Question

Remove one hydrogen atom from C_1 the hexane model, and one H atom plus its bond from C_6. Bend the carbon chain around and join C_1 to C_6 to make cyclohexane. Mentally designate one carbon as C_1. Place the model in the chair conformation by tipping C_1 up and C_4 down. Check the conformation with the instructor. Identify the axial and equatorial hydrogens and describe their orientation relative to the plane of the ring. Look down each successive C-C bond (as in step 16) and describe the conformation of the hydrogens (eclipsed, staggered, skewed, etc) for each bond by filling in the table Put the cyclohexane model in the boat conformation, by tipping both C_1 and C_4 up. Identify the axial, equatorial and "flagpole" hydrogens, describe their orientations relative to the boat. Repeat step 21 and give the conformation of the hydrogens around each bond. Which is the more stable conformation of cyclohexane (boat or chair)? Why? Put the model back into the chair conformation and replace two hydrogens with chlorines to make trans-1, 2-dichlorocyclohexane with the chlorines. Are the chlorines axial or equatorial to the plane of the ring? Now, switch the model to the other chair conformation. Describe the orientation of the chlorines (axial or equatorial; cis or trans). Now switch the chlorines to make cis-1, 2 dichlorocyclohexane and describe the orientation of the C_1 and C_2 chlorines (axial or equatorial for each) Now switch the model to the other chair conformation and describe orientation of the chlorines at C_1 and C_2 individually (axial or equatorial for each). Summarize the effects of conformational shifts (alternate chairs) on the axial-equatorial and cis-trans orientation of the chlorines.

Answer 1

26. Which is the more stable conformation of cyclohexane (boat or chair)? Why?

Cuclohexane Chair Boat vansannuiar straun equartoial H axial H

The chair cyclohexane is the more stable conformation of cyclohexane. Cyclohexane adopts a spatial arrangement in chair form to avoid eclipsing between its 12 hydrogens, while the bond angles remain close to 109°. If we look along a C-C bond we can see an alternating arrangement of all substituents. This all reduces the potential energy of the conformation and makes it the most stable.

The boat shape is less stable because of the eclipsing of the 8 hydrogens at the bottom of the boat and the repulsion between the hydrogens projecting into the boat (transannular strain).