[Lo.Lee.Ta.]

When "inversion" occurs, does this mean that an existing bond broke and a new bond formed in its place- causing an R molecule to become an S molecule, or vice versa.

(Also- WHEN do you know inversion has to occur? Will it always occur when an Sn2 or Sn1 reaction takes place?)

And does "optically pure" mean that everything is only R molecules or only S molecules?

...Is that right?

Thanks!

[discodermolide]

You don't have to have bond breaking and re-forming to have inversion.
Usually Sn2 reactions give inversion of configuration unless something else is going on.
Can you think of when an Sn2 may give retention?
Optically pure means there is only one enantiomer present.

[Lo.Lee.Ta.]

Every problem and example in my book of an Sn2 reaction shows inversion ocurring.

I think Sn2 reactions pretty much always create inversion...?

Walden's Cycle of turning (+)-malic acid into (-)-malic acid and vice versa was what made me think bonds had to break and then form in order to create inversion.

In the mechanism for Walden's Cycle, it looks like every time stuff like OTos or CH3COOH bonds to the overall molecule and then breaks off, the overall molecule moves in a way that slowly makes the shape closer and closer to that of the other enantiomer,
e.g. (+)-malic acid  (-)-malic acid

I have been working some problems concerning Sn1 reactions as well.
It looks like inversion also occurs in that reaction.
But does inversion always ocurr in Sn1 reactions?

Thanks!

[discodermolide]

Inversion is usually the result of a Sn2 reaction.
Walden's cycle is something a bit different if you look here:
http://en.wikipedia.org/wiki/Walden_inversion
Inversion does not usually happen in Sn1 reactions as the reaction goes through a carbonium ion which are usually planar so attack of the nucleophile can occur from either side. If there is a special case where this cannot happen then one may expect retention.

[orgopete]

When "inversion" occurs, does this mean that an existing bond broke and a new bond formed in its place- causing an R molecule to become an S molecule, or vice versa.

That's how I think of it. (You may find some instances in which the ordering of the groups may actually result in an S becoming an S, but that is not because an inversion didn't occur).