Bad Berkeley Review question is Bad?

[t1me]

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This is L-Isoleucine.

The question is from Berkeley Review Ochem ch3 passage 1 question 2:

"If the side chain chiral center were changed, the new structure would be switched to which of the following?

a. an enantiomer of isoleucine.
b. a diastereomer of isoleucine.
c. an epimer of isoleucine.
d. identical to isoleucine."

My answer was c, but BR's answer is b. I thought the definition of an epimer is a diastereomer that only differs by 1 stereocenter and has a hydrogen substituent. See orgohack's post. What gives? Bad question? Do I have an outdated version of the book? BerkeleyReviewTeach care to comment?

 

[Rabolisk]

Bad question as the answer is clearly both b and c. One thing to note is that the term epimer is mostly used in the mcat when talking about sugars.

Actually, I can make the case for a as well. Nitrogen is actually sp2 hybridized in an amino acid.

 

[PiBond]

This is L-Isoleucine.

The question is from Berkeley Review Ochem ch3 passage 1 question 2:

"If the side chain chiral center were changed, the new structure would be switched to which of the following?

a. an enantiomer of isoleucine.
b. a diastereomer of isoleucine.
c. an epimer of isoleucine.
d. identical to isoleucine."

My answer was c, but BR's answer is b. I thought the definition of an epimer is a diastereomer that only differs by 1 stereocenter and has a hydrogen substituent. See orgohack's post. What gives? Bad question? Do I have an outdated version of the book? BerkeleyReviewTeach care to comment?

Since there's only two chiral centers in the molecule, the "better" answer is B. We don't say that two molecules with the relationship RS and SS are epimers, we say they are diastereomers.

Bad question as the answer is clearly both b and c. One thing to note is that the term epimer is mostly used in the mcat when talking about sugars.

Actually, I can make the case for a as well. Nitrogen is actually sp2 hybridized in an amino acid.

this is false. the nitrogen isn't adjacent to an sp2 atom and therefore has no where to donate its lone pair through. therefore it's sp3. you must be confusing this with the nitrogen in an amide bond (which is sp2)

 

[Rabolisk]

Since there's only two chiral centers in the molecule, the "better" answer is B. We don't say that two molecules with the relationship RS and SS are epimers, we say they are diastereomers.



this is false. the nitrogen isn't adjacent to an sp2 atom and therefore has no where to donate its lone pair through. therefore it's sp3. you must be confusing this with the nitrogen in an amide bond (which is sp2)

Yeah I should clarify, I meant nitrogen in a peptide where amino acids are bonded together, like an amide. Thanks for noticing.

 

[t1me]

Since there's only two chiral centers in the molecule, the "better" answer is B. We don't say that two molecules with the relationship RS and SS are epimers, we say they are diastereomers.

Thanks for the reply, but could you elaborate on the official definition of epimer? So far including this new information you've provided, I've gathered that an epimer is:

1) a diastereomer that differs in configuration by only 1 stereocenter.
2) the stereocenter of difference needs to have a hydrogen substituent.
3) when there are only 2 stereocenters, rule 1 is nullified and the molecule is a diastereomer.

Anything else? Not to doubt your credibility, but maybe you can link to a page that explains the all the epimer rules as an extra resource. That would be very helpful. Thanks!

 

[Don Draper]

pointless MCAT studying occurring now.

Keep to the basics guys.

For MCAT - epimer deals with sugars
For MCAT - you know what a diastereomer is if you've taken Ochem. Some but not all of chiral centers are different.

 

[t1me]

pointless MCAT studying occurring now.

Keep to the basics guys.

For MCAT - epimer deals with sugars
For MCAT - you know what a diastereomer is if you've taken Ochem. Some but not all of chiral centers are different.

Thanks for the reply Don.

I just don't see why none of the MCAT books I used could just straight out say "epimers deals with sugars; if it's not a sugar and you see it on the MCAT, it's not an epimer".

 

[Don Draper]

Thanks for the reply Don.

I just don't see why none of the MCAT books I used could just straight out say "epimers deals with sugars; if it's not a sugar and you see it on the MCAT, it's not an epimer".

because they don't want to come out and make a generality. You will get a better feel once you take FL practice tests. The material will all the sudden quickly narrow down into 1/3 of what BR was going over.

I took all 7 BR FLs and MOST of their content doesn't reach their FLs. BR likes to cover everything in depth but that's not what the MCAT is really about. Make sure to take real FLs to get a feel (not only doing BR passages which are mostly hard and often overboard).