Cyclic Tertiary Amine= Chiral??

[Kam3725]

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Is a cyclic tertiary amine, such as the one shown in the attachment, chiral? If so, would it be counted in the 2^n rule?

 

[MD Odyssey]

Is a cyclic tertiary amine, such as the one shown in the attachment, chiral? If so, would it be counted in the 2^n rule?

Not sure what you mean by a 2^n rule, but the nitrogen in that molecule is a chiral center. It has 4 unique substituents - build a model and it should be pretty obvious that its mirror image is non-superimposable.

 

[salim271]

Not sure what you mean by a 2^n rule, but the nitrogen in that molecule is a chiral center. It has 4 unique substituents - build a model and it should be pretty obvious that its mirror image is non-superimposable.

It only has 3, if the nitrogen had four substituents it would be charged.

 

[Kam3725]

the lone pair counts as a substituent; I'm referring to the formula that determines the number of possible stereoisomers by raising 2 to the nth power where n is the number of stereocenters.

 

[Kam3725]

the reason I ask is that in one of the TPR practice passages (Orgo Passage 1: Science Workbook), they fail to count a similar nitrogen as a stereocenter.

 

[MD Odyssey]

It only has 3, if the nitrogen had four substituents it would be charged.

There's a lone pair on the nitrogen, which gives that particular molecule a trigonal pyramidal shape, so the methyl group winds up being either above the ring or below it. That's why it's a chiral center. However, it's not permanent, due to nitrogen inversion (see my next statement).

I'm referring to the formula that determines the number of possible stereoisomers by raising 2 to the nth power where n is the number of stereocenters.

I had forgotten about this, but I suspect that the reason that TPR isn't calling it a chiral center or including it in your rule is because of nitrogen inversion. At normal temperatures, nitrogen compounds actually invert themselves, which is why the drawing you posted earlier doesn't indicate any chirality. Without really low temperatures, it's probably not possible to isolate an R or S variant.

This is somewhat analagous to how molecules undergo conformational changes at normal temperatures, although it is different in that the actual structure changes. One way to think about it is that it is in equilibrium with its stereoisomer at room temperatures.

 

[MD Odyssey]

This is somewhat similar to how molecules undergo conformational changes at normal temperatures, although it is different in that the actual structure changes. One way to think about it is that it is in equilibrium with its stereoisomer at room temperatures.

I checked and, for something like ammonia, the barrier for inversion is about 25 kJ/mol. For cyclohexane, the energy required to go between conformations is something like 10 kJ/mol. So, it doesn't look like it takes much to get it to flip from one configuration to the other.

Also, if you had some giant molecule where the nitrogen atom configuration was locked in place, I'd imagine that it would be appropriate to refer to it as chiral. Probably not something to worry about for the MCAT though.

 

[salim271]

There's a lone pair on the nitrogen, which gives that particular molecule a trigonal pyramidal shape, so the methyl group winds up being either above the ring or below it. That's why it's a chiral center. However, it's not permanent, due to nitrogen inversion (see my next statement).

Ah, never heard of that before, guess its something to keep in mind.

 

[Integrated MCAT Course]

There's a lone pair on the nitrogen, which gives that particular molecule a trigonal pyramidal shape, so the methyl group winds up being either above the ring or below it. That's why it's a chiral center. However, it's not permanent, due to nitrogen inversion (see my next statement).

I had forgotten about this, but I suspect that the reason that TPR isn't calling it a chiral center or including it in your rule is because of nitrogen inversion. At normal temperatures, nitrogen compounds actually invert themselves, which is why the drawing you posted earlier doesn't indicate any chirality. Without really low temperatures, it's probably not possible to isolate an R or S variant.

This is somewhat analagous to how molecules undergo conformational changes at normal temperatures, although it is different in that the actual structure changes. One way to think about it is that it is in equilibrium with its stereoisomer at room temperatures.

Great post

 

[Kam3725]

There's a lone pair on the nitrogen, which gives that particular molecule a trigonal pyramidal shape, so the methyl group winds up being either above the ring or below it. That's why it's a chiral center. However, it's not permanent, due to nitrogen inversion (see my next statement).

I had forgotten about this, but I suspect that the reason that TPR isn't calling it a chiral center or including it in your rule is because of nitrogen inversion. At normal temperatures, nitrogen compounds actually invert themselves, which is why the drawing you posted earlier doesn't indicate any chirality. Without really low temperatures, it's probably not possible to isolate an R or S variant.

This is somewhat analagous to how molecules undergo conformational changes at normal temperatures, although it is different in that the actual structure changes. One way to think about it is that it is in equilibrium with its stereoisomer at room temperatures.

Thanks for the help!