Penultimate Carbon

[MedPR]

Which one is it? The Alpha carbon? Beta Carbon? 🙁

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[mcloaf]

Well, penultimate means next to last, so whether it's alpha or beta or whatever is going to depend on the molecule in question.

I assume you're asking about carbohydrates, in which case the penultimate carbon is that which determines L or D.

 

[MedPR]

Well, penultimate means next to last, so whether it's alpha or beta or whatever is going to depend on the molecule in question.

I assume you're asking about carbohydrates, in which case the penultimate carbon is that which determines L or D.

Yea I'm talking about carbohydrates. I know that an R penultimate carbon = D, and S = L, but which carbon is the penultimate carbon? Is it the first chiral carbon?

 

[mcloaf]

Yea I'm talking about carbohydrates. I know that an R penultimate carbon = D, and S = L, but which carbon is the penultimate carbon? Is it the first chiral carbon?

Looking at a regular Fisher (Fischer?) projection the carbons are going to be numbered from top to bottom starting at the top because of the carbonyl group. So the penultimate (second to last carbon) carbon is the last chiral carbon of the chain. The alcohol group attached to this carbon is that which attacks the carbonyl during cyclization of a sugar. See the picture here: http://www.harpercollege.edu/tm-ps/chm/100/dgodambe/thedisk/carbo/yback3.htm

 

[MedPR]

Looking at a regular Fisher (Fischer?) projection the carbons are going to be numbered from top to bottom starting at the top because of the carbonyl group. So the penultimate (second to last carbon) carbon is the last chiral carbon of the chain. The alcohol group attached to this carbon is that which attacks the carbonyl during cyclization of a sugar. See the picture here: http://www.harpercollege.edu/tm-ps/chm/100/dgodambe/thedisk/carbo/yback3.htm

Thanks again.