Axial Equatorial Orgo Question

[dnovikov]

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A little confused with the explanation to question 2 from passage I in the second chapter of TBR orgo part 1 (pg 147). The answer states that "the most stable orientation (most stable chair confirmation) has as many deuterium atoms with axial orientation as possible. I thought that the most stable confirmation had it's bulkiest substituents equatorial...

If you're confused the question goes as follows:
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?

The picture is a cyclohexane with 3 deuteriums attached on 3 consecutive carbons of a cyclohexane all pointing out of the page. For reference, this question is found on page 147 of TBR Orgo part 1.

 

[SaintJude]

I think there was a post on this earlier in the forum. The clue they gave you is that the C-D bond is SHORTER than the C-H bond in this case. Thus it is not the bulkiest group in the molecule and thus will be in the axial orientation.
The C-H bond, the "bulky" group will be in the equatorial orientation, because the most stable form is that which has the bulkiest group in equatorial position.

 

[ljc]

If the D bond is shorter, it's taking up less space. This means it can squeeze into an axial position better than a hydrogen, because the hydrogen will experience more interference with the groups around it. Thus, D in axial is lower energy than H in axial.

 

[MedPR]

This must be a good question. It's giving quite a few people trouble.

I've seen 3 new threads about it since December!

 

[gettheleadout]

This must be a good question. It's giving quite a few people trouble.

I've seen 3 new threads about it since December!

SaintJude explained it well, too. Just because deuterium is heavier doesn't make it bulkier.