I'm trying to find the R/S for the carbon marked with the asterisk. As I branched out to from the asterisk carbon to the red carbons, the methyl group has the 3rd priority (with the hidden hydrogen 4th). Here comes the confusion. In order to find the first point of difference for the two carbons, for one of the carbon, I can simply move outwards to the next green carbon (C, C, H). But for the other carbon, there are two paths I can take, one which gives (C, H, H) and the other (C, C, H). Now, which path should I take?

Assuming I take the path that gives me the (C, C, H) carbon. This gives me a tie. Again, I have two paths to take (indicated by the green arrow). Which path should I take? If I choose to go 'down', the two arrow clash at a single carbon.

I have consulted numerous textbooks but none of them seem to even surface this as an issue. In general, if there are multiply paths, which path should one take?

Extension of question: In a scenario that there are two carbon, C1 and C2 that 'ties', and it so happens that each of the two carbons has 'two paths' to take: C1 with path 1 and path 2, and C2 with path 3 and path 4. What happens if the priority of taking the different paths is not clear. Eg. Priority when taking path 1 is higher than the path taken by path 3, but path 2 has a lower priority then when taking path 4?

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  • 1
    $\begingroup$ There is no "which". You don't choose paths; you always take them all at once. $\endgroup$ – Ivan Neretin Apr 30 '19 at 20:52

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