BR Organic Chemistry Chapter 2 Passage 1 #2

[dougkaye]

Curious about BR's logic here. The molecule in quest is 1,2,3D-cycohexanol (all facing out of page). it asks how many deuterium will have axial orientation in most stable conformation of the molecule.

They say that the C-D bond is shorter than C-H. My problem with this question is that a shorter bond doesn't really tell you whether or not the D or H is the more sterically hindered moiety. I'm assuming from the question that they think H is more sterically hindered than D, since the correct answer has 2 deuterium axial, and therefore only one equatorial. I guess you could assume that the longer bond for C-H makes it more "bulky" but from orgo class I know that you can't always easily predict the steric bulk of side-groups off cyclohexane.

any clarifications? I'm pretty good on cyclohexane/ring-flips/etc but this one caught me for a loop. Thanks.

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[jtw8]

The atoms attached to the cyclohexane molecule are not stationary, they are vibrating about their bond with each carbon atom. The D-C bond is shorter that the H-C bond so as each atom vibrates in space the longer bonded atoms (H) will move closer and farther from one another in space. This is the steric hindrance that is being referenced.
Hope this helps!! Good Luck

 

[TwoPaddles]

Curious about BR's logic here. The molecule in quest is 1,2,3D-cycohexanol (all facing out of page). it asks how many deuterium will have axial orientation in most stable conformation of the molecule.

They say that the C-D bond is shorter than C-H. My problem with this question is that a shorter bond doesn't really tell you whether or not the D or H is the more sterically hindered moiety. I'm assuming from the question that they think H is more sterically hindered than D, since the correct answer has 2 deuterium axial, and therefore only one equatorial. I guess you could assume that the longer bond for C-H makes it more "bulky" but from orgo class I know that you can't always easily predict the steric bulk of side-groups off cyclohexane.

any clarifications? I'm pretty good on cyclohexane/ring-flips/etc but this one caught me for a loop. Thanks.

sterically hindered side chains have equatorial orientation..and deuterium is heavier than hydrogen..however, deuterium atoms are preferred on axial position and hydrogen atoms on equatorial due to something called deuterium isotope effect...this is just a special case.

hope this helps.