ochem - fischer projections

[capn jazz]

Okay, I am having a TON of difficulty with using Fischer projections for reactions. I can evaluate them by themselves, but I get totally overwhelmed when a passage comes with 10 projections. Does anyone have any suggestions for making Fischer projections bend to my will? I really dislike them and don't understand how they're helpful in depicting anything. Why can't we just use the dashed line/wedge depiction??

But really... any suggestions on what to do would be great. I have the EK OChem book, the Kaplan bio notes, and the TPR science workbook.

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[sv3]

Are you just slow to figure out the configuration of chiral centers, or are you getting them wrong? And what reactions are you talking about for example? I've only come across Fischers on a consistent basis in the carbohydrate section of ORGO. Otherwise I rarely see them - I've gone through TPR and Ek so perhaps they didn't focus on Fischers.....

Practice obviously helps with speed. Not sure what your problems are but the following helped me start off right......

If the lowest priority group is coming at you (on a horizontal line) don't bother trying to manipulate the figure. Just get the configuration and the opposite is the answer - this is b/c the lowest priority is coming at you, not away from you.

I know with dash/wedge you can actually switch two groups in order to make the low priority group go into the page, and then again you would take the opposite of the configuration you get b/c switching two groups necessarily changes the configuration (S vs R). I am not sure if this trick is applicable to Fischer diagrams but I'm sure someone will confirm or deny this soon enough on this board.

perhaps you know this already but i couldn't tell what your specific issue was.....

 

[chemtopper]

In Fischer projection--vertical line is giving you groups away or pointing behind the plane. Horizontal line groups pointing towards us or in front of the plane .Decide about the R and S without troubling your brain (means no rotation),if lowest priority group is on horizontal line just take the opposite configuration (if it is R than take it S).If lowest group is on vertical line than no need of change.If you are getting R ,it is R only.

 

[capn jazz]

Thanks guys, just finding R/S and reversing it if the H is on the horizontal axis is much easier than trying to rotate the molecule.

I guess my biggest problem is visualizing rxns with Fischers. If inversion of configuration happens via SN2, does that just mean that whatever C is attacked, the 2 horizontal substituents flip and the attacking molecule shows up on the opposite side of where it attacked? How do I learn to draw changes in stereochemistry due to rxns with Fischer projections?

Thanks!

 

[sv3]

where you are finding that you need to visualize these reactions? I'm sure its nice to be able to but I've gone through TPR and Ek Orgo and never needed to visualize it. I think whats important is knowing what the stereochemistry is of the products and reactants. When given a diagram as an answer choice, just get the stereochemistry of the chiral carbons and if it matches what you expect, then pick it (and try to eliminate the other choices). For example, if you expect Sn2 to invert config at a carbon, and the molecule is S before, then look for an answer choice that has R (and of course the expected substituents).

Perhaps if you could explain in what type of problems you need to visualize fisher reactions someone could be of more help. You might be making this tougher than it needs to be. So far, what has worked for me when encountering tough problems is to remember that they shouldn't take that long to figure out given the time constraints.

cheers
steve