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Provide absolute configuration to sulfur atom.

[(E)-2-(Phenylsulfinyl)vinyl]benzene

I know that oxygen atom will get the highest priority by Cahn–Ingold–Prelog rule, but what about the next priority? I believe it should be phenyl, but all my classmates say it is the alkene phenyl. Please provide an explanation for the sulfur configuration within the the R/S system.

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    $\begingroup$ Were you taught some kind of rules for resolving this kind of ambiguity? $\endgroup$ Jan 22, 2022 at 14:15
  • $\begingroup$ Also, did you look for other resources on the Internet, which explain how to resolve ties when the first atom directly bonded to the stereocentre is the same? $\endgroup$ Jan 22, 2022 at 14:16
  • $\begingroup$ Also, what makes you believe it is phenyl? Did you follow the CIP rules, or did you make a guess, or something else? $\endgroup$ Jan 22, 2022 at 14:17
  • $\begingroup$ Yes I followed the rules. Since there are 2 carbons attached to Sulphur it's a tie for priority $\endgroup$
    – Sushant
    Jan 23, 2022 at 3:35
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    $\begingroup$ Now, Phenyl carbon has only carbons attached to it whereas the alkene carbon has one Hydrogen atom attached to it Thus, phenyl carbon should get priority $\endgroup$
    – Sushant
    Jan 23, 2022 at 3:36

1 Answer 1

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Phenyl gets the second highest priority. To break the tie, check the level of substitution at the carbon bonded to sulfur. Duplicate the doubly bonded carbon. Hence in phenyl it is bonded to three carbon atoms (C,C,C) whereas in alkenyl phenyl it is bonded two carbons and one hydrogen (C,C,H). Phenyl carbon is highly substituted and hence gets highest priority. Lowest priority is lone pair. Counterclockwise, lowest priority is behind the plane hence 'S' configuration.

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