Question

Could you please explain how can I get the answer of Q13 "only (b) "

13. (a) The conformations of some highly substituted cyclohexanes were studied (Dalton Trans 2013, 42, 13404). The two chair conformations for compound J differ by 18 kcal/mol, draw the more stable chair conformation of compound J. Include all 6 substituents and the hydrogen atoin groups associated with each vertex. Take care to create a clear drawing (11o11-boricding electrons do not need to be shown). N(CH3)2 ОН OH HO (H3CN. (H3C) NY N(CH3)2 (H3C)2N N(CH3)2 Он conpoLund J H NH, (b) Another one of the structures studied was compound K. Is the difference in energy between the two chair conformations of compound Knole likely equal to 0.5, 16, 20, or 35.5 kcal/mol? Select which one it is. The energy difference between the two chair conformations of K is most like to be (circle one): 0.5 (16) 20 35.51 H2NYNH OH compound K

Answer 1

Substituent's on the chair conformation prefer to be in the equatorial position. The reason is that when substituents are in the axial position, there tends to be more unfavorable interactions with other axial atoms on the same side. These unfavorable interactions are called 1,3-diaxial interactions. When substituents are in the equatorial position, they are farther away from each other. This increases the stability of the conformation.

In our case, there is a possibility of hydrogen bonding also. Usually hydrogen bonding will increse the stability of confirmation.

Conformation-1 is having three OH groups on axial positions (circles in the figure), though there is a possibility for hydrogen bonding, there will be repulsion between lone pairs, hence conformation-1 is less stable.

In conformation-2 repulsions are weak/less compare to conformation 1. Hence conformation 2 is more stable than conformation-1

OH OH NH HO NH NH2 H2N Chair -2 Chair-1