[Winga]

Winga, do you have an example of where you would have to pick between an R and an S sidechain?  I'm trying to think of an example where the compound wouldn't be meso because of a plane of symmetry.M is like R, P is like S.  For axial chirality you also sometimes see aR and aS instead of just R and S.

With respect to Blueshawk's question, Winga is correct.  The substituents on the "front" ring are automatically given higher priority than those on the "back" ring.  So, the isopropyl groups are priority 1 and 3, respectively.  The H atoms on the other side of the rings are 2 and 4, respectively.

Xylitol has R and S side chains but it's also a meso compound due to a plane of symmetry.
If both are either R or S, then it's chiral.

I'm not quite sure what example you are thinking of. You mean a compound containing R and S side chains without a plane of symmetry?


About assigning the priority of the 2nd structure, what H atoms we need to count?
Is it only the H atoms opposite to isopropyl groups?

As you said isopropyl groups are priority 1 and 3 while H atoms are 2 and 4, should we follow the path 1 --> 2 --> 3 --> 4? Or 1 --> 3 then, 2 --> 4?

[movies]

I was trying to understand what you meant by your earlier question:

About the 2nd question, I mean when two of the substituents which are R/S relationship to each other, which one has a higher priority, R or S?

For your other question, the H atoms in question are at the other ortho position to the biphenyl linkage (meta to the isopropyl groups).  For determining the R/S configuration, you go 1 --> 3 --> 2 --> 4, clockwise is R, counterclockwise is S.

Or, if you prefer: isopropyl in front --> isopropyl in back --> H in front --> H in back

[Winga]

I was trying to understand what you meant by your earlier question:For your other question, the H atoms in question are at the other ortho position to the biphenyl linkage (meta to the isopropyl groups).  For determining the R/S configuration, you go 1 --> 3 --> 2 --> 4, clockwise is R, counterclockwise is S.

Or, if you prefer: isopropyl in front --> isopropyl in back --> H in front --> H in back

Aren't isopropyl groups also at the ortho postion to the biphenyl linkage?

[movies]

Yes.  I'm talking about the other ortho position where there are hydrogens.

[laotree]

Big discuss! Just what we need!

Thanks, Guys!