[Astrokel]

Good day chemists I am reading an organic text by myself and have many doubts with me. I really want to clear some basic doubts before moving on.

I am having difficulties imagining the rotation, mirror plane and so on in stereochemistry. I am thinking of getting a gumdrop model set but i have no idea where to get. Is it common or i have no talent in stereochemistry? lol

When determing R/S for an enantiomer, there are two types - when the lowest priority group is in the back and when it is not.

1) When the lowest priority group is in the back
- What do they mean by in the back? In term of 3D model i think it meant bond going away from you. But how do i know which group should be dashed/wedges/solid?
- See image. Ethyl group has highest priority but it is in the back? The book says they are same molecules. :S

2) When the lowest priority group is not in the back
- From the text, it says exchange and put the lowest priority group in the back. Then exchange and exchange again. lol I don't get it at all.

Hope to learn something from all of you!

[azmanam]

start here:

http://www.cem.msu.edu/~reusch/VirtTxtJml/sterism3.htm#isom13

[gloinddark]

to start with, as far as my knowledge goes, NH₂ should have the highest priority, that is #1... since you consider the atom immediately attached to the chiral C.

it's commonly difficult to visualise it but close your eyes and try to see it and rotate in 3d. you can also use your fingers kind of instead of models.

for your 2nd question, exchange, find R/S configuration, and if you get R than it is actually S, and vice versa - got it? since you have done an exchange the result will the the opposite. at least that is how i do it...

[Astrokel]

I'm getting it. But why must the lowest priority be in the back in order to assign?

[gloinddark]

i guess that is just a rule to keep things standard. lowest priority group at the back.

[Astrokel]

Man i got it finally. Thanks. I will move on to fischer for now and be back later.

[azmanam]

The lowest priority absolutely does not need to be in back to determine stereochemistry.  In the mnemonic you're learning, it's easiest to evaluate the stereochemical relationship of the compound under observation when lowest priority is in back.  There are other mnemonics.  If you like the 'lowest priority in the back' mnemonic, stick with it.  If it's confusing, google your way to other mnemonics and find one you like.

Tip: when evaluating Fisher Projections (or Newman projections, etc), I don't think it's useful to try to determine stereochemistry fro those projections alone.  I always advocate re-drawing the molecule in wedge/dash form.  See here for ways to do so:
http://www.chemicalforums.com/index.php?topic=34001.0

[Astrokel]

I tried the method of coverting to 3D form and it worked in answering the practices from the text. Would the same 'back method' works directly from Fisher Projections? It seems to work as long as i reverse the direction.