{"id":10128,"date":"2016-09-08T17:27:53","date_gmt":"2016-09-08T21:27:53","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=10128"},"modified":"2022-10-31T05:08:34","modified_gmt":"2022-10-31T10:08:34","slug":"conjugation_and_color","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2016\/09\/08\/conjugation_and_color\/","title":{"rendered":"Conjugation And Color (+ How Bleach Works)"},"content":{"rendered":"

Conjugation And Color<\/strong><\/p>\n

Why are tomatoes red? Why are carrots orange? Why are egg yolks yellow? \u00a0 And… why is\u00a0Vulcan\u00a0blood green?<\/p>\n

\"spock<\/p>\n

OK, I’m not going to\u00a0touch that last one, but as for the first three: great question.<\/p>\n

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

    \n
  1. These Highly Colored Molecules Have Highly Conjugated Pi Bonds<\/a><\/li>\n
  2. Removing Pi Bonds Changes The Color (Or Removes It Entirely)<\/a><\/li>\n
  3. How Bleach Works:\u00a0 By Destroying Pi\u00a0 Bonds<\/a><\/li>\n
  4. So How Exactly Is Conjugation Related To Color?<\/a><\/li>\n<\/ol>\n
    \n

    There are actually 15 chemical causes\u00a0of color<\/a>, but today\u00a0we’ll begin to explore the\u00a0reason for the characteristic colors of tomatoes, egg yolks, carrots, and many other pigments from nature, such as the green color of leaves and the red color of blood (heme). And yes, there is a spectroscopy angle here – but that’s going to wait until the next post.<\/p>\n

    Before getting into the\u00a0“why”, I always like to look at the “what”. Specifically, here are some examples of highly coloured molecules from everyday life.\u00a0Do you notice something that\u00a0all of these molecules have in common?<\/p>\n

    \"representative<\/p>\n

    Yes, they have lots of double bonds. \u00a0But having lots of double bonds is not sufficient<\/b> for a molecule to be strongly colored.<\/p>\n

    For instance, natural rubber latex can have hundreds or thousands of pi bonds, and yet it is milky white [Note 1<\/a>]:<\/p>\n

    \"natural<\/p>\n

    <\/a>1. These Highly Colored Molecules Have Many Conjugated<\/em>\u00a0Pi<\/em>\u00a0Bonds<\/strong><\/h2>\n

    There’s something\u00a0special<\/em> about the way the pi bonds are arranged in lycopene, lutein, and b-carotene, as opposed to natural rubber latex: the pi bonds are\u00a0conjugated.\u00a0<\/em><\/p>\n

    What does that mean? [See: Are these alkenes conjugated<\/a><\/strong>?].<\/em><\/p>\n

    Quickie review on “conjugated” versus “non-conjugated” alkenes:<\/p>\n

    \"conjugated<\/p>\n