Assigning R/S to Cylical Compounds

[To be MD]

The image is from TPR's Demo Exam Q17.

I do not understand how to go about figuring out which group are "towards" or "away" from you when in an cyclical ring like this. I get assigning priorities, but counterclockwise wouldn't lead to R like on the left side of the molecule... so how do I know when to use the "reverse" rule??

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[aldol16]

Don't worry about "reverse" rules. And ignore all parts of the molecule except the tetrahedron that you're interested in (after assigning priorities to the points of the tetrahedron, of course). So just imagine the tetrahedron on the left. Put the highest priority group in the "back." In this case, the highest priority group is facing you, so you have to imagine viewing the adamantane ring system from behind the plane of the page. In that case, the order of substituents is in fact clockwise and thus R.

 

[victorias]

I am not sure if this is the correct way but can you say that group 4's are part of the chair conformation so they are along the plane of the page and thus, the lowest priority group is not facing away from you so you have to switch configuration.

 

[To be MD]

Don't worry about "reverse" rules. And ignore all parts of the molecule except the tetrahedron that you're interested in (after assigning priorities to the points of the tetrahedron, of course). So just imagine the tetrahedron on the left. Put the highest priority group in the "back." In this case, the highest priority group is facing you, so you have to imagine viewing the adamantane ring system from behind the plane of the page. In that case, the order of substituents is in fact clockwise and thus R.

Excellent! Thank you. I can do that in my head.

 

[NextStepTutor_2]

Excellent! Thank you. I can do that in my head.

Your brain has enough to do on test day without trying to visualize more info. When in doubt WRITE IT OUT!
As stated above, take each sp3 carbon on its own, and draw it (or visualize for those who can) with the lowest priority group facing away from you. A few extra seconds of investment is worth it if you can pick up relatively easy points like this on test day!

hope this helps, good luck!

 

[7331poas]

Reverse rules are way faster than redrawing and putting the lower group in the back imo

 

[To be MD]

Reverse rules are way faster than redrawing and putting the lower group in the back imo

It's fine to say 'reverse rules' but if you don't know that the lower group is in the back, you don't know if those reverse rules should be applied in the first place.

 

[7331poas]

Maybe I am confused by what you mean by reverse rules. If the lowest priority group is in the front, then the answer is the opposite of the current configuration. Its quicker than physically flipping the compound, then assigning R/S

 

[aldol16]

Maybe I am confused by what you mean by reverse rules. If the lowest priority group is in the front, then the answer is the opposite of the current configuration. Its quicker than physically flipping the compound, then assigning R/S

I think what he/she is trying to say is that to see that the lowest priority group is in the front or back you have to be able to visualize it. If you can't convert 2D to 3D, no rule is going to help.

 

[Astra]

Question, in this case, how do you know which carbon has the higher priority? ( carbon 2 v 3 v 4)

 

[7331poas]

Question, in this case, how do you know which carbon has the higher priority? ( carbon 2 v 3 v 4)

molecular weight of each alkyl chain. There are rules online if you need it

http://www.chem.ucla.edu/harding/tutorials/stereochem/rsez.pdf