Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: toadesque on September 23, 2009, 07:11:14 PM
-
Everything seemed to be going well up until now, I have my first exam next week and this stuff is giving me problems :(
"How many stereoisomers exist for 1,3-cyclopentanediol?"
I need some advice on what the best method of approaching these problems are. What do I do first? Draw the structure? I would appreciate if someone could list out step by step what you have to do. Like A) Do this first, B) Then do this, C) Then this.....etc
I actually think my biggest problem is that I don't know how to draw the structure. After that all you do is just count how many stereocenters there are and then do 2^n right? I get confused on the best way to draw the structure for these cyclo ones...
-
draw the flat (i.e. no stereochemistry defined) structure, just so you know what it looks like.
Start with one of the stereocenters. Arbitrarily draw it in the 'up' position. Go to the next stereocenter - there are only two choices for that one. Up or down. draw both possibilities (keeping the first one 'up'). Then switch the first stereocenter to 'down'. go to the next stereocenter - again, only two possibilities. Up and down. draw them both.
You'll have drawn all 4 maximum possibilities (remember the 2n rule).
The last thing to do is see if any of them are the same - that is if you've drawn two identical compounds (that can be drawn the same through rotations and flipping). If two of the structures are the same, then because they're the same there is one fewer total stereoisomer.
Based on that, what do you think for 1,3-cyclopentanediol?