[cms2002]

So you have two butane compounds given as Newman projections.  If you find the chiral carbon and assign R and S, you find that they are enantiomers, the butane on the left being "R"; the butane on the right is "S".  Makes sense.  What I can't visualize is how these two are enantiomers (I understand by definition a compound with one stereocenter is chiral, I just can't visualize it).  A molecular model placing the chiral carbons as mirror images does not place the position-4-carbons in mirror image position (which seems would make them diastereomers).  Are you assuming rotation about the C2-C3 bond (and hence this is why double and triple bonds are also considered stereocenters - no rotation allowed, and thus interchanging groups changes a stereoisomer) or are non-chiral atoms completely ignored when determining mirror images? Please help.

(EDITED for better terminology.  Please correct me if I misuse anything.)

[gfunk]

Firstly, it's not butane, it's 3-hexanol (there are six carbons!).

Rotate about one axis, then rotate the front face, and voila, you have a mirror image!  You have to manipulate it around -- the question won't give the answer to you easily.  You need to work it.

Asymmetric carbon centres have four different substituents attached to it.  In the case of 3-hexanol, the four unique substituents are hydrogen, hydroxyl, propyl, and ethyl.

[cms2002]

Ah, hexanol, I knew that! 

I wasn't sure if you were allowed to rotate one face and not the other, but since the two faces are connected by a single bond, I suspected this was possible.  Thanks for the explanations and diagrams!