7 (23+6.7)KJ mol-1 more stable than the boat conformation, 23KJ mol-1 more be than the twist boat conformation. A twist boat conformation is only about 6.7KJ mol-1 more stable are the boat conformation. The higher energy barrier is the transition state conformation called the if chair’ which is the least stable of all as these possess considerable angle strain, torsional strain in steric strain.
Equatorial and Axial bonds in Cyclohexane:
The twelve C-H bonds in cyclohexane are of two types. Six of them lie parallel to the axis of the cyclohexane ring are called ‘axial hydrogen atoms and the bonds holding these hydrogens are called ‘axial bonds’. These are designated by ‘a’. The other six hydrogen atoms which extend radially outwards at an angle of 109.5° situated along the equator of the ring are called ‘equatorial hydrogen atoms’ and the bonds are called ‘equatorial bonds’. These are represented by ‘e’.
Out of six axial hydrogen atoms, three lie above the plane of the ring. Similarly three equatorial hydrogen atoms also lie above the plane of the ring. On each carbon one bond is axial and the other is equatorial.
Monosubstituted Cyclohexane :
The two chair forms of cyclohexane have equal energy. At room temperature one chair form is readily converted into the other chair form. Since the two positions are equivalent, there is no existence of conformational isomers. When some groups other than hydrogens are attached to the carbon both chair forms are no longer equivalent. For example in methyl cyclohexane, the methyl group can occupy either the equitorial or the axial position and the two form are no longer identical. The reasons behind this are explained below:
In the equitorial conformer, the methyl group extends into space away from the rest of the molecule(II). In an axial conformer, the methyl group is close to the other two axial hydrogens on the same side of the molecule. There is 1,3 diaxial interactions which repel each other. Consiquently, the axial methyl group makes it less stable. However, the extend of destabilisation is depend upon the size of the substituent. This 1,3 diaxial interaction is similar to that in gauche conformation of n-butane. The axial methyl group in methyl cyclohexane is gauche to two of the ring carbons (c) and when in equitorial position it is anti to the ring(d)
Thus, equatorial conformer is more stable than the axial. equitorial-methy(e-methyl) cyclohexane is expected to be about 7.1K Jmol-1 more stable than axial-methyl(a- methyl) cyclohexane. At room temperature, about 95% of molecules of the chair conformation are with remaining 5% having the axial methyl conformation. In tert.-butyl cyclohexane about 99.9% have equatorial tert.-butyl group.
