[AAMC 8] Stereogenic center of glycerol

[sc4s2cg]

When glycerol reacts with three different fatty acids, how many stereogenic centers does the product triacylglycerol contain?

A) 0
B) 1
C) 2
D) 3

I thought there was 0, since a triacylglycerol looks like:

CH2 - CO2 - R
CH - CO2 - R'
CH2 - CO2 - R"

So outer two carbons are out, because they have two hydrogens. My thinking was that the inner carbon is out as well, because it is attached to two CH2s. But AAMC says the answer is 1 because of the middle carbon.

What am I missing?

---

Members don't see this ad.

 

[rjosh33]

When glycerol reacts with three different fatty acids, how many stereogenic centers does the product triacylglycerol contain?

A) 0
B) 1
C) 2
D) 3

I thought there was 0, since a triacylglycerol looks like:

CH2 - CO2 - R
CH - CO2 - R'
CH2 - CO2 - R"

So outer two carbons are out, because they have two hydrogens. My thinking was that the inner carbon is out as well, because it is attached to two CH2s. But AAMC says the answer is 1 because of the middle carbon.

What am I missing?

You have to look at the whole group attached to the carbon in question. Those methylene groups are then attached to different fatty acid chains (as the question stem tells us), so the middle carbon is considered to be attached to 4 different groups. That makes it chiral.

 

[sc4s2cg]

Ah. So just like in cyclic compounds, if there is a similarity then we move on to the next attached atoms? So here CH2s are the same, then two COO-, and finally two different R groups.

 

[rjosh33]

Ah. So just like in cyclic compounds, if there is a similarity then we move on to the next attached atoms? So here CH2s are the same, then two COO-, and finally two different R groups.

Exactly.

 

[sc4s2cg]

Thank you for the help.

 

[sehri1234]

Hey guys,
I am also a little confused about this. I initially it would be 0 because even if you move to the next attached atom - the first and the third chain are exactly the same though and have the same atoms in same sequence. So, wouldn't that just count as having like a double bond?

 

[theonlytycrane]

All of the fatty acids attached are different, so the chains are different.