TBR O-Chem Section II Passage I

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kg21

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The question has shows a picture of a cyclohexane with 3 deuterium substituents, all cis to each other (1,2,3-triD-cyclohexane?). "A carbon-deuterium bon 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?"

C-D bonds are shorter than C-H bonds, so I am looking for the chair conformation that places as many deuterium atoms in the axial position as possible, which would minimize the amount of hydrogen atoms in the axial position, because the steric interactions from C-H bonds are greater than C-D bonds?

The answer explains "the most stable orientation has a many deuterium atoms with axial orientation as possible". This is because the deuterium atoms create less steric hindrance than hydrogen atoms in the axial position? The answer shows a chair conformation with two deuterium in the axial position, one in the equatorial position, with two equatorial hydrogen and one axial hydrogen.

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The question has shows a picture of a cyclohexane with 3 deuterium substituents, all cis to each other (1,2,3-triD-cyclohexane?). "A carbon-deuterium bon 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?"

C-D bonds are shorter than C-H bonds, so I am looking for the chair conformation that places as many deuterium atoms in the axial position as possible, which would minimize the amount of hydrogen atoms in the axial position, because the steric interactions from C-H bonds are greater than C-D bonds?

The answer explains "the most stable orientation has a many deuterium atoms with axial orientation as possible". This is because the deuterium atoms create less steric hindrance than hydrogen atoms in the axial position? The answer shows a chair conformation with two deuterium in the axial position, one in the equatorial position, with two equatorial hydrogen and one axial hydrogen.

CD bonds are shorter and provide less steric hindrance. You want as many CD bonds axial as possible. Axial CH bonds are unfavorable and you want to minimize them. Because all of the bonds are coming out of the page, and are on 3 carbons in a row, you can only have 2 of the 3 CD bonds axial. You can't have 3 CD axial bonds in a row. So the order, on each carbon, goes like this: 1 axial CD/1 equatorial CH, 1 equatorial CD/1 axial CH, 1 axial CD/1 equatorial CH.
 
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