[Kalibasa]

I'm trying to understand Fischer projections in biochem and I'm really confused! I think I'm having trouble orienting the molecule at the beginning. They say that the horizontal lines represent bonds coming out of the page (toward the viewer) and that vertical lines represent those going away.  But if a molecule is tetrahedral, how can you even decide which bonds are supposed to be pointing which way? Isn't this arbitrary- couldn't you just get a different arrangement by looking at the molecule from a different angle?

This picture in Wikipedia is like the one in my book, and this is the step I don't get.

http://en.wikipedia.org/wiki/File:Fischer_projection.png

Why couldn't you have W and Y, for instance, along the horizontal lines, or even W and X? How do you know that it's X and Z? Every time I do this I get the wrong isomer...

Thank you!

[Schrödinger]

i think this picture is self explanatory.

http://en.wikipedia.org/wiki/File:Fischer_Projection2.svg

this one should help you visualize the molecule.

[Kalibasa]

No, I still don't get it. If you have a chiral carbon, all the groups shown at the bottom would be different and equal in importance, wouldn't they? Why couldn't you sight A on the top and, say, D on the bottom? Again, I don't understand how you can be this specific with a tetrahedral molecule.

[renge ishyo]

The key to a Fischer projection is to place the most oxidized species on the sugar at the top of the projection. You use this to orient the rest of the molecule. For example, for glucose the aldehyde group is placed at the top. If you build a ball and stick model of straigt chain glucose you can see easily how this works. Place the aldehyde at the top and then try to "flatten" the molecule with the groups facing towards you. It will give the Fischer projection with the correct stereochemistry.

Also, keep in mind that your "R" and "S" configs for the stereochemistry of the tetrahedral carbons will still work on the Fischer projections so this is one way to see if you have kept the right stereochemistry when you draw them.

[Kalibasa]

Ok, that helps. I think in trying to simplify the things (using generic letters and looking at only one carbon) they made me even more confused!

Thanks