Deuterium - Axial vs Equatorial on cyclohexane

[RichardParker]

Will Deuterium have any preference for equatorial or axial position on a cyclohexane in chair conformation? Its heavier than Hydrogen (but not necessarily bulkier?) so I was leaning towards equatorial. But the answer is axial and I don't know why ?

For anyone who has the BR orgo book, its pg 147Q2.

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[BerkReviewTeach]

Will Deuterium have any preference for equatorial or axial position on a cyclohexane in chair conformation? Its heavier than Hydrogen (but not necessarily bulkier?) so I was leaning towards equatorial. But the answer is axial and I don't know why ?

For anyone who has the BR orgo book, its pg 147Q2.

This is known as the deuterium isotope effect. Because D is heavier than H, when the C-D bond vibrates, it stretches less than a C-H bond, making it shorter at its full stretched state. Because it is shorter, C-D bonds take up less space than C-H bonds and are therefore less sterically hindered. The result is that CH3 is a bulkier group than CD3.

The question you are considering is C-D vs. C-H for equatorial orientation. Because a C-H bond is longer (and more sterically hindering) than a C-D bond, the C-H bond chooses equatorial aligment pushing the C-D to axial.

Hope this helps.

 

[RichardParker]

Ohh, Yes it makes sense now.
Thanks for the answer.

 

[ajs121]

This is known as the deuterium isotope effect. Because D is heavier than H, when the C-D bond vibrates, it stretches less than a C-H bond, making it shorter at its full stretched state. Because it is shorter, C-D bonds take up less space than C-H bonds and are therefore less sterically hindered. The result is that CH3 is a bulkier group than CD3.

The question you are considering is C-D vs. C-H for equatorial orientation. Because a C-H bond is longer (and more sterically hindering) than a C-D bond, the C-H bond chooses equatorial aligment pushing the C-D to axial.

Hope this helps.

Wow! Great answer...I would have thought the opposite had I not read your explanation. Thanks!