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enter image description here

Can somebody tell me how why they changed/choose the groups for vertical and horizontal lines. As far as I know the vertical lines represent group going away from us and horizontal depicts coming towards us but I can't figure out how to choose which groups lie vertically and which groups horizontally... I am hopelessly stuck in Fischer projections.

EDIT: What I am having difficulty is how to choose where o arrange the groups in the horizontal line.

For eg:

enter image description here

Why is the H bond on the C1 is arranged to the right while the OH is arranged to the left while the opposite is in C2 in the fischer projections It would make more sense for OH to be on the right as it is coming out of the plane and would appear at right when seen from above. Why does dash line is at right while wedge line to the left. How to interpret?

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  • $\begingroup$ This depiction tries to show how 3D tetrahedral structure is projected onto the paper plane, so the structure above is not meant to be a formula but a 3D model. Try to imagine you look at the structure from above, you get X−(C)−Z pointing towards you horizontal, and W−(C)−Y pointing down, vertical. $\endgroup$ – mykhal Sep 18 '18 at 15:14
  • $\begingroup$ I am confused about the positions of Z and W.... How to know which will be in vertical line?Whever I try I always switch Z and W so it gives me wrong answers. Can you provide an answer on how to view this $\endgroup$ – Aladdin Sep 19 '18 at 0:12
  • $\begingroup$ .. They did not change anything. For each crossing of the Fischer projection, the molecule had to be oriented (or your point of view changed) such that you see the cross, and the horizontal groups point towards you, vertical outside. On your 1st image, it's already oriented, and you look from above. On the 2nd image it's also oriented, but you look from the right side. If one can't see that, he/she probably has to train a 3D spatial imagination. $\endgroup$ – mykhal Sep 19 '18 at 7:39
  • $\begingroup$ If I see from right then I see cho-c-c-ch2oh as a line, the oh bond towards me and the hydrogen bond away for the C1, that would be opposite to the given fischer. Can you tell me what I did wrong here $\endgroup$ – Aladdin Sep 19 '18 at 9:08
  • $\begingroup$ Not enough, imagine you are moving closer and closer to the paper at the right side and looking to left at the molecule, until you cross the paper, then you should see OH (from C1) pointing left and towards you, H (from C1) pointing right and towards you. $\endgroup$ – mykhal Sep 19 '18 at 10:20
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The whole point of the projection is that you project your 3-D structure onto a lower dimension representation, but it doesn't say how you should orient for the projection. In other words, any orientation here is fine. Just like when you look at a hand in any orientation, it's still the same hand. That is, there is no absolutely correct perspective to look at it.

For the example provided, you could cycle any three groups, and the resulting project is valid. For example, W -> X -> Y -> W. This corresponds to a rotation along the axis to the remaining atom, so the stereochemistry is unchanged. Since you're still depicting the same structure, the resulting projection looks different but represents the same thing.

EDIT: Note that when you have a complex structure, the rest of the structure can and does impose a preference on which groups should go where. For example, for carbohydrates, you should orient each carbon center such that the carbon backbone is vertical.

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  • $\begingroup$ I'll quibble a bit. Quoting from Wikipedia "The use of Fischer projections in non-carbohydrates is discouraged..." The whole point of Fisher projections for carbohydrates is that the projection does specify how the projection should be oriented. That was the whole point. To draw all the different carbohydrates in a uniform way so that the 2D structures could be easily compared. $\endgroup$ – MaxW Sep 18 '18 at 16:09
  • $\begingroup$ @MaxW That's hardly quibbling since you are 100% correct. I'll edit my answer. Thanks for the suggestion. Wish I could donate some of rep for the assist there... $\endgroup$ – Zhe Sep 18 '18 at 17:27
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The easiest way to analyze fischer projections is this:

  1. Draw the projection on a page.
  2. Consider the vertical groups to be inside the page and the horizontal groups to be outside the page.

This technique works for me flawlessly. I hope it does for you too.

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  • $\begingroup$ How do I decide which groups are inside the plane or outside from the wedge dash. Can you elaborate more on this... I think I am confused at that particular point $\endgroup$ – Aladdin Sep 18 '18 at 23:34
  • $\begingroup$ Any group shown by a wedge bond is always outside the plane. Any group shown by dash bond is always inside the plane. Any group shown by a line is always on that plane. It’s that simple. $\endgroup$ – Som V. Tambe Sep 19 '18 at 1:23

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