[Sophia7X]

In my notes, it says nonsuperimposable stereoisomers that are mirror images are called enantiomers... this makes me think that there might be superimposable stereoisomers? Is that possible?

[discodermolide]

In my notes, it says nonsuperimposable stereoisomers that are mirror images are called enantiomers... this makes me think that there might be superimposable stereoisomers? Is that possible?

No, I don't think so. If they are imposable then they will be the same compound and not stereoisomers.

[AWK]

http://en.wikipedia.org/wiki/Stereoisomers

[Sophia7X]

In my notes, it says nonsuperimposable stereoisomers that are mirror images are called enantiomers... this makes me think that there might be superimposable stereoisomers? Is that possible?

No, I don't think so. If they are imposable then they will be the same compound and not stereoisomers.

Oh, that make sense. Thanks.

[fledarmus]

Well, yes and no. The definition of stereoisomers includes diastereomers. In general, you would expect that with n stereocenters you would expect 2ⁿ stereoisomers, but if you have meso structures, some of those stereoisomers will be superimposable on their mirror-images, lowering the total number of stereoisomers possible.

So, if you have two stereocenters, you can expect to have 4 stereoisomers. This would break down into two pairs of mirror-images. If one of the pairs is a meso structure, it will be superimposable and that particular structure does not have enantiomers. The other pair, which are nonsuperimposable mirror images, are enantiomers of each other as well as diastereomers.

[Dan]

Well, yes and no.

This is misleading. The answer is a definite no. If two structures are superimposable than they are the same compound and not isomers of any kind.

The mirror image of a meso compound X is compound X, not a superimposable stereoisomer of X.

[fledarmus]

Well, yes and no.

This is misleading. The answer is a definite no. If two structures are superimposable than they are the same compound and not isomers of any kind.

The mirror image of a meso compound X is compound X, not a superimposable stereoisomer of X.

I'm sorry, perhaps I didn't make myself clear. I was responding specifically to the wording

nonsuperimposable stereoisomers that are mirror images are called enantiomers... this makes me think that there might be superimposable stereoisomers?

which reminded me very strongly of the way I was originally taught to figure out how many stereoisomers were possible in a structure containing multiple stereocenters. The solution was to write up (or visualize) all the possible mirror image pairs and figure out which ones were superimposable and which ones were not superimposable. If they were not superimposable, they were enantiomers; if they were superimposable they were identical and as you say, not isomers of any kind. However, in my notes at the time, and I suspect in the original poster's notes, the professor was using this formalism of "superimposable mirror image" and "nonsuperimposable mirror image" to refer to pairs of drawings which were either identical (thus lowering the total possible number of stereoisomers) or enantiomeric.

I apologize if I've caused any confusion by answering in this manner.

[Sophia7X]

A little confusion on meso compounds...

My teacher said if there is an internal plane of symmetry (for meso compounds), then the "mirror image" compound can be rotated so it can be superimposed on the original compound.

          CH3                                      CH3
      H--|--OH       is the same as   HO--|--H
      H--|--OH                             HO--|--H
          CH3                                       CH3
  
I understand this but...

          CH3                                            CH3
      H--|--OH       is NOTthe same as   HO--|--H
    HO--|--H                                       H--|--OH
          CH3                                            CH3

I know that there's no internal plane of symmetry, but it seems like you could simply spin the structure 180 degrees (flipping it on its other side) and it would be superimposable with its enantiomer?

[Babcock_Hall]

Sophia,

I remember a lab where the students made molecular models of these sorts of compounds.  It is actually a very good way to see whether or not two molecules really are superimposable.

[sjb]

A little confusion on meso compounds...

My teacher said if there is an internal plane of symmetry (for meso compounds), then the "mirror image" compound can be rotated so it can be superimposed on the original compound.

          CH3                                      CH3
      H--|--OH       is the same as   HO--|--H
      H--|--OH                             HO--|--H
          CH3                                       CH3
  
I understand this but...

          CH3                                            CH3
      H--|--OH       is NOTthe same as   HO--|--H
    HO--|--H                                       H--|--OH
          CH3                                            CH3

I know that there's no internal plane of symmetry, but it seems like you could simply spin the structure 180 degrees (flipping it on its other side) and it would be superimposable with its enantiomer?

Look into the way Fischer projections are created, perhaps?