{"id":12435,"date":"2019-05-21T21:40:42","date_gmt":"2019-05-22T02:40:42","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=12435"},"modified":"2026-04-18T05:05:13","modified_gmt":"2026-04-18T10:05:13","slug":"how-to-determine-r-and-s-configurations-on-a-fischer-projection","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2019\/05\/21\/how-to-determine-r-and-s-configurations-on-a-fischer-projection\/","title":{"rendered":"How To Determine R and S Configurations On A Fischer Projection"},"content":{"rendered":"

Determining R<\/em> and S<\/em> Configurations On Fischer Projections<\/strong><\/p>\n

From time to time you may be asked to determine R\/S on a Fischer projection.<\/p>\n

\"determining-r-s-in-fischer-projection\"<\/p>\n

So how do you do that?<\/p>\n

The Fischer projection is an early solution to the problem of drawing 3-dimensional molecules on a 2-dimensional page. Think of a light source illuminating a 3-dimensional object that casts a shadow on a flat surface. If these objects are always arranged in a consistent way, we can infer the 3-D structure of the object from the shadow. In the days before dashes, wedges, and the CIP rules, that’s what Fischer was going for.<\/p>\n

[Note: This\u00a0post was co-authored with Matthew Pierce of\u00a0Organic Chemistry Solutions<\/a>.\u00a0 Ask Matt about scheduling an online tutoring session\u00a0here<\/a>. ]<\/em><\/span><\/p>\n

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

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

    \n
  1. Interpreting A Fischer Projection<\/a><\/li>\n
  2. Step 1: Identify The Chiral Centers<\/a><\/li>\n
  3. Assign Cahn-Ingold-Prelog (CIP) Priorities On The Fischer Projection<\/a><\/li>\n
  4. Trace A Path From The 1, 2, and 3 Priority Groups. Clockwise or Counterclockwise?<\/a><\/li>\n
  5. How Do We Assign R\/S When The Priority #4 Group Is In The Front?\u00a0<\/a><\/li>\n
  6. With Fischer Projections, “Reverse Rules” Will Apply 99% Of The Time<\/a><\/li>\n
  7. Converting A Fischer Projection To A Line Diagram<\/a><\/li>\n
  8. Summary: Determining R\/S On A Fischer Projection<\/a><\/li>\n
  9. Notes<\/a><\/li>\n
  10. Quiz Yourself!<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. Interpreting A Fischer Projection<\/strong><\/h2>\n

    Recall how the Fischer projection works. The longest carbon chain is arranged vertically and the substituents are drawn out to the side. Although the “projection” is drawn<\/em> flat, it’s important to remember that the carbons are still tetrahedral. The convention is that the horizontal groups on the side are “wedges” – that is, as my teacher Professor Kazlauskas used to say, “the arms come out to hug you”. Or strangle you<\/em>, as one of my students once suggested.<\/p>\n

    \"how-to-interpret-a-fischer-projection-the-arms-come-out-to-hug-you\"<\/p>\n

    How do you determine R\/S on a Fischer? Let’s use the first molecule (a) as an example.<\/p>\n

    <\/a>2. Identify The Chiral Centers On The Fischer Projection<\/h2>\n