[reed]

The chemical in question is (1R,3S,5R)-3-(1-methylethyl)-5-(2-methylpropyl)cyclohexanol

the correct answer is:

but I got:

now let me explain why I assigned the priorities that I did to each chiral carbon:

all 4's are hydrogen, therefore lowest priority

1a - O is heaviest atom

2a- side of ring leads to bigger chain of carbon atoms

3a- side of ring leads to smaller chain of carbon atoms

so if I was correct I should have gotten R with the OH pointing down (which is incorrect, OH is supposed to be pointing up)

1b - side of ring leads to oxygen while others lead to just carbon

2b - leads to bigger chain

3b - leads to smaller chain

(this was the only correct one)

1c- leads to oxygen while others repeat carbon chains

2c- leads to bigger chain

3c- leads to smaller chain

(this one was incorrect also)

Can anyone tell me which priorities I assigned wrongly?

Thanks!

[enahs]

You have a ring. So, even though it is not a planar ring, lets look at it, as it is.

If we were able to ignore the ring of the molecule, you are correct. For instance, your OH is drawn in the R configuration as you have it, if you are allowed to look at it from the other side. But if you look at it from the other side, you have to look at all your chiral centers from the other side as well.

When they are in a ring like that, you can not just randomly decide to look at it from one side or the other, make them consistent.

Do you understand what I am saying?

[fledarmus]

Hello Reed,

I think the problem is that you are looking at the entire structure of the molecule. The Cahn-Ingold-Prelog rules don't work that way - you only look at one atom at a time.

Looking in the direction you've labeled 2a - the first atom is a carbon. Same with 3a - no difference.

Next bond is two hydrogens and a carbon for 2a. Same with 3a - no difference.

Next bond... see where I'm going with this? The total size of the side chain doesn't matter, only the first point of difference.

Let me know if this makes sense.