Sort the following cyclohexane conformations according to stability.
Sort the following cyclohexane conformations according to stability.
⇒Each carbon in chair conformation has an axial bond and an equatorial bond.
⇒ Axial bonds are perpendicular to the plane of the ring equatorial bonds are in the plane of the ring.
⇒If axial bond on carbon - 1 is above the plane of the ring then axial bond on carbon - 2 will be below the plane of the ring.
Thus
C - 1, C - 3 and C - 5 axial bonds are above
C - 2, C - 4 and C - 6 axial bonds are below
⇒ Thus C - 1 axial and C - 2 axial are trans to each other. Similarly C - 1 and C - 5 axials are cis to each other.
⇒If axial bond on carbon-1 will be above the plane then equatorial bond on this carbon will be below the plane.
(a) Thus C - 1 equatorial and C - 2 equatorial bonds are trans.
(b) C - 1 axial and C - 2 equatorial will be cis.
⇒ As a result of rotation about carbon-carbon single bonds cyclohexane rapidly interconverts between two stable chair conformations. This inter-conversion is know as ring - flip. When the two chair forms interconvert, axial bonds become equatorial and equatorial bonds become axial.
⇒ Cyclohexane can also exist in a boat conformation. Like the chair conformation the boat conformation is free of angle strain. However, the boat conformation is less stable than the chair conformation by 11 kcal/mole. Boat conformation is less stable because some of the carbon-hydrogen bonds in boat conformation are eclipsed.
The boat conformation is further destabilized by the close proximity of the flagpole hydrogens. These hydrogens are 1.8 Å apart but the vander Waal's radii is 2.4 Å. The flagpole hydrogens are also known as trans nuclear hydrogens.
Note: The selective stabilities of the four conformations of cyclohexane decrease in the order:
Chair > twist boat > boat > half chair
⇒Each carbon in chair conformation has an axial bond and an equatorial bond.
⇒ Axial bonds are perpendicular to the plane of the ring equatorial bonds are in the plane of the ring.
⇒If axial bond on carbon - 1 is above the plane of the ring then axial bond on carbon - 2 will be below the plane of the ring.
Thus
C - 1, C - 3 and C - 5 axial bonds are above
C - 2, C - 4 and C - 6 axial bonds are below
⇒ Thus C - 1 axial and C - 2 axial are trans to each other. Similarly C - 1 and C - 5 axials are cis to each other.
⇒If axial bond on carbon-1 will be above the plane then equatorial bond on this carbon will be below the plane.
(a) Thus C - 1 equatorial and C - 2 equatorial bonds are trans.
(b) C - 1 axial and C - 2 equatorial will be cis.
⇒ As a result of rotation about carbon-carbon single bonds cyclohexane rapidly interconverts between two stable chair conformations. This inter-conversion is know as ring - flip. When the two chair forms interconvert, axial bonds become equatorial and equatorial bonds become axial.
⇒ Cyclohexane can also exist in a boat conformation. Like the chair conformation the boat conformation is free of angle strain. However, the boat conformation is less stable than the chair conformation by 11 kcal/mole. Boat conformation is less stable because some of the carbon-hydrogen bonds in boat conformation are eclipsed.
The boat conformation is further destabilized by the close proximity of the flagpole hydrogens. These hydrogens are 1.8 Å apart but the vander Waal's radii is 2.4 Å. The flagpole hydrogens are also known as trans nuclear hydrogens.
Note: The selective stabilities of the four conformations of cyclohexane decrease in the order:
Chair > twist boat > boat > half chair
