Need help on a OC problem?

[Merisa15]

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So I happened to get (1R, 2R, 3S)-2-ethyl-3-methylcycloehexanol. But that is not the answer. Can someone be so kind as to explain why that 2nd carbon is S?

 

[FeralisExtremum]

In all three cases, the atom directly adjacent to carbon 2 is another carbon (attached to other groups). So we have to look at the attachments on those carbons to figure out the highest rank. Look at it this way in terms of ranking:

1. The carbon to the immediate left of carbon 2 is attached to an O. This makes it highest priority.

2. The remaining two carbons attached to carbon 2 are both attached to other carbons - so we have to distinguish priority. The first carbon of the ethyl group (which you've written out as CH2 above) is only attached to one more carbon, the CH3 at the end of the ethyl group. Meanwhile, the other carbon that needs to be ranked (carbon 3, directly to the right of carbon 2) is attached to a CH3 from the methyl group and the CH2 of the adjacent carbon the cyclohexane ring. Since carbon 3 is attached to two additional carbons, and the carbon in the ethyl group is only attached to one additional carbon, carbon 3 ranks higher in priority.

This makes the order of priority on carbon 2's attachments:
C1 > C3 > ethyl carbon

As you'd expect, this gives us a counter-clockwise S ranking for C2.

Hope that made sense!