R/S, E/Z, Axial Equatorial

[Moka11]

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Is it safe to say that R/S, E/Z and Axial Equatorial/Newman Projections are ranked on a different set of rules?

For example regarding Newman Projections CH3 would rather be anti to CH3 vs Br on a carbon that has CH3, H and Br.

And for the chair conformations of cyclohexane: Br vs CH3. CH3 Gets equatorial because it has more atoms. However for R/S and E/Z Br would be ranked higher. Am I correct? Slightly confused.

 

[Paperclipz]

Is it safe to say that R/S, E/Z and Axial Equatorial/Newman Projections are ranked on a different set of rules?

For example regarding Newman Projections CH3 would rather be anti to CH3 vs Br2 on a carbon that has CH3, H and Br.

And for the chair conformations of cyclohexane: Br vs CH3. CH3 Gets equatorial because it has more atoms. However for R/S and E/Z Br would be ranked higher. Am I correct? Slightly confused.

I guess yes to answer your first question.

Remember that for E/Z....this is just german for "entgegen = opposite" and "zusammen=together". Here you are not really ranking but it's seeing how the two groups are in comparison to one another across a double bond.

In axial/equatorial, yes the heaviest groups would take the equatorial position because they are bigger and the smaller would take the axial position.

For R/S, given your example of CH3 and Br, you have to look at their weight and then rank them from heaviest to lightest.

I hope I answered your question. I'm going over the material as well for my upcoming exam. GL!

 

[KyoPhan]

R/S and E/Z are not determined by ranking their weight. You have to look at the atomic # of their attachments.

 

[Moka11]

Thanks paperclipz

R/S and E/Z are not determined by ranking their weight. You have to look at the atomic # of their attachments.

Thanks. Whereas Newman and Axial/Eq we ARE looking for weight?

 

[KyoPhan]

Thanks paperclipz

Thanks. Whereas Newman and Axial/Eq we ARE looking for weight?

The ranking for stability in the Newman and Axial/Eq also follow the same rule, which is determined by steric strain. Here was my response a while ago to one of your question. 😉 I still don't know what the rule is for determining steric strain. Maybe someone else can clear that up.

Just did random google search to try to find a rule in finding steric strain. Couldn't really find a rule. But this is from www.people.vcu.edu/~s2cmring/Chapter 3.doc

These are for a ring, but the rule is pretty much the same; you want the largest group furthest apart.
"If the ring is substituted, you want to get the largest substituent in the equatorial position. This minimizes steric strain. The largest substituent is not defined by molecular weight but by physical size
Ex. CH3 is bigger than Br, even though it weighs less"

I remember chad mentioning in his video that CH3 is bigger than Br/Cl/OH.

Also here's his response to Br vs CH3 in a ring.
"The methyl group it turns out does indeed have the priority here and should get to be equatorial before the Br. There's no intuitive way to know this; it's just something we determined empirically and now you have to remember it."

If you want the source of where chad says that, copy and paste that quote in google and it should turn up.

 

[patricklin27]

How do you know something is bigger by not looking at molecular weight? Like how do you know ch3 is bigger than be? Is it because Ch3 has more bonds than br?

 

[KyoPhan]

How do you know something is bigger by not looking at molecular weight? Like how do you know ch3 is bigger than be? Is it because Ch3 has more bonds than br?

That's the part I never understood. I just committed it to memory.

 

[roquer2]

That's the part I never understood. I just committed it to memory.

👍

Like my Ochem professor said, "It is what it is, people. Either accept it or get over it."

I didn't even stress about E/Z. I didn't memorize Entgegen, but Z. Because, "Ze moleculez iz on Ze Zame Zide"