How many chiral centers. AAMC Sample test Q2

[Life2dmax]

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Hey sorta basic question but this one tripped me up. I know the basics of a chiral center and that nitrogen with a lone pair can be one. Why are there only 5 for this molecule? I counted 3 nitrogens and 4 carbons

 

[aldol16]

You missed a carbon. Nitrogens are generally not involved for the purposes of the MCAT and in this case, if you're interested, it's because the barrier to inversion for trigonal pyramidal nitrogens is so low that you don't get any optical activity due to those nitrogens. In other terms, the stereochemistry at the nitrogens are not fixed at all.

 

[Life2dmax]

You missed a carbon. Nitrogens are generally not involved for the purposes of the MCAT and in this case, if you're interested, it's because the barrier to inversion for trigonal pyramidal nitrogens is so low that you don't get any optical activity due to those nitrogens. In other terms, the stereochemistry at the nitrogens are not fixed at all.

Hm. Didn't know that Nitrogens didn't count...but why would the barrier for inversion be so low? I thought if anything the barrier would be higher because of how big and bulky they are. Is this a general rule for the MCAT or just under some situations?

These are the 5 carbons if i'm not mistaken

 

[aldol16]

Hm. Didn't know that Nitrogens didn't count...but why would the barrier for inversion be so low? I thought if anything the barrier would be higher because of how big and bulky they are. Is this a general rule for the MCAT or just under some situations?

Well, the first thing that should tip you off is that in the picture, the stereochemistry is shown very specifically. It would defeat the purpose of the picture if they just didn't draw the stereochemistry at the nitrogens (assuming the nitrogens had any).

Nitrogen inversion is actually quite well known in organic chemistry and has many implications. If you're not familiar with the concept of nitrogen inversion, Wikipedia may provide some brief insight: https://en.wikipedia.org/wiki/Nitrogen_inversion. Note that the kinetic barrier for reactions to occur spontaneously at room temp is around 24 kcal or so. This is more comprehensive: http://www.chem.ucla.edu/harding/tutorials/nitrogen_inversion.pdf.

For more advanced study, you may want to look at some early papers - these cover theoretical and experimental inversion barriers in aziridines as well as piperidine.

http://pubs.acs.org/doi/abs/10.1021/ja01552a048
http://pubs.acs.org/doi/abs/10.1021/ja00450a064