stereogenic centres

[maria]

Okay I need some clarification with this stuff.

 

I was told that a stereogenic centre is a carbon with 4 DIFFERENT groups attached onto it so for example a carbon with a bromine, a hydrogen, R, and R' would be a sterogenic centre. But if a carbon is attached to say R1, CH2, and double bonded to an oxygen, this would not be stereogenic correct since only three groups are attached? Another example would be C attached to 2 H's, a CH and a C=O since again there are three different groups attached.

 

I really an confused with this and get it right on some practice questions and wrong on others all the wile using the exact same method. Any help would be appreciated. Thanks!

[supafield]

Okay I need some clarification with this stuff.

 

I was told that a stereogenic centre is a carbon with 4 DIFFERENT groups attached onto it so for example a carbon with a bromine, a hydrogen, R, and R' would be a sterogenic centre. But if a carbon is attached to say R1, CH2, and double bonded to an oxygen, this would not be stereogenic correct since only three groups are attached? Another example would be C attached to 2 H's, a CH and a C=O since again there are three different groups attached.

 

I really an confused with this and get it right on some practice questions and wrong on others all the wile using the exact same method. Any help would be appreciated. Thanks!

 

Stereogenic centres are sp3 hybridized with 4 different molecules...

sounds like you're doing it right

[maria]

I think so too but on AAMC #9 there was a question about the number of sterogenic centres that triacyglycerol has and the answer was 1 but I thought it was o since all the carbons are either attached to two hydrogens and hence only three different groups or they are sp2 hybridized. hmmmm.....

[supafield]

I think so too but on AAMC #9 there was a question about the number of sterogenic centres that triacyglycerol has and the answer was 1 but I thought it was o since all the carbons are either attached to two hydrogens and hence only three different groups or they are sp2 hybridized. hmmmm.....

 

For a triglyceride? the middle carbon on the glycerol molecule is chiral as long as the fatty acids are different.

[Jochi1543]

No, stereoGENIC centers (i.e. centers that can GENERATE stereospecific - chiral - molecules) are carbons with 3 bonds (often 1 double and 2 single ones). If you add another element to this carbon, it gets 4 bonds (the double turns into a single) - so if the original carbon had 3 different substituents, and the 4th substituent is different yet again, this carbon turns into a chiral center.

 

For example, if you have a carbon Ca, in a set-up like this:

 

C-Ca-H

II

O

 

(that's the Ca bonded to a carbon, a hydrogen, and double-bonded to oxygen)

 

An addition of, say, F to that Ca will produce a set-up like this:

 

 

F

I

C-Ca - H

I

O

 

This is a chiral center now. Therefore, carbon Ca is stereogenic if a substituent other than C, H, or O is added.

 

 

 

EDIT: crap, my awesome diagrams got shifted. Anyway, I hope the verbal expression conveys what I'm talking about.

[maria]

ooooooooh......so a chiral centre is a C with four different groups on it and a stereogenic centre is one that could potentially lead to a chiral centre when another molecule is added?

 

 

That makes sense now. Thanks so much.

[supafield]

Yup, you got it!

 

Really? Princeton said they were synonyms!! haha good to know with my MCAT starting in about 13 hours

[jpc43]

i tend to disagree with jochi.

 

the question the op is referring to shows the triacylglycerol molecule, and it clearly has one chiral (or stereogenic) carbon. It may be the case that these two terms are not synonymous in more advanced organic chemistry, but for the sake of the MCAT they do mean the same thing.

 

Question 119 of aamc #8

 

kaplan answer:

"This item can be considered a pseudo-discrete, since we do not need information from the passage to answer it.Glancing at the answer choices, we are looking for a numerical answer that will reveal the number of stereogenic centers in the triacylglycerol shown in Reaction 1. First, we must understand what a stereogenic center implies. A stereogenic center, or chiral carbon, is a carbon atom bound to four different groups or atoms. In our case, the only carbon in triacylglycerol that is attached to four different groups is the second carbon in the glycerol backbone of triacylglycerol; C2 is thus considered to be a stereogenic center. The remaining carbon atoms are not. Why is that the case? The C1 and C3 of glycerol are bound to two hydrogen atoms—by definition, these atoms cannot be stereogenic. The first carbon of each fatty acid chain is bound to the R group and two oxygen atoms, also precluding any classification as stereogenic centers. The correct answer is, again, choice (."