My lecture notes describe epimers as compounds which differ by configuration at only one carbon and gives the example of D-erythrose and D-threose as being epimers.
However isn't that the same thing as diastereomers? My understanding is that epimers are supposed to be some kind of subset of diastereomers, however I am not sure how or why. I tried searching the web but I got more confused with one site saying that carbohydrates in open form can have two different chirality centres and still be epimers.
Answer
epimers as compounds which differ by configuration at only one carbon
Yes, at only one centre.
However isn't that the same thing as diastereomers?
No, not completely. Diastereomers differ at least at one, but at less than all stereocentres. If two compounds would differ at all stereocentres, they would be enantiomers.
Update
As far as epimerism in open form carbohydrates is concerned, examining the Fischer projections using a German mnemonic derived from the signal horn sounds of police cars (ta-tü-ta-ta) might help. Yes, sounds weird ;-)
Let ta and tü be $\ce{OH}$ on either side of the projection.
Then, glucose is ta-tü-ta-ta, whereas galactose is ta-tü-tü-ta. Apparently, they differ at one stereocentre. Consequently, they are epimers.
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