{"id":7217,"date":"2013-04-12T17:45:29","date_gmt":"2013-04-12T22:45:29","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=7217"},"modified":"2025-07-07T21:00:40","modified_gmt":"2025-07-08T02:00:40","slug":"addition-hbr-alkenes-roor-peroxides-free-radical","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2013\/04\/12\/addition-hbr-alkenes-roor-peroxides-free-radical\/","title":{"rendered":"A Fourth Alkene Addition Pattern – Free Radical Addition"},"content":{"rendered":"

Free Radical Addition Of HBr To Alkenes With ROOR (Peroxides)\u00a0<\/strong><\/p>\n

We’ve seen that there are three major alkene reactivity patterns [carbocation<\/a>, three membered ring<\/a><\/strong>, and concerted<\/a><\/strong>], but there are two minor pathways as well. This post discusses one of them: free-radical addition of HBr to alkenes, which shows the opposite regioselectivity (anti-Markovnikov) than “normal” addition of HBr to alkenes (Markovnikov) which follows the “carbocation” pathway.<\/p>\n

\"Summary-Free<\/a><\/p>\n

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

    \n
  1. Free Radical Addition Of HBr To Alkenes Leads To “Anti-Markovnikov” Products<\/a><\/li>\n
  2. An Outline Of The Free Radical Mechanism For Addition Of HBr To Alkenes In The Presence Of Peroxides<\/a><\/li>\n
  3. Initiation Of The Free-Radical Process Through Homolytic Cleavage Of Peroxides By Heat Or Light<\/a><\/li>\n
  4. Formation Of The Bromine Radical From The Alkoxide Radical And\u00a0 HBr<\/a><\/li>\n
  5. Propagation Step #1 : Addition Of Bromine Radical To The Alkene Occurs So As To Give The Most Stable Carbon Radical<\/a><\/li>\n
  6. Propagation Step #2: The Resulting Carbon Radical Removes A Hydrogen Atom From H\u2013Br, Regenerating The Bromine Radical<\/a><\/li>\n
  7. The Termination Step<\/a><\/li>\n
  8. Summary: Free-Radical Addition Of HBr To Alkenes<\/a><\/li>\n
  9. Notes<\/a><\/li>\n
  10. Quiz Yourself!<\/a><\/li>\n
  11. (Advanced) References and Further Reading<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. Free Radical Addition Of HBr To Alkenes Leads To “Anti-Markovnikov” Products<\/strong><\/h2>\n

    As discussed previously, alkenes normally react with HBr to give products of “Markovnikov” addition; the bromine ends up on the most substituted carbon of the alkene, and the hydrogen ends up on the least substituted carbon. However, something interesting happens when the same reaction is performed in the presence of peroxides and \u00a0heat \/ light: the pattern of addition changes!<\/p>\n

    Instead of Br ending up on the\u00a0most\u00a0<\/strong>substituted carbon of the alkene, it ends up on the least. [The stereochemistry of the reaction, however, is unchanged: it still gives a mixture of “syn” and “anti” products.]<\/em><\/span><\/p>\n

    \"-free<\/a><\/p>\n

    This so-called “anti-Markovnikov” addition is intriguing. What difference could the presence of peroxides, and furthermore heat (or light) make to this reaction?<\/p>\n

    <\/a>2. An Outline Of The Free Radical Mechanism For Addition Of HBr To Alkenes In The Presence Of ROOR (Peroxides)<\/h2>\n

    This reaction occurs through a free-radical process<\/strong>. (For a primer on free radical chemistry, you might want to check out this introductory article on Free Radical Reactions<\/a>).\u00a0 Here is an outline of the mechanism:<\/em><\/span><\/p>\n