[webguy54]

I'm having trouble comparing newman and fischer projections.  Here's a recent homework problem that I can't figure out.  I have no problem when comparing fischer-to-fischer or newman-to-newman projections.  I can't figure out how to tranlate a fischer projection into a newman projection and vice versa.  Thank you, any help would be greatly appreciated.

Questions:
a) Which two are the same molecule?
b) Which is an isomer of erythrose?
c) Which two pairs are enantiomers?
d) Which two stereoisomers would Fischer have called L- ___ & ___? (the blanks don't refer to names or molecules, just number representing molecules from picture)

[Yggdrasil]

In this case it would be helpful to build models of these molecules, so that you can play around with them and practice visualizing them in your head.

[AWK]

It is difficult to do comparisons using different projections.
Besided building models you can convert Neman projections to Fisher ones and then compare them. It is a good advice to convert Neman to sawhorse, then to Fisher projection.

http://en.wikipedia.org/wiki/Fischer_projection
http://en.wikipedia.org/wiki/Newman_projection

[alexanderdundua]

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[AWK]

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3 and 4 are different - you drawed the same

[Yggdrasil]

Here's an example of how to do #3 and the mental rotations that I go through to convert the Newman projection into a Fischer projection.  Try doing #4 using the same strategy.

[webguy54]

Thanks everybody!  I was able to see where I went wrong using the methods suggested.  I have another question, but I think this might simply be a book error.

A question in the text listed Newman Projections #1 thru 4.  The first part of the question asked "which represent the same compound?"  I first found the following configurations...
#1 - 2R,3R
#2 - 2S,3S
#3 - 2R,3S
#4 - 2R,3S

I came to the conclusion that #3 & #4 were the same compound based on the fact that they have the same configurations.  The solution manual said that #1 & #4 are the same compound.  I've been going crazy trying to figure out how!  I just realized that the solution manual may have found the answer based on a different Newman Projection for #4. 

I listed the solution manual version of #4 below the four projections listed in the text.  Am I correct in saying that both Newman Projects listed as #4 are projections of two different compounds and not simply the same compound rotated?  Based on the projection given in the solution manual I found that the configuration was 2R,3R and that projections #1 and #4 are in fact the same compound.

[Yggdrasil]

You are correct.  #3 and #4 are identical, #1 and #4 (solution manual) are identical, and #4 and #4 (solution manual) are different.

Stereochemistry is hard enough without errors in the book.  Kudos to figuring out that the book is wrong though.  That's when you know that you understand things well.

[webguy54]

Thanks for the reply Yggdrasil.  I have my OChem final this week and I feel good about it so far.