{"id":10655,"date":"2017-04-11T14:54:08","date_gmt":"2017-04-11T18:54:08","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=10655"},"modified":"2026-05-06T19:01:53","modified_gmt":"2026-05-07T00:01:53","slug":"more-on-12-and-14-additions-to-dienes","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2017\/04\/11\/more-on-12-and-14-additions-to-dienes\/","title":{"rendered":"More On 1,2 and 1,4 Additions To Dienes"},"content":{"rendered":"

Two More Diene Reactions: Free-Radical Addition of HBr to Dienes, and Addition of Br2<\/sub> to Dienes<\/strong><\/p>\n

In an earlier post we covered 1,2 and 1,4 additions to dienes,<\/a> specifically the addition of strong acid (e.g. HCl or HBr) to dienes.<\/p>\n

You might rightfully ask, “is that it?”. Are there any other examples of reactions that give “1,2” vs “1,4” products?<\/p>\n

Why yes, thank you for asking. Today we’ll go through two more: free-radical addition of HBr to dienes, and the addition of Br2<\/sub> to dienes.
\nOh, and we’ll also work through three 1,2- versus 1,4- addition product practice problems.<\/p>\n

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

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

    \n
  1. Recap: “1,2” vs. “1,4” Addition To Dienes, And “Kinetic Control” vs “Thermodynamic Control “<\/a><\/li>\n
  2. Free-Radical Addition of HBr to Dienes Is Also Subject To Kinetic And Thermodynamic Control<\/a><\/li>\n
  3. In Radical Addition Of HBr To Butadiene, The 1,2- Product Is The Kinetic Product And The 1,4- Product Is The Thermodynamic Product<\/a><\/li>\n
  4. Addition of Br2<\/sub> (And Other Halogens) to Dienes<\/a><\/li>\n
  5. 1,2- And 1,4- Products In Addition Of Br2<\/sub> To Butadiene<\/a><\/li>\n
  6. The Role Of Temperature In Determining The Product Ratio Of 1,2- And 1,4- Addition Of Br2<\/sub><\/span> To Butadiene<\/span><\/span><\/a><\/li>\n
  7. A Different Rationalization For Why The 1,2- Product Is The Kinetic Product: Ion Pairing<\/a><\/li>\n
  8. Thinking Through “Kinetic” and “Thermodynamic” Control – Three Practice Problems<\/a><\/li>\n
  9. Question #1. Cyclopentadiene<\/a><\/li>\n
  10. Question #2 (2,5 dimethyl hexa-2-4-diene)<\/a><\/li>\n
  11. Question #3: 1-Methyl Cyclohexadiene<\/a><\/li>\n
  12. Conclusion<\/a><\/li>\n
  13. Notes<\/a><\/li>\n
  14. Quiz Yourself!\u00a0<\/a><\/li>\n
  15. (Advanced) References and Further Reading<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. Recap: “1,2” vs. “1,4” Addition To Dienes, And “Kinetic Control” vs “Thermodynamic Control”<\/h2>\n

    To recap: In Org 1 we learned that addition of HCl and HBr to normal, isolated alkenes (such as 1-butene) just gives one product – the Markvonikoff product (“1,2-addition”) where the H and the nucleophile (e.g. Br-) are on adjacent carbons, and Br has added to the most substituted carbon.<\/p>\n

    However, addition of acid to a diene<\/strong> results in a resonance-stabilized carbocation<\/strong>, which can undergo attack by the nucleophile at two<\/strong> possible positions, and thus can lead to two<\/strong> different products.<\/p>\n

    The\u00a0two different products are the “1,2-addition” product, where the H and Br are on adjacent carbons, and the “1,4-addition” product, where the carbons bearing H and Br are separated by a double bond.<\/p>\n

    Temperature plays a key role in determining the product distribution. At low temperatures, the reaction is irreversible and the product that proceeds through the\u00a0lowest-energy\u00a0transition state (i.e. the product that is formed fastest!) \u00a0is favoured. We rationalized this by saying that attack occurred at the carbon best able to stabilize positive charge (i.e the “resonance form” with the most stable carbocation). At higher temperatures, the reaction becomes reversible, and the favoured product will be the one with the most substituted double bond (not unlike\u00a0Zaitsev’s Rule<\/a>)\u00a0\u00a0We call these two conditions, respectively, kinetic control \u00a0and thermodynamic control.<\/p>\n

    We finished up the post with a quiz, which I’m reproducing here.<\/p>\n

    \"practice<\/p>\n

    We’ll dig into the answers for these questions\u00a0at the bottom.<\/p>\n

    But first, for completeness, let’s cover two other reactions of dienes that can lead to 1,2- and 1,4- addition products.<\/p>\n