TBR O-Chem - Ch. 3 Stereochemistry - Passage I Q. 2

[J3TTY]

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

The answer is diastereomer. Why is it not an epimer of isoleucine? I thought epimer would be more specific and thus, the better answer since only 1 stereocenter was changed.

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[MedPR]

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

The answer is diastereomer. Why is it not an epimer of isoleucine? I thought epimer would be more specific and thus, the better answer since only 1 stereocenter was changed.

I thought epimers were diastereomers that differed by the location of a hydroxyl group. Like a-glucose and b-glucose.

Anything else is just a plain diastereomer.

 

[Pisiform]

hmmm I also thought if there was a change to only one stereogenic center then epimer would be more accurate answer ,,, can someone clarify please?

 

[MedPR]

hmmm I also thought if there was a change to only one stereogenic center then epimer would be more accurate answer ,,, can someone clarify please?

There was another question today about epimer vs diastereomer. I thought epimers only referred to hydroxides on one stereogenic center.. like in alpha and beta glucose

Edit: Haha, nevermind this is the that thread.

 

[Pisiform]

There was another question today about epimer vs diastereomer. I thought epimers only referred to hydroxides on one stereogenic center.. like in alpha and beta glucose

Edit: Haha, nevermind this is the that thread.

lol ...MedPR is high

Ohh so yea that might be the problem only hydroxyl grp ... thanks

 

[J3TTY]

There was another question today about epimer vs diastereomer. I thought epimers only referred to hydroxides on one stereogenic center.. like in alpha and beta glucose

Edit: Haha, nevermind this is the that thread.

Yeah, this is the conclusion I came to as well because the 1001s used glucose as an example where an epimer of it was a better answer than a diastereomer. The only problem I have with this explanation is that according to google, amino acids can exist in epimeric forms as well. If only threonine can exist in epimeric form, that would make sense but, if isoleucine can as well, then our reasoning would be wrong :/. Can anyone confirm that it's the OH group switching that defines the epimer?

Wait no, isn't the alpha and beta called anomer and not epimer?

 

[MT Headed]

Geez guys, wikipedia isn't that far away.

"In chemistry, epimers are diastereomers that differ in configuration of only one stereogenic center."
http://en.wikipedia.org/wiki/Epimer

"In carbohydrate chemistry, an anomer is a special type of epimer. It is one of two stereoisomers of a cyclic saccharide that differs only in its configuration at the hemiacetal or hemiketal carbon, also called the anomeric carbon."
http://en.wikipedia.org/wiki/Anomers

 

[SaintJude]

Mt Headed is correct. To clarify in plain ol' English, which is more useful for me personally:

An anomer is a cyclical stereoisomer that differs in configuration at Carbon 1 (--only cabon "1"! a.k.a hemiacetal carbon).

Note the distinction b/w an anomer and an epimer: Yes an anomer is a special type of epimer but the configuration can only differ at C1. In an epimer the configuration can differ at ANY one of the stereogenic centers in a group (there are C-2, C-3, C-4 epimers)

Also, an anomer inolves only a hemiacetal carbon. The carbon atom bearing the two oxygen atoms (from the alcohol and alkyl group) is the hemiacetal carbon.

Naming anomers:
In a 6-membered ring, if the hydroxy group attached to the C1 and the substituent attached to C5 (the carbon adjacent to C1 ) are trans --> molecule is alpha-anomer. If both groups are cis---> molecule is beta-anomer.

 

[J3TTY]

Geez guys, wikipedia isn't that far away.

"In chemistry, epimers are diastereomers that differ in configuration of only one stereogenic center."
http://en.wikipedia.org/wiki/Epimer

"In carbohydrate chemistry, an anomer is a special type of epimer. It is one of two stereoisomers of a cyclic saccharide that differs only in its configuration at the hemiacetal or hemiketal carbon, also called the anomeric carbon."
http://en.wikipedia.org/wiki/Anomers

Thanks, but that just adds confusion as to why in the amino acid isoleucine, the change at one stereocenter was said to be a diastereomer but, there was a choice of epimer which I thought was the better answer. Why is this not the case? Thanks.

 

[MedPR]

Geez guys, wikipedia isn't that far away.

"In chemistry, epimers are diastereomers that differ in configuration of only one stereogenic center."
http://en.wikipedia.org/wiki/Epimer

"In carbohydrate chemistry, an anomer is a special type of epimer. It is one of two stereoisomers of a cyclic saccharide that differs only in its configuration at the hemiacetal or hemiketal carbon, also called the anomeric carbon."
http://en.wikipedia.org/wiki/Anomers

I think this is one of the times when wikipedia is wrong.

If compound A has 2 chiral centers (stereogenic centers) and you invert the absolute configuration of both, you have the enantiomer. If you invert the absolute configuration of only one, then you have the diastereomer. That's exactly what my undergrad taught, and exactly what TBR teaches. Every organic chemistry definition I've seen of an epimer is when one stereocenter differs in the orientation of a hydroxyl.

 

[MT Headed]

I think this is one of the times when wikipedia is wrong.

Every organic chemistry definition I've seen of an epimer is when one stereocenter differs in the orientation of a hydroxyl.

Hmm, from doing a google search about 2/3 of the definitions say an epimer is when only one carbon is inverted. The rest say that the inverted carbon also has to have an -OH group. So whom do we accept as our authority? Wikipedia uses IUPAC, and when it comes to the exact definition of chemical terms I don't think there is a higher authority than them.

"EPIMERS - Diastereoisomers that have the opposite configuration at only one of two or more tetrahedral stereogenic centres present in the respective molecular entities."
http://goldbook.iupac.org/E02167.html


I think team hydroxyl loses this round, and J3TTY was correct in selecting epimer for isoleucine.

 

[MedPR]

Nevermind, just read the part about tetrahedral in IUPAC.

So epimers are (R,R) and (R,S) isomers and every combination.

 

[MedPR]

TBR hints that epimers can only exist in sugars. In example 3.16 of TBR Orgo "The compound is not a sugar, so choice D (epimers) is eliminated."

If the term epimer pops up on the MCAT, should we go with the IUPAC definition? This makes two examples in TBR where diastereomer and epimer are both answer choices in a question where, by IUPAC definition, the answer should be epimer but TBR says diastereomer.

The question is:

The addition of alkyl magnesium bromide to a carbonyl in ether adds a new alkyl substituent to the carbonyl carbon, resulting in conversion of the carbonyl into an alcohol. The addition of CH3MgBr to R-2-methylcyclohexanone in diethylether yields which products?

A. meso compound
B. two diastereomers
C. two enantiomers
D. two epimers

The products will differ only at carbon 1. We still consider cyclohexane carbons to be sp3 and tetrahedral, correct? So by the IUPAC definition, the products are two epimers. Is it possible that the IUPAC definition recently changed? If so, is it also possible that the AAMC will (is) be using the old definition of epimer (whatever that may be)?