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Question

What is the name of the cyclohexane conformation?


  1. Chair 

  2. Half chair

  3. Twist Boast

  4. Boat

  5. Anti


Solution

The correct option is D

Boat


This cyclohexane conformation is known as boat.

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 gthe 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.

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