AAMC Unscored C/P Question #2: chirality centers

[Gurby]

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The question asks you to find how many stereoisomers of this compound exist. The formula is 2^n where n is number of stereocenters. The answer they want is 2^5 = 32.

I don't understand why the atoms marked in red are not stereocenters? A nitrogen bound to three different groups and a lone pair should have tetrahedral shape and be chiral, no?

 

[exacto]

For MCAT purposes, only think of chiral centers involving a central carbon and 4 different groups.

 

[Gurby]

For MCAT purposes, only think of chiral centers involving a central carbon and 4 different groups.

But why? Am I wrong, or did they arbitrarily decide to change the laws of chemistry?

 

[aldol16]

But why? Am I wrong, or did they arbitrarily decide to change the laws of chemistry?

I don't understand why the atoms marked in red are not stereocenters? A nitrogen bound to three different groups and a lone pair should have tetrahedral shape and be chiral, no?

They didn't change it completely arbitrarily. As the poster above said, for MCAT purposes, nitrogens are not stereocenters. But according to chemistry, trigonal pyramidal centers exhibit rapid inversion - even at room temperature. So these centers will not be optically active. (These are trigonal pyramidal and not tetrahedral).

 

[Gurby]

They didn't change it completely arbitrarily. As the poster above said, for MCAT purposes, nitrogens are not stereocenters. But according to chemistry, trigonal pyramidal centers exhibit rapid inversion - even at room temperature. So these centers will not be optically active. (These are trigonal pyramidal and not tetrahedral).

Ahhhhh I see, thank you.

 

[7331poas]

Unless the nitrogen is locked in a three members ring and thus the inversion is thermodynamically unfavorable!

Man, isn't organic fun.

 

[aldol16]

Unless the nitrogen is locked in a three members ring and thus the inversion is thermodynamics lily unfavorable!

But I doubt aziridines or even azetidines will come up on the MCAT. They're pretty obscure and only used by chemists - they have few applications in biological chemistry.

 

[7331poas]

But I doubt aziridines or even azetidines will come up on the MCAT. They're pretty obscure and only used by chemists - they have few applications in biological chemistry.

Oh yea there is no way. But knowledge is fun.

 

[aldol16]

Oh yea there is no way. But knowledge is fun.

It is 😉 Especially chemistry knowledge. I know, I'm a nerd (but I don't even enjoy organic chemistry that much!).

 

[theonlytycrane]

if a nitrogen atom had 4 bonds, could it be considered a chiral center?

 

[aldol16]

if a nitrogen atom had 4 bonds, could it be considered a chiral center?

Yes.

http://www.chem.ucla.edu/~harding/tutorials/nitrogen_inversion.pdf

 

[7331poas]

No.

http://www.chem.ucla.edu/~harding/tutorials/nitrogen_inversion.pdf

Your source says otherwise. The reason nitrogen a aren't chiral centers is because of inversion of its lone pair.

If the nitrogen is quartenary then there is no inversion phenomenon, making it a stereo center.

 

[aldol16]

Your source says otherwise. The reason nitrogen a aren't chiral centers is because of inversion of its lone pair.

If the nitrogen is quartenary then there is no inversion phenomenon, making it a stereo center.

Sorry, I have edited above. For some reason I was thinking the poster was saying "tetrahedral" instead of "4 bonds."