[tamubsa]

I am having trouble understanding why these 2 protons are enantiotopic (the picture is attached). When I replace the H's with Cl's, I get S chiral centers in both cases. In addition, if I flip over the molecule, the  protons seem to be the same. My question is, why are they enantiotopic (which is in the answer key of my textbook) and not homeotopic?

[k1mng]

Perhaps do not flip the molecule when trying to get the enantiomer, but make a mirror image of the molecule on the page.

[tamubsa]

I still get S chiral centers for both H's.

[k1mng]

I think there may be a confusion about the solution in the way you approaching the problem. We substitute protons with other atoms (you use Cl) to produce our stereocenters in case they are not already present. Here, we have stereocenters for both our protons already so we can go about determining their relationship by mirror-imagining our molecule.

I do get (S,S) for the given molecule, however upon mirror imaging it it becomes (R,R). Therefore, the two protons are related enantiomerically and thus enantiotopic, as opposed to diastereotopic in which (S,S) and (R,S) would be so. I hope this makes sense.

EDIT: As a side note, the protons would only be homotopic if the compound was a meso-compound (see attached).

[Dan]

Hmmm...

I am confused here as well. Defining this relationship in C2 symmetric systems is not something that I remember ever needing to do, but given the question I would do what the OP has done: Separately substitute each H and examine the relationship between the two new hypothetical compounds.

If you do this, they are the same (see diagram below). I would have concluded that the two Hs are homotopic as well. I have always taken this as the standard method, but perhaps it is an oversimplification... Do you have a comment on this k1mng? At the moment I think the book is wrong, but I will consult Eliel & Wilen this weekend and post what I find.

EDIT: As a side note, I think that in the meso example you added, the Hs are enantiotopic, not homotopic (by analysis analogous to that in the attached diagram).

[mjc123]

I think that is right. If the protons are equivalent by rotational symmetry, then replacing them gives molecules that can be superimposed by simple physical rotation, i.e. they are the same - maybe chiral, but the same enantiomer, so the protons are not enantiotopic. If they are equivalent only by reflection symmetry (an operation that cannot be physically performed), replacing them gives mirror-image molecules that cannot be superimposed, i.e. enantiomers. So the protons in the meso compound above are enantiotopic.

[Irlanur]

I think they are homotopic.

[mjc123]

Which "they"?

[Irlanur]

sorry, in the indicated H's in the original post

[k1mng]

Hey guys. Sorry! I think I was wrong. I've been taught slightly differently in determining stereotopicity and perhaps have misguidedly figured a faulty way of working at these problems. So it's me that's having issues approaching this type of problem and I'll work on that. Thanks everyone