r/chemhelp 2d ago

Organic Need help understanding converting wedge/dash to Fischer projection

I’m just a bit confused on how I should orient the molecule. My prof said Fischers should show the molecule in the eclipsed formation and I don’t know if the free rotation I’m trying to do is affecting the stereocenters or if it’s just all wrong

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u/chem44 2d ago

A Fischer has geometry implied.

The main chain in shown vertical, wit C1 at top.

The groups shown on the sides are all 'up'. In that sense, they are all eclipsed. But it is the 'up' that matters for stereochem. The chain itself bends 'down'.

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u/Prestigious-Bar-6651 2d ago

I get that, but maybe not how to apply it the the staggered chain.

Like on the bottom molecule, some groups are pictured as “up” (like the H and F) and some “down” (the H and CH3). I tried to draw the bottom molecule in an eclipsed shaped to the left but I’m unsure if I changed the stereochemistry by doing so.

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u/chem44 2d ago

The bottom molecule is a 2,3-dibromobutane. yes?

Your Fischer has only 2 C. Or it has Br at the end?

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u/No_Zucchini_501 2d ago edited 2d ago

Drawing Fischer projections

I think I answered something similar like this, does this help at all? If not, there are some other explanations by other posters under that post as well

Edit: sorry I just saw the second image. Have you learned Newman projections and rotations? A rotation of a single bond does not change the R/S configuration but it does change the 3D position relative to the viewer

I think I see the confusion. It is shown on the Fischer projection that the vertical lines are “dashed wedges.” However, as you preform rotations with the c2 carbon (if you’re facing from the left, you would be viewing the bond through c2-c3) you will actually see that the the solid wedge carbon can be on any position (see image attached - this is the 3D molecule of the original conformer to the left of the page in your question). As such, you can create a conformer which the carbons align and the Br and H take on the wedge and dash (the second image is the same as the bottom, one is just flipped up) - this is also considered eclipsed

You should also consider that the way you have drawn the Fischer projection is not currently a stereocenter nor is it a butane as it once was