{"id":10347,"date":"2017-01-17T16:54:36","date_gmt":"2017-01-17T21:54:36","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=10347"},"modified":"2026-05-06T20:24:33","modified_gmt":"2026-05-07T01:24:33","slug":"determining-rs-2-the-method-of-dots","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2017\/01\/17\/determining-rs-2-the-method-of-dots\/","title":{"rendered":"Assigning Cahn-Ingold-Prelog (CIP) Priorities (2) – The Method of Dots"},"content":{"rendered":"

Cahn Ingold Prelog Priorities (2): Determining R\/S On Rings, Chains, And More<\/strong><\/p>\n

In this post we’ll expand on the Cahn-Ingold-Prelog (CIP) rules and show how to assign\u00a0R<\/em>\/S<\/em> in some trickier cases that involve rings, multiple chiral centers, multiple bonds, and isotopes. We’ll see that the “method of dots” is extremely valuable for breaking ties in determining CIP priorities!<\/p>\n

\"summary-breaking<\/a><\/p>\n

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This post was co-authored with Matt Pierce of\u00a0Organic Chemistry Solutions<\/a>.\u00a0 Ask Matt about scheduling an online tutoring session\u00a0here<\/a>.<\/em><\/p>\n

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Table Of Contents<\/strong><\/p>\n

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  1. Recap: Determining CIP Priorities In Straightforward Cases<\/a><\/li>\n
  2. How Do We Assign CIP Priorities In Trickier Cases Involving Rings, Multiple Chiral Centers, Multiple Bonds, And Isotopes?<\/a><\/li>\n
  3. Reminder: Cahn-Ingold-Prelog Priorities Are Determined At The First Point Of Difference<\/a><\/li>\n
  4. Assigning CIP Priorities: An Application Of The “Method Of Dots”<\/span><\/span><\/a><\/li>\n
  5. How Do We Determine R\/S Configuration In Rings?<\/a><\/li>\n
  6. Determining R\/S Configuration In Rings: A Common “Trick Question”<\/a><\/li>\n
  7. Assigning R\/S Configurations To Molecules With Multiple Chiral Centers<\/a><\/li>\n
  8. Assigning Cahn-Ingold-Prelog (CIP) Priorities With Multiple Bonds<\/a><\/li>\n
  9. Expanding Out Multiple Bonds With “Phantom Atoms”<\/a><\/li>\n
  10. Cahn-Ingold-Prelog Priorities With Isotopes<\/a><\/li>\n
  11. Conclusion: Assigning R\/S Configurations With “The Method Of Dots<\/a><\/li>\n
  12. Notes<\/a><\/li>\n
  13. Quiz Yourself!<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. Recap: Determining CIP Priorities In Straightforward Cases<\/h2>\n

    In our first\u00a0post <\/a>on the Cahn-Ingold-Prelog (CIP) Rules, we gave a broad overview of how to assign R<\/em> or S<\/em> to chiral centers in a molecule. We followed this up with a subsequent post on how to use the CIP rules to determine priorities in alkenes<\/a>\u00a0in order\u00a0to determine whether they are\u00a0E or<\/em>Z<\/em>.<\/p>\n

    As with most introductory posts, the examples chosen were pretty simple. \u00a0It’s fairly\u00a0straightforward to assign priorities in a molecule like the molecules below\u00a0where the central carbon is attached to four atoms with different atomic weights. And as we discussed previously, once you learn the single swap rule,<\/a> it’s easy enough to determine R\/S in cases when the #4 priority substituent isn’t pointing behind the plane of the page.<\/p>\n

    \"simple-4-atoms\"<\/p>\n

    In cases of stereocenters which are attached to identical atoms, we saw that it’s also pretty straightforward to use the “method of dots” we used for E\/Z configuration in alkenes to break ties and move down the chain. (See:<\/span> The Method of Dots<\/a><\/span>)<\/span><\/em><\/p>\n

    More on that in a minute: here are some exercises you can practice on if you like.<\/p>\n

    \"simple<\/p>\n

    <\/a>2. How Do We Assign CIP Priorities In Trickier Cases Involving Rings, Multiple Chiral Centers, Multiple Bonds, And Isotopes?<\/h2>\n

    This post is about some of the further complications we see in assigning priorities. For example, how do you assign priorities in the following cases:<\/p>\n