epimers

[t1me]

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Are epimers necessarily sugars (as suggested per Example 3.16 BR Ochem solution "The compound is not a sugar, so ["two epimers"] is eliminated)?

Also on a side note, does Cahn-Ingold-Prelog rules dictate atomic mass or number as the first rule to priority? It obviously is not that important of a distinction because the "important" atoms are ordered the same way by either convention. I just wanted to know out of curiosity about the official rule since I've seen both in a number of places.

 

[Rabolisk]

Epimers are diastereomers that are different at only one stereogenic center. Therefore, epimers are not necessarily sugars.

CIP rules dictate atomic number as the first rule, then mass number.

 

[orgohacks]

Epimers are diastereomers that are different at only one stereogenic center. Therefore, epimers are not necessarily sugars.

CIP rules dictate atomic number as the first rule, then mass number.

More specificially, epimers differ at one stereogenic center where one of the substituents is hydrogen. Most commonly referred to when you can convert, say, (R) into (S) by treatment with base followed by quench.
Probably won't come up, but that's what it means.

Rabolisk is right that atomic number comes first then mass number. So for instance, deuterium (D) is favored over protium (H).