BR O Chem Section 2 Passage 1

[EmpathMan]

Hey all, quick question here.

"A carbon-deuterium bond is shorter than a carbon-hydrogen bond. Using this idea, how many deuterium atoms assume axial orientation in the most stable conformation of the following molecule?"

A) 0
B) 1
C) 2
D) 3

The molecule is a cyclohexane with 3 Deuteriums pointing up (wedge shaped) and all 3 are side by side. I thought the answer was 1, because you wanted as many equatorial orientations as possible. But the answer says you want as many axials as possible?

Thank you very much for your time!

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[Bru]

As stated in the passsage, the C-D bond is shorter then C-H. Therefore in terms of 1,3-diaxial costs, the C-D axials will be more preferred over the C-H axial. Basically the C-D bonds are going to be able to swing around and cause less steric issues as compared to the C-H ones which though small are larger then the C-Ds. It would be as if the C-D is a small group like a methyl and the C-H is a larger group like an ethyl in terms of preference.