{"id":10487,"date":"2017-02-24T18:02:01","date_gmt":"2017-02-25T00:02:01","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=10487"},"modified":"2026-05-06T19:36:56","modified_gmt":"2026-05-07T00:36:56","slug":"optical-purity-and-enantiomeric-excess","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2017\/02\/24\/optical-purity-and-enantiomeric-excess\/","title":{"rendered":"Optical Purity and Enantiomeric Excess"},"content":{"rendered":"

All About Optical Purity and Enantiomeric Excess<\/strong><\/p>\n

What is a racemic mixture? What is “optically active”, “optically inactive”, and “optically pure”. What do the terms, “l”\u00a0<\/em>and “d<\/em>” mean? What is the formula for optical purity? What is the difference between optical purity and enantiomeric excess? [OK, that one’s quick: they’re the same thing].<\/p>\n

In this post we answer all these burning questions, and do some worked examples of optical purity and enantiomeric excess besides.<\/p>\n

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

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

    \n
  1. Louis Pasteur and The Discovery Of “Enantiomers”<\/a><\/li>\n
  2. A Racemic Mixture Is An Equal Mixture Of Two Enantiomers And Has A Specific Rotation of 0\u00b0 (Optically Inactive)<\/a><\/li>\n
  3. An “Optically Pure” Solution Contains Only One Enantiomer<\/a><\/li>\n
  4. The Terms “l-<\/em>” And “d-<\/em>” Refer To The “Levorotatory (\u2013)” And “Dextrorotatory (+)” Forms Of Each Enantiomer<\/a><\/li>\n
  5. The Formula For Optical Purity<\/a><\/li>\n
  6. Using “Optical Purity” To Determinine The Molar Composition Of A Mixture<\/a><\/li>\n
  7. Optical Purity = Enantiomeric Excess<\/a><\/li>\n
  8. Another Practice Problem For Enantiomeric Excess<\/a><\/li>\n
  9. An Optical Purity Practice Problem<\/a><\/li>\n
  10. Notes<\/a><\/li>\n
  11. Quiz Yourself!<\/a><\/li>\n<\/ol>\n
    \n

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

    \n

    <\/a>1. Louis Pasteur And The Discovery Of\u00a0 “Enantiomers”<\/h2>\n

    In our last post on optical rotation (See post: Optical Rotation, Optical Activity, and Specific Rotation<\/a><\/span>)<\/em>\u00a0we saw that when Louis Pasteur crystallized a salt of a compound then known as “racemic acid” he discovered that it formed two different types of crystals.<\/p>\n

    When redissolved in water, one set of crystals rotated plane-polarized light clockwise (dextrorotatory, “to the right”).<\/p>\n

    The other set of crystals rotated plane-polarized light to exactly the same extent<\/strong>, but\u00a0counterclockwise (levorotatory, “to the left”).<\/p>\n

    This was surprising for two reasons.<\/p>\n