[miaou5]

After learning about meso compounds and mirror images and internal planes of symmetry, I'm completely confused! I just had a few questions:

For instance, given tartaric acid, why is 2R, 3S tartaric acid the same as 2S, 3R tartaric acid? I get that they're mirror images of each other, but I don't see how they're superposable? I think that's probably my main issue--being able to see how two images are superposable. Is there a formula that makes this easier

Also, if something has an internal plane of symmetry, does it necessarily mean that it will be superposable onto its mirror image? How about vice versa? (In other words, can these two effects occur independently of each other, or do they always occur together?)

One final unrelated question: for induction, is the effect only relegated to the adjacent atom, or can the effect extend beyond directly neighboring atoms (e.g., 2 atoms over, etc.)?

Thank you all so much

-Grace

[Dan]

For instance, given tartaric acid, why is 2R, 3S tartaric acid the same as 2S, 3R tartaric acid? I get that they're mirror images of each other, but I don't see how they're superposable? I think that's probably my main issue--being able to see how two images are superposable.

You have to re-orientate (rotate) the molecule in 3D to see it. This can be quite challenging to do in your head at first - I recommend working with models for a bit until you can do it mentally.

Also, if something has an internal plane of symmetry, does it necessarily mean that it will be superposable onto its mirror image?

Yes. Finding a plane of symmetry is the easiest what to see that a compound is achiral.

How about vice versa? (In other words, can these two effects occur independently of each other, or do they always occur together?)

Not always. If a compound is achiral (i.e. has superimposable mirror images), then it must possess a plane of symmetry and/or a centre of symmetry.

For example:          (IGNORE the fact that is says "chiral" in the picture, it's a SMILES bug - it is achiral)

This compound does not have a plane of symmetry, but it does have a centre of symmetry and it is achiral (superimposable mirror images). It is a good idea to build models for the mirror images of these to see they are superimposable.

One final unrelated question: for induction, is the effect only relegated to the adjacent atom, or can the effect extend beyond directly neighboring atoms (e.g., 2 atoms over, etc.)?

The magnitude of the effect drops off with distance, but can have an effect through more than one bond - for example, compare the pKa of ethanol with 2,2,2-trifluoroethanol.