AAMC7 question about stereoisomers

[Meredith92]

Question #95 has a picture of a compound with 4 chiral carbons and one group that is R-NHCHO. The question asks what is the max possible number of stereoisomers
This nitrogen I believe counts as a stereocenter! But the answer says there are only 4 stereocenters.... why is this?

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[sazerac]

Question #95 has a picture of a compound with 4 chiral carbons and one group that is R-NHCHO. The question asks what is the max possible number of stereoisomers
This nitrogen I believe counts as a stereocenter! But the answer says there are only 4 stereocenters.... why is this?

Because the energy requirement to "pop" the lone pair of electrons to the other side of a nitrogen is so small, for the purposes of the MCAT all nitrogens are racemized. There are some obscure cases where stereohinderance would cause the orientation of the nitrogen to be fixed (and therefore a stereocenter), but this is beyond the scope of the MCAT. For the MCAT, only carbons are stereocenters.

Here is another explanation:

http://www.chem.ucla.edu/harding/ec_tutorials/tutorial05.pdf

 

[MD Odyssey]

I seem to remember making the same mistake on that practice exam as well. In a sense, it's a good thing that question is on the exam so that you can make the mistake and learn not to count nitrogen atoms. Better to make it here and not on the real thing.

 

[Meredith92]

I seem to remember making the same mistake on that practice exam as well. In a sense, it's a good thing that question is on the exam so that you can make the mistake and learn not to count nitrogen atoms. Better to make it here and not on the real thing.

Agreed! I'm glad this came up now instead of on the actual exam thanks for your help!

 

[MaDA]

Question #95 has a picture of a compound with 4 chiral carbons and one group that is R-NHCHO. The question asks what is the max possible number of stereoisomers
This nitrogen I believe counts as a stereocenter! But the answer says there are only 4 stereocenters.... why is this?

How did you get 4 stereoisomers for the question? I keep getting 3 for some reason.

I have the one that has the CH3 out of the page, the one at the top of the 5C ring and the one with the isopropyl group.... which one am I missing?

Thanks

**** Never mind! I just looked at it after I posted and found what I missed.... ahaha