Hexabromocyclododecane has 16 possible stereo-isomers. So far so good.

I found that:

Now, I have two more CAS which are a bit opaque to me:

  • 678970-15-5

    • Pubchem says it's the same as 134237-50-6
    • However, ChemIDplus says it's alpha-Hexabromocyclododecane, (-)-
  • 138257-18-8

    • Pubchem says it's the same as 134237-51-7
    • However, ChemIDplus says it's beta-Hexabromocyclododecane, (-)-

Question: Describe each of the CAS different chemicals or are 678970-15-5 and 138257-18-8 just synonyms. If they are different chemicals, what does (-)- mean here?


1 Answer 1


The (−)- in the chemical name refers to the optical activity of the compound, that is to tilt the plane of linerarly polarized light to the left (levo-rotatory), or to the right (dextro-rotatory) as determined with a polarimeter. It may be either (−)- (minus sign) or (+)- (plus sign), as seen, for example, for two of the three forms of tartaric acid. Compounds with stereogenic centers which do not tilt the plane of the polarization (e.g., meso-compounds) may be labeled by (±)-, too.

Similar to this, and equally used for molecules with stereogenic centers − especially carbohydrates − is the the use of (D)-, (L)- or (DL)- which however relates to the spatial arrangement around the sterogenic atom of highest priority (there are rules how to determine this just from reading structure formula). It may be used independently as well as simultaneously to the former system of (−)- and (+)-; but note the knowledge what to write within former system does not translate 1:1 into what to use in the later system.

As for the CAS number, this is a book-keeping number (see here); contrasting to a chemical name, it does not convey structural or even chemical meaning. CAS for example systematically attributes more than one registry number to the same molecule if there is evidence that it crystallizes in different polymorphs. This «already» happens with inorganic molecules like $\ce{TiO2}$ which may crystallize in the form of the mineral anatase (CAS 1317-70-0), or rutile (CAS 1317-80-2). Thus, if your literature search is about the isolated molecule alone (say, molecular mass), it does not matter which entry you follow further. Yet if you are interested in say the melting point of your compound or crystal structure, you have to treat the CAS numbers separate, like referring to different compounds. Thus, the relationship between CAS number and a structure is not bijective.

(Occasionally, e.g. for some of the more substituted fullerenes, you may encounter many CAS numbers for one and the same compound in a database like Scifinder or Reaxys. Possibly the difficulty to draw their formulae contributed to create whole sets of CAS numbers.)


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