{"id":7428,"date":"2013-07-30T07:24:51","date_gmt":"2013-07-30T11:24:51","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=7428"},"modified":"2025-11-05T03:57:20","modified_gmt":"2025-11-05T09:57:20","slug":"free-radical-reactions","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2013\/07\/30\/free-radical-reactions\/","title":{"rendered":"Free Radical Reactions"},"content":{"rendered":"<p><strong>Free Radical Reactions &#8211; Chlorination of Methane<\/strong><\/p>\n<ul>\n<li>Alkanes are pretty boring, chemically speaking.\u00a0 They don&#8217;t tend to undergo many reactions.<\/li>\n<li>However, when they are treated with Cl<sub>2<\/sub> and <strong>light <\/strong>(<em>h\u03bd<\/em>), a <strong>substitution<\/strong> of H for Cl occurs.<\/li>\n<li>As we hope to show here, this is NOT an acid-base reaction.\u00a0 In\u00a0 fact, it proceeds through a pathway we haven&#8217;t explored yet, called, &#8220;<strong>free radical substitution<\/strong>&#8220;.<\/li>\n<li>In this post we introduce the concepts\u00a0 of <strong>homolytic bond breakage<\/strong>, <strong>single-barbed curved arrows<\/strong>, <strong>free radicals<\/strong>, and <strong>free-radical substitution<\/strong> through the example of\u00a0 chlorination of\u00a0 alkanes with Cl<sub>2<\/sub> and light, which\u00a0 will lay the\u00a0 foundation for this series on free-radical chemistry.<\/li>\n<\/ul>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-38573\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/11\/0-summary-free-radical-substitution-passes-through-free-radical-intermediates-which-are-neutral-but-highly-reactive.gif\" alt=\"summary-free radical substitution passes through free radical intermediates which are neutral but highly reactive\" width=\"640\" height=\"451\" \/><\/a><\/p>\n<p><strong>Table of Contents<\/strong><\/p>\n<ol>\n<li><a href=\"#one\">Heterolytic Bond Cleavage Versus Homolytic Bond Cleavage<\/a><\/li>\n<li><a href=\"#two\">Chlorination of Methane With Cl<sub>2<\/sub> And Light (<em>h\u03bd<\/em>) Breaks C\u2013H and Forms C\u2013Cl<\/a><\/li>\n<li><a href=\"#three\">Why Chlorination of Methane With Cl<sub>2<\/sub> Cannot Be An Acid-Base Reaction<\/a><\/li>\n<li><a href=\"#four\">The Reaction Proceeds Through A Neutral\u00a0 Intermediate<\/a><\/li>\n<li><a href=\"#five\">The Chlorine-Chlorine Bond Is Weak. So How Does It Break?<\/a><\/li>\n<li><a href=\"#six\">&#8220;Homolytic&#8221; Chlorine-Chlorine Bond Breaking Is Depicted With &#8220;Single-Barbed&#8221; Curved Arrows<\/a><\/li>\n<li><a href=\"#seven\">The Chlorine Atom Is Neutral And Has An Unpaired Electron<\/a><\/li>\n<li><a href=\"#eight\">Summary: &#8220;Free\u00a0 Radicals&#8221;Are Highly Reactive Chemical Species Containing An Unpaired Electron<\/a><\/li>\n<li><a href=\"#notes\">Notes<\/a><\/li>\n<li><a href=\"#quizzes\">Quiz Yourself!<\/a><\/li>\n<li><a href=\"#references\">References and Further Reading<\/a><\/li>\n<\/ol>\n<hr \/>\n<h2><a id=\"one\"><\/a>1. Heterolytic Bond Cleavage versus Homolytic Bond Cleavage<\/h2>\n<p>With rare exceptions, until now every reaction we&#8217;ve discussed (acid-base, substitution, elimination, addition) has involved the formation of bonds between an <strong>electron pair donor<\/strong> (Lewis base) and an<strong> electron pair acceptor<\/strong> (Lewis acid) or the breakage of bonds to generate the same [this is called &#8220;<strong>heterolytic<\/strong>&#8221; cleavage, by the way, since one bonding partner gets two electrons and the other gets zero].<\/p>\n<p>In this series of posts we&#8217;ll take a detour into a corner of organic chemistry where bonds are formed by the <strong>combination of<\/strong> <strong>single electrons<\/strong> and bonds break through<strong> &#8220;homolytic&#8221; cleavage<\/strong> [that is, each bonding partner receives an <strong>equal<\/strong> number of electrons]. As we&#8217;ll see, these reactions are generally referred to as <strong>free radical reactions.<\/strong><\/p>\n<h2><a id=\"two\"><\/a>2. Chlorination Of Methane With Cl<sub>2<\/sub> And &#8220;Light&#8221; Breaks C-H and Forms C-Cl<\/h2>\n<p>It all starts with a simple observation. Take an ordinary hydrocarbon gas &#8211; methane, for example, although any alkane hydrocarbon will be suitable here. \u00a0When we combine this hydrocarbon with chlorine gas,\u00a0<b>in the dark<\/b>, nothing happens.<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-41379\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-methane-plus-chlorine-gives-no-products-in-the-absence-of-light-or-heat.gif\" alt=\"methane plus chlorine gives no products in the absence of light or heat\" width=\"640\" height=\"176\" \/><\/a><\/p>\n<p>Here&#8217;s the interesting part. Flick a switch &#8211; or remove the cover &#8211; such that visible light can enter the flask, and suddenly our methane is consumed such that carbon-hydrogen bonds are replaced with carbon-chlorine bonds. The final product depends on the number of equivalents of chlorine gas &#8211; let&#8217;s use Cl<sub>2<\/sub> in very small quantities to start with, to keep things simple.<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-41380\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-methane-plus-chlorine-and-light-gives-methyl-chloride-zero-point-25-equivalents.gif\" alt=\"methane plus chlorine and light gives methyl chloride zero point 25 equivalents\" width=\"640\" height=\"138\" \/><\/a><\/p>\n<p>What&#8217;s going on here? Note that &#8220;h\u03bd&#8221; means &#8220;light&#8221;.<span style=\"color: #993366;\"><em> [We can also do this reaction with heat alone, although it requires higher temperatures].<\/em><\/span><\/p>\n<p>Before trying to understand why this happened, let&#8217;s make sure we&#8217;re clear on <span style=\"text-decoration: underline;\">what<\/span> has happened.<\/p>\n<p>Let&#8217;s look at what bonds have formed and what bonds have broken. Notice that we&#8217;re <strong>breaking<\/strong> Cl-Cl, C-H, and<strong> forming<\/strong> C-Cl.<\/p>\n<h2><a id=\"three\"><\/a>3. Why Chlorination of Methane With Cl<sub>2<\/sub> Cannot Be An Acid-Base Reaction<\/h2>\n<p>What reactions have we seen so far that would be capable of such a transformation? Well, we&#8217;ve seen that all acid-base reactions involve the cleavage of a bond between H and some atom. So maybe, you might say, it&#8217;s possible that somehow a hydrogen is being pulled off the carbon by a strong base, and then the carbon attacks chlorine. But does that make sense here?<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-41381\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-methane-plus-base-does-not-give-methyl-anion-equilibrium-not-favored.gif\" alt=\"methane plus base does not give methyl anion equilibrium not favored\" width=\"640\" height=\"357\" \/><\/a><\/p>\n<p>Note that methane [CH<sub>4<\/sub>] would have to be the acid, and chloride ion [Cl- ] would have to be the base. As we&#8217;ve seen before, this makes NO sense as an acid base reaction, because we&#8217;d be going from a very weak acid [CH<sub>4<\/sub>] to a very strong acid [HCl] and likewise a weak base [Cl] to a strong base.<\/p>\n<p>This is like trying to get Niagara Falls to flow in reverse. Not gonna happen!<\/p>\n<h2><a id=\"four\"><\/a>4. The Reaction Proceeds Through A Neutral Intermediate<\/h2>\n<p>Furthermore, a second piece of evidence should give pause. If the reaction proceeded through some kind of charged intermediate like Cl<sup>&#8211;<\/sup> or CH<sub>3<\/sub><sup>\u2013<\/sup>\u00a0, we would expect that the reaction would\u00a0 proceed more quickly in <strong>polar<\/strong> solvents [<span style=\"color: #993366;\"><em>that can stabilize charge<\/em><\/span>] as opposed to nonpolar solvents [<span style=\"color: #993366;\"><em>which do not stabilize charge<\/em><\/span>]. Instead, we find that the rate of the reaction is almost completely independent of solvent polarity. It proceeds just about as quickly in [nonpolar] carbon tetrachloride as it does in a polar solvent such as methanol.\u00a0<b><br \/>\n<\/b><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-41382\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/4-radical-reactions-not-as-subject-to-solvent-effects-since-radicals-are-neutral.gif\" alt=\"radical reactions not as subject to solvent effects since radicals are neutral\" width=\"640\" height=\"217\" \/><\/a><\/p>\n<p>So what does this mean? It is consistent with the reaction proceeding through <b>neutral<\/b> intermediates rather than <b>polar<\/b> ones.<\/p>\n<p>Alright &#8211; so how, then, might we get a neutral intermediate?<\/p>\n<h2><a id=\"five\"><\/a>5. The Chlorine-Chlorine Bond Is Weak. So How Does It Break?<\/h2>\n<p>Let&#8217;s think first about that chlorine-chlorine bond, which is relatively weak. Imagine that by heating it up or by shining light on it [<span style=\"color: #993366;\"><em>recall that light is a form of energy!<\/em><\/span>] it might break somehow. How might it break?<\/p>\n<p>Option #1 would look something like this &#8211; &#8220;heterolytic&#8221; cleavage, where one atom receives both electrons from the bond, while the other does<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-41383\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/5-how-does-light-break-cl-cl-bond-not-through-heterolytic-cleavage-because-this-would-lead-to-ions.gif\" alt=\"how does light break cl cl bond not through heterolytic cleavage because this would lead to ions\" width=\"640\" height=\"258\" \/><\/a><\/p>\n<p>However if this was the case, then we should expect to see a faster reaction in polar solvents, which is <b>not<\/b> the case.<\/p>\n<p>So what else might happen here?<\/p>\n<p>Recall that many of the molecules we&#8217;ve been discussing have a dipole. That is,\u00a0 two atoms sharing a bond have unequal electronegativities, and thus unequal electron densities &#8211; one is electron rich and one is electron poor.<\/p>\n<p>However here we have two chlorine atoms. They have equal electronegativity. So we would not expect that one chlorine should &#8220;win&#8221; the tug of war of electrons over the other.<\/p>\n<h2><a id=\"six\"><\/a>6. &#8220;Homolytic&#8221; Chlorine-Chlorine Bond Breakage Is Depicted With &#8220;Single-Barbed&#8221; Curved Arrows<\/h2>\n<p>So we are left with this: <b>What if the chlorine chlorine bond breaks such as to give each chlorine a single electron?\u00a0<\/b><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-41384\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-depiction-of-homolytic-cleavage-of-cl-cl-with-single-barbed-arrows.gif\" alt=\"depiction of homolytic cleavage of cl cl with single barbed arrows\" width=\"640\" height=\"311\" \/><\/a><\/p>\n<p>Weird, when you first see it. One note &#8211; when using the arrow pushing notation, we modify it somewhat, such that we have <b>single fishhooks<\/b> showing where the electrons go. Note that we can use these to show a single electron going to each of the two chlorines.<\/p>\n<p>But let&#8217;s look at this a bit more closely.<\/p>\n<h2><a id=\"seven\"><\/a>7. The Chlorine Atom Is Neutral And Has An Unpaired Electron<\/h2>\n<p>What would be the charge of the chlorine atom? It has seven valence electrons.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-41385\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/7-formal-charge-of-free-radicals-is-zero-chlorine-has-seven-valence-electrons-and-neutral-highly-reactive-partially-filled-orbital.gif\" alt=\"formal charge of free radicals is zero chlorine has seven valence electrons and neutral highly reactive partially filled orbital\" width=\"640\" height=\"275\" \/><\/a><\/p>\n<p>This species is <b>neutral,<\/b> which is consistent with the solvent data.<\/p>\n<p>Would we expect such a species to be stable? If we think back to the octet rule, we see that\u00a0<b> each chlorine atom bears less than a full octet of electrons.\u00a0<\/b> In other words, it is electron-deficient.<\/p>\n<h2><a id=\"eight\"><\/a>8. Summary: Species With An Unpaired Electron Are Called &#8220;Free Radicals&#8221; And Are Highly Reactive<\/h2>\n<p>We would expect such a species to be highly reactive, <strong>since there will be a strong driving force to form the full octet.<\/strong><\/p>\n<p>The whole point of this article is to say that yes, such species <b>do<\/b> exist, and they are called &#8220;<strong>free radicals<\/strong>&#8221; <span style=\"color: #993366;\"><em>[the etymology of this term is interesting, but a little old-fashioned and not crucial for us right now]<\/em><\/span><\/p>\n<p>There are additional pieces of evidence collected over the past hundred-odd years to support the existence of free radicals. They aren&#8217;t crucial to understand, but if you&#8217;re curious you could try reading about <a href=\"https:\/\/en.wikipedia.org\/wiki\/Electron_paramagnetic_resonance\">EPR<\/a>, <a href=\"http:\/\/en.wikipedia.org\/wiki\/CIDNP\">CIDNP<\/a>, or <a href=\"http:\/\/en.wikipedia.org\/wiki\/Moses_Gomberg\">Moses Gomberg<\/a>, the father of free radical chemistry.<\/p>\n<p>In the next few posts we&#8217;ll go through the following topics:<\/p>\n<ul>\n<li><span class=\"Apple-style-span\" style=\"line-height: 13px;\">What factors stabilize (or destabilize) free radicals ?<\/span><\/li>\n<li>How do we &#8220;know&#8221; when a free radical reaction might be occurring?<\/li>\n<li>What is the mechanism of free radical substitution reactions (such as that above?)<\/li>\n<li>What useful reactions can we perform using free-radical chemistry?<\/li>\n<\/ul>\n<p>See you next post!<br \/>\n<strong>Next Post: <a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/08\/02\/3-factors-that-stabilize-free-radicals\/\">3 Factors Which Stabilize Free Radicals<\/a><\/strong><\/p>\n<hr \/>\n<h2><a id=\"notes\"><\/a>Notes<\/h2>\n<div class=\"related-articles\"><p><strong>Related Articles<\/strong><\/p><ul><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/08\/02\/3-factors-that-stabilize-free-radicals\/\" class=\"\"><span>3 Factors That Stabilize Free Radicals<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/08\/14\/bond-strengths-radical-stability\/\" class=\"\"><span>Bond Strengths And Radical Stability<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/08\/30\/radical-initiation-why-is-light-or-heat-required\/\" class=\"\"><span>Free Radical Initiation: Why Is \u201cLight\u201d Or \u201cHeat\u201d Required?<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/09\/06\/initiation-propagation-termination\/\" class=\"\"><span>Initiation, Propagation, Termination<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/09\/17\/monochlorination-isomers-from-free-radical-reactions\/\" class=\"\"><span>Monochlorination Products Of Propane, Pentane, And Other Alkanes<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/10\/31\/selectivity-in-free-radical-reactions-bromine-vs-chlorine\/\" class=\"\"><span>Selectivity in Free Radical Reactions: Bromination vs. Chlorination<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/organic-chemistry-practice-problems\/radicals-practice-quizzes\/\" class=\"\"><span>Free Radicals Practice Quizzes (MOC Membership required)<\/span><\/a><\/li><\/ul><\/div>\n<p><strong><a id=\"noteone\"><\/a>Note 1<\/strong>. The activation energy for the removal of H from H-CH<sub>3<\/sub> (methane) by chlorine radical is about 3.8 kcal\/mol.<\/p>\n<hr \/>\n<h2><a id=\"quizzes\"><\/a>Quiz Yourself!<\/h2>\n\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/3496-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <\/p>\n\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/3497-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <\/p>\n\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/3495-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <\/p>\n\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/3499-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <\/p>\n\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/3498-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <\/p>\n<hr \/>\n<h2><a id=\"references\"><\/a>References<\/h2>\n<p>For a great historical context on the development of free radical reactions, see Ingold, &#8220;Structure and Mechanism in Organic Chemistry&#8221;,\u00a0<strong>1969<\/strong>, p. 257 and also p. 586.<\/p>\n<ul>\n<li>Nernst (Z. Elekrtochem.\u00a0<strong>1918<\/strong>,\u00a0<em>24<\/em>, 335) proposed the first specific chain mechanism.<\/li>\n<\/ul>\n<ol>\n<li><strong>The Study of Chlorine Atom Reactions in the Gas Phase<\/strong><br \/>\nH. O. Pritchard, J. B. Pyke, and A. F. Trotman-Dickenson<br \/>\n<em>Journal of the American Chemical Society<\/em> <strong>1955<\/strong> 77 (9), 2629-2633<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/pubs.acs.org\/doi\/10.1021\/ja01614a088\">10.1021\/ja01614a088<\/a><br \/>\nThe activation energy for the removal of H from H-CH<sub>3<\/sub> by chlorine radical is about 3.8 kcal\/mol.<\/li>\n<\/ol>\n","protected":false},"excerpt":{"rendered":"<p>Free Radical Reactions &#8211; Chlorination of Methane Alkanes are pretty boring, chemically speaking.\u00a0 They don&#8217;t tend to undergo many reactions. However, when they are treated <\/p>\n","protected":false},"author":1,"featured_media":38573,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1411],"tags":[310,381,463,933,279,934],"post_folder":[],"class_list":["post-7428","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-free-radical-reactions","tag-chlorination","tag-free-radicals","tag-initiation","tag-propagation","tag-substitution","tag-termination"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.7 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Free Radical Reactions &#8211; Master Organic Chemistry<\/title>\n<meta name=\"description\" content=\"The reaction of alkanes with Cl2 and hv (light) results in a new C-Cl bond. But it&#039;s not an acid-base reaction! 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