[Wald_ron]

    F                   Is a fisher projection, is it R or S? I thought it was R, but it's S.
    |
Cl-C - H
    |
    Br

I think my book is wrong.
The only thing that makes sense to me is if it wasn't a fisher projection, then it would be S.

[Dan]

If that is a Fischer projection then it is definitely S. If it is not a Fischer projection then the stereochemistry is undefined and the question cannot be answered.

Draw the molecule in a "standard" wedge style and it will be easier to see. If you can provide a diagram indicating why you think it's R then we can see where you went wrong.

[Wald_ron]

Thank you for replying Dan

Priority (R and S)
Br>Cl>F>H

     3
     |
2 - C - 4
     |
     1

Clockwise therefore R

[Dan]

Ok you haven't taken into account that the H substituent is pointing towards you. The clockwise = R, anticlockwise = S is true if the lowest priority points away from you, the rule is reversed if the lowest priority points towards you. I advise that you convert it into a wedge style representation first, then try to assign the absolute configuration and see what you get.

[Wald_ron]

So,

          F
       |out|
Cl -in- C -in- H
       |out|
         Br

So it would then be


   2
   |
3-C-4
   |
   1

and therefore S , correct?

[Juicebox]

Yep, it's all in the visualization. Make sure the lowest ranked group is pointing away from you and you should be golden.

[Doc Oc]

So,

          F
       |out|
Cl -in- C -in- H
       |out|
         Br

So it would then be


   2
   |
3-C-4
   |
   1

and therefore S , correct?

The priority assignment here is not correct, it was correct in your 2nd post.  You're confused because of the visualization of it.  If you have a model set it's helpful to make this molecule and see why it's S.  Then you will know how to rearrange things in your head from a Fischer projection to properly determine configuration.

[Dan]

So,

          F
       |out|
Cl -in- C -in- H
       |out|
         Br

No. In a Fischer projection the substituents on the vertical axis point away from the viewer (in/down) and the horizontal substituents point towards the viewer (out/up).
The reason you got S this time is that you assigned priority incorrectly (see J-Bone's post).

See diagram below:

[Wald_ron]

No. In a Fischer projection the substituents on the vertical axis point away from the viewer (in/down) and the horizontal substituents point towards the viewer (out/up).
The reason you got S this time is that you assigned priority incorrectly (see J-Bone's post).

Thank you very much for your patience Dan, I understand now where I went wrong, you've helped me out a lot with my R/S problem. Now I'll need to practice on my own.

[AbsoluteConfig]

If that is a Fischer projection then it is definitely S. If it is not a Fischer projection then the stereochemistry is undefined and the question cannot be answered.

Draw the molecule in a "standard" wedge style and it will be easier to see. If you can provide a diagram indicating why you think it's R then we can see where you went wrong.

The RS configuration can be answered without knowing the stereochemistry, just as long an assignment of priorities can be established.  I'll post an image later today to explain how to do it and it is very simple, and useful for every question concerning the naming of a sterocenter with an R or S configuration

[Dan]

The RS configuration can be answered without knowing the stereochemistry,

Fischer to R/S is one thing, but you are claiming to be able to assign R/S with no stereochemical information - ie. to an undefined asymmetric centre. I'm sure that's impossible.

I am aware that you can go directly from a Fischer to R/S (since asymmetric centres are defined in a Fischer projection), it is just easier to convert to wedge representations for most people.

[AbsoluteConfig]

The method used above with the fingers, hand and arm is located at:  
www.AbsoluteConfiguration.com or www.RSconfiguration.com

Hope it helps the students that are having trouble with the mental rotation of the priority groups.

As most of us understand, the Fischer Projection is, for the most part, the only molecular representation that the stereochemistry (dashes, wedges, etcetera) does not have to be written.

GREAT Information on Chemical Forums  

[AbsoluteConfig]

Sorry about that Dan, that's correct, some sort of stereochemistry must be noted if it is not a Fischer Projection, otherwise the RS configuration about a stereocenter can not be determined.  

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