{"id":7801,"date":"2013-12-09T08:00:00","date_gmt":"2013-12-09T14:00:00","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=7801"},"modified":"2026-05-07T04:54:49","modified_gmt":"2026-05-07T09:54:49","slug":"in-summary-free-radicals","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2013\/12\/09\/in-summary-free-radicals\/","title":{"rendered":"In Summary: Free Radicals"},"content":{"rendered":"

A Summary Of Topics\u00a0 For Free-Radical Reactions<\/strong><\/p>\n

So what have we learned in this series about free radicals? Lots of things. We’ve seen that\u00a0 they are neutral species with a half-filled valence shell that tend to be highly reactive with alkanes\u00a0 and double bonds.\u00a0 We’ve seen that they are stabilized by electron donors and delocalization.\u00a0 We’ve shown their mechanisms proceed through three distinct stages (initiation, propagation, termination) and that although reactions tend to be selective for formation of the more stable free radical, bromine radicals are far more selective than chlorine radicals in halogenation of alkanes. Finally we’ve also explored allylic (and benzylic) halogenation and seen examples of allylic rearrangement.<\/p>\n

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

    \n
  1. What Are Free Radicals?<\/a><\/li>\n
  2. Homolytic Cleavage And “Single-Barbed” Curved Arrows<\/a><\/li>\n
  3. Factors That Affect Free Radical Stability<\/span><\/span><\/a><\/li>\n
  4. Factors That Destabilize Free Radicals<\/a><\/li>\n
  5. A Handy Shortcut For Determining Radical Stability<\/a><\/li>\n
  6. How Do Free Radical Reactions Work? Initiation, Propagation, Termination<\/a><\/li>\n
  7. Free-Radical Halogenation Leads To Mixtures Of Isomers – But There Is Some Selectivity<\/a><\/li>\n
  8. Bromination Is More Selective Than Chlorination<\/a><\/li>\n
  9. Allylic Bromination,\u00a0 And Allylic Bromination With Allylic Rearrangement<\/a><\/li>\n
  10. Free-Radical Addition Of Radicals To Alkenes<\/a><\/li>\n
  11. Notes<\/a><\/li>\n
  12. Quiz Yourself!<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. What Are Free Radicals?\u00a0<\/strong><\/h2>\n

    Free radicals<\/a> are chemical species that contain a singly occupied orbital. They are\u00a0neutral<\/span> and tend to be highly reactive.
    \nOne of the most common classes of free-radical reactions is\u00a0free-radical substitution<\/span>. Here is an example of the reaction between methane and Cl2<\/sub>:<\/p>\n

    \"free-radical-substitution-reaction-of-methane-with-cl2-giving-methyl-chloride-initiated-with-light\"<\/p>\n

    <\/a>2. Homolytic Cleavage And “Single-Barbed” Curved Arrows<\/strong><\/h2>\n

    Unlike many of the reactions we have seen previously, free radical reactions do not involve the donation or acceptance of an electron\u00a0lone pair<\/span> and they are not\u00a0ionic<\/span>. Instead, free radical reactions operate through\u00a0homolytic cleavage<\/span> – that is, bonds break such that equal numbers of electrons are distributed to each atom.\u00a0\u00a0The curved arrow formalism is modified here: we draw curved arrows with a single barb, which shows the movement of a single electron:<\/p>\n

    \"homolytic-cleavage-of-chlorine-single-barbed-arrows-giving-chlorine-radicals\"<\/p>\n

    <\/a>3. Factors That Affect Free Radical Stability<\/strong><\/h2>\n

    Like carbocations, \u00a0free radicals are electron-deficient<\/span> species. [Helpful to know: \u00a0the factors which affect the stability of free radicals are the same which influence the stability of carbocations.]\u00a0They can be stabilized through donation of electron density by neighbours; for this reason, radical stability increases in the order methyl < primary < secondary < tertiary . [Radicals are also stabilized by adjacent atoms with lone pairs<\/a>, such as oxygen and nitrogen]. [See post: 3 Factors Which Stabilize Free Radicals<\/a>]<\/em><\/span><\/p>\n

    \"order-of-radical-stability-methyl-least-stable-tertiary-most-stable\"
    \nA second important factor which stabilizes free radicals is “delocalization” – that is, if the radical can be spread out over two or more carbons. A more familiar way of saying this is that free radicals are stabilized by resonance<\/span>.<\/p>\n

    \"delocalization-of-electrons-stabilizes-radicals-through-resonance-allylic-and-benzylic\"<\/p>\n

    <\/a>4. Factors That Destabilize Free Radicals<\/h2>\n

    As electron-deficient species, it is also helpful to keep in mind some of the factors which\u00a0destabilize<\/span> free radicals. There are several important trends to keep in mind [See post: Which Factors Destabilize Free Radicals<\/a>?]<\/em><\/span><\/p>\n