TBR's method for determining stereochemistry

[Neutral]

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I have a question regarding TBR's method for determining stereochemistry -

It is split up into 4 situations:

If lowest priority group (LPG) is going away, then you do everything normaly
If LPG is going toward you, then you invert stereochemistry
If LPG is planar and adjacent to the group going toward you, do everything normally
If LPG is planar and adjacent to the group going away from you, invert stereochemistry

What is confusing me is that a molecule can be written two ways like this. The group going toward you can either be pointing off to the left, or can be pointing off to the right, and the group going away will be written opposite to that.

If you write a molecule that has the group going toward you going off to the left, then you would treat the stereocenter differently than if it was going off to the right even though it would be the exact same molecule.

Does anyone know why this is?

 

[sps27]

So you have to imagine the projection of the group on the plane of paper. So if you have a group going outside the plane of paper towards you, or inside the plane of paper away from you and pointing to the right, the projection of that or u can say the shadow of that on the plane of paper will be to the right regardless of whether they are going in or out, similarly for a group going out of the plane of paper or away from the plane of paper and towards left, the projection of that on the plane of paper will be to the left. And then there will be two other groups as well, one going directly perpendicular to the plane of paper pointing away from you, and another directly perpendicular to the plane of paper and pointing towards you. I hope this helps.

 

[Neutral]

I don't really understand what you mean by that so I drew a picture in paint to illustrate what I'm saying

Even though these are the same molecules, TBR's method would assign them opposite stereochemistry

EDIT: So after reading your post again, I think what you are saying is if you rotate the molecules in your head? So that the LPG is going away from you? In that case, yes, that is the way to do it but TBR's method supposedly works so that you don't have to do that in your head since that takes time and is prone to mistake.

 

[Jepstein30]

^^ those aren't the same molecules...

Draw an arrow from priority group 1 to priority group 3. If you had to cross priority group 4 that's sticking out at you.. then invert.

EDIT: Isn't it as simple as inverting R/S if the lowest priority group is sticking out at you?

 

[sps27]

I don't really understand what you mean by that so I drew a picture in paint to illustrate what I'm saying

Even though these are the same molecules, TBR's method would assign them opposite stereochemistry

EDIT: So after reading your post again, I think what you are saying is if you rotate the molecules in your head? So that the LPG is going away from you? In that case, yes, that is the way to do it but TBR's method supposedly works so that you don't have to do that in your head since that takes time and is prone to mistake.

The left figure: 1) Flourine 2) Oxygen 3) Carbon. Correct. Which makes it S. Now reverse that since H is next to the group going inside the plane of paper, which makes it R.
The right figure: 1) Flourine 2) Oxygen 3) Carbon. Correct. Which makes it R. Don't invert as the H is next to the group going outside the plane of paper. So in both cases we have R. Which means they are one and the same. Let me know if I am wrong.

Thanks for the figure. It makes is so much easier. I perhaps misunderstood what you are asking. I still don't quite know what you are referring to but I do think TBR's method is solid and works. I follow that instead of rotating molecules in head, which is hard for me, most of the times.

 

[Jepstein30]

The left figure: 1) Flourine 2) Oxygen 3) Carbon. Correct. Which makes it S. Now reverse that since H is next to the group going inside the plane of paper, which makes it R.
The right figure: 1) Flourine 2) Oxygen 3) Carbon. Correct. Which makes it R. Don't invert as the H is next to the group going outside the plane of paper. So in both cases we have R. Which means they are one and the same. Let me know if I am wrong.

Thanks for the figure. It makes is so much easier. I perhaps misunderstood what you are asking. I still don't quite know what you are referring to but I do think TBR's method is solid and works. I follow that instead of rotating molecules in head, which is hard for me, most of the times.

I could've sworn that the rule is swapping any two groups automatically makes an enantiomer..

that's the trick to use.. if the lowest priority group isn't in the back.. swap it with the group in the back.. find the chirality and invert it.

 

[Neutral]

The left figure: 1) Flourine 2) Oxygen 3) Carbon. Correct. Which makes it S. Now reverse that since H is next to the group going inside the plane of paper, which makes it R.
The right figure: 1) Flourine 2) Oxygen 3) Carbon. Correct. Which makes it R. Don't invert as the H is next to the group going outside the plane of paper. So in both cases we have R. Which means they are one and the same. Let me know if I am wrong.

Thanks for the figure. It makes is so much easier. I perhaps misunderstood what you are asking. I still don't quite know what you are referring to but I do think TBR's method is solid and works. I follow that instead of rotating molecules in head, which is hard for me, most of the times.

Yes! Thank you so much it is now crystal clear! 🙂 Much appreciated.

I could've sworn that the rule is swapping any two groups automatically makes an enantiomer..

that's the trick to use.. if the lowest priority group isn't in the back.. swap it with the group in the back.. find the chirality and invert it.

The groups aren't actually switched. If you think of it in 3-D, nothing was switched, the only difference is that they are written slightly differently.

 

[Jepstein30]

Yes! Thank you so much it is now crystal clear! 🙂 Much appreciated.



The groups aren't actually switched. If you think of it in 3-D, nothing was switched, the only difference is that they are written slightly differently.

oh god.. dunno what I was thinking earlier.. been awhile since I've bothered with Orgo but that's pretty brutal not to see I was treating the line in the middle as a mirror for some odd reason..

anyways, that trick does work and may be easier to use since its only one thing to remember..

Left: swap F and H. Stereochemistry is S so invert to R.
Right: swap F and H. Stereochemistry is S so invert to R.

I can't quite remember what I used on the test.. I definitely didn't use either of those but didn't rotate in my head either.. gah