Question

1. Draw the different conformations of cyclohexane and indicate which conformer is more stable and explain the reason why it is more stable.
2. Draw the saddle conformation of cyclohexane and indicate the axial and equatorial positions of the hydrogen atoms.
3. Identify the chiral carbon with an asterisk in the following molecule. Say how many stereoisomers you can have. CH3CH(OH)CH(Br)CH3

Answer 1

The different conformations of cyclohexane are :

• chair conformation
• boat conformation
• twist boat conformation
• half chair conformation

in chair conformation there is no steric hinderance or dipole dipole interactions between the non bonded atoms. thus this conformation is strain free and the most stable conformation of cyclohexane.

in boat conformation, the hydrogen on four of the carbon atome ( C2 , C3 and C5 ,C6 ) are ecplised. as a result , there is torsional strain. also , due to two hydrogens at C1 and C4 are very close to each other and the interaction is called flagpole interaction. due to these strains, boat conformation is less stable than chair conformation.

twist boat conformation : in this conformation , flagpole interactions are minimized , and the torsional strain on C2 , C3 and C5, C6 is allso reduced. as a result , twist boat conformation is more stable than boat conformation by about 6.7 KJ/mol although it is less stable than chair conformation.

in between chair conformation and twist boat conformation lies yet another conformation known as half chair conformation. it has both angle and torsional strains. in fact it represents merely a tranistion state conformation.

so the relative stability of different are :

chair > twist boat > boat > half chair