1. If cyclohexane were planar, how many of the three types of strain (torsional,

Question

1. If cyclohexane were planar, how many of the three types of strain (torsional, angle, steric) would it have, and what feature(s) of its structure would cause each type of strain?

2. Does the chair conformation of cyclohexane have any of these types of strain?

3. For each type of strain in which chair cyclohexane has less strain than planar cyclohexane, what is the difference in the structure of the chair that allows it to have less strain?

4. What is the role of 1,3-diaxial interactions in contributing to the energy of a cyclohexane with a substituent?

5. How can you describe the two conformations of cis-1-bromo-2-ethylcyclohexane, Conformer A and Conformer B, in a way that clearly distinguishes them?

6. Which group has the larger steric impact, ethyl or bromo?

7. Which of the two chair conformations of cis-1-bromo-2-ethylcyclohexane is more thermodynamically stable and why, or are they equivalent in energy?

8. Which of the two chair conformations will be the major component at equilibrium, or will there be a 50:50 mixture?

Explanation / Answer

1. If the carbons of a cyclohexane ring were placed at the corners of a regular planar hexagon, all the C-C-C bond angles would have to be 120". Because the expected normal C-C-C bond angle should be near the tetrahedral value of 109.5", the suggested planar configuration of cyclohexane would have angle strain at each of the carbons.

Number of angle strain = 6

Also, in planar structure, the C and Hs will be in favourable eclipsed position. Total number of steric strain = 6

Torsional strain arises due to the atoms which are close to each other. In cyclohexane, C-H bonds are close to each other. Number of torsional strain = 6

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