How do I determine whether the central carbon of this compound is (R) or (S)?


On both the sides of carbon, the groups are same. They differ only in the relative position of hydrogen atoms. On what basis do I assign priority in these groups?

Does the R/S nomenclature for the middle carbon really depend on E/Z nomenclature of the alkenes, in such cases of stereoisomerism? Or is there an entirely different argument in such a case?


1 Answer 1


In order to arrive at an unambiguous description of stereoisomers, the priority for the groups attached to the chirality center is established by the application of ‘Sequence Rules’. In the hierarchical Sequence Rules given in Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), we find

P- Sequence Rule 3

When considering double bonds and planar tetraligand atoms ‘seqcis’ = ‘Z’ precedes ‘seqtrans’ = ‘E’ and this precedes nonstereogenic double bonds.

In the given example, however, the stereochemical configuration at the central carbon atom is not defined. Note that solid wedges should be used to indicate bonds that project above the plane of the paper and hashed wedges to indicate bonds that project below the plane of the paper.

  • 4
    $\begingroup$ Maybe it's a Fischer projection. $\endgroup$
    – EJC
    Commented Aug 15, 2015 at 14:00
  • 2
    $\begingroup$ According to IUPAC Recommendations (2006): “The use of Fischer projections in non-carbohydrates is problematic. Any drawings of this type are necessarily ambiguous since it is unclear whether the author of the drawing intended to make a Fischer projection or simply happened to draw a diagram in that orientation. (…) Accordingly, (…), similar depictions of non-carbohydrates should be used only when the viewer of the diagram is certain to understand that the diagram was intended to represent a Fischer projection. Fischer projections should be avoided where ambiguity is possible, (…)” $\endgroup$
    – user7951
    Commented Aug 16, 2015 at 11:58

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