Cis/Trans in Newman projections

[rippinitez]

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What is the easiest way to determine if substituents are in CIS/TRANS when looking at a newman projection?

 

[TexasOMFS]

Hmm typically Newman projections are used for looking down a C-C bond, usually of a n-alkane. Cis/Trans designation is for alkenes or possibly cycloalkanes. You wouldn't ever have to designate Cis or Trans for a cycloalkane based on the Newman projections. Typically, the projections are just used to map relative stabilities of different conformations of the molecule based on steric and eclipsing interactions.

 

[rippinitez]

The only reason I asked is because one of the problems in Ochem Odyssey shows them determine whether it is cis or trans based on the newman. There was no explanation, so I was a little confused.

 

[TexasOMFS]

Mind scanning and posting the question?

 

[RSD2014]

What is the easiest way to determine if substituents are in CIS/TRANS when looking at a newman projection?

Do you have an example?

 

[darkwalk]

If you are good at PAT, then this should be simple. I mentally rotate the projection so the lowest priority is either in the back or the front. Then, assign the priorities and see if the substituents are rotating left or right. Flip the result if the lowest priority is in the back.

 

[rippinitez]

I obviously can't post the actual problem from the book. But here is an example I made up that is also "trans".

Also, can you tell from this example whether the H and Br are anti?

 

[darkwalk]

This one is tricky. I don't think you'll get something like this in the DAT, but doesn't hurt to know.

Using the priority rules:
http://en.wikipedia.org/wiki/Cahn_Ingold_Prelog_priority_rules

The carbon closest to the viewer is R.
1 is bonded to Br, so that takes priority
2 is bonded to 2 real carbons and 1 ghost carbon, taking priority away from 3 which is bonded to 2 carbons and 1 H.

The carbon away from the viewer is R.
The first atom attached are C,C,C,H, so we must look at the atoms attached to them
1 is attached to Br, so that must take priority regardless of the double bond in 2
2 takes priority compared to 3 because it has double bond, therefore, replace with "ghost" atoms

 

[rippinitez]

So based on this, can you tell if the H and Br are anti to allow for an E2 reaction to take place?

 

[darkwalk]

The proton being attacked must be anti with Br.

Since you have two phenyl group, I assume they must be anti with each other due to strain. (I'm not 100% positive about this)

http://en.wikipedia.org/wiki/Gauche_effect

If they are anti, Br and H are not, so E2 cannot take place.

However, if enough energy exists to Gauche the structure, then E2 can take place.

Anyone want to chime in?

I think this question is great for studying, but I don't think you will encounter such difficult question.