{"id":7771,"date":"2013-12-02T15:03:17","date_gmt":"2013-12-02T20:03:17","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=7771"},"modified":"2026-04-20T05:40:30","modified_gmt":"2026-04-20T10:40:30","slug":"bonus-topic-allylic-rearrangements","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2013\/12\/02\/bonus-topic-allylic-rearrangements\/","title":{"rendered":"Bonus Topic: Allylic Rearrangements"},"content":{"rendered":"<p><strong>Allylic Rearrangements &#8211; Allylic Bromination With Rearrangement<\/strong><\/p>\n<ul>\n<li>In allylic bromination reactions, the C-H bond of an allylic carbon breaks, and a new C-Br bond is formed.<\/li>\n<li>This reaction sometimes occurs with allylic rearrangement. That is, the new C-Br bond forms on the end carbon of the pi-bond, and the pi bond moves over such that the original sp<sup>3<\/sup>-hybridized C-H carbon becomes an alkenyl carbon.<\/li>\n<li>This can be favored in cases where a more substituted double bond results (i.e. Zaitsev&#8217;s rule)<\/li>\n<\/ul>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-38591\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/11\/0-summary-allylic-bromination-with-allylic-rearrangement-using-nbs-and-formation-of-rearranged-product-more-substituted-alkene.gif\" alt=\"summary-allylic bromination with allylic rearrangement using nbs and formation of rearranged product more substituted alkene\" width=\"640\" height=\"561\" \/><\/a><\/p>\n<ul>\n<li>Note &#8211; in the real world, given how sensitive allylic bromides can be to rearrangement all on their own, it&#8217;s important to distinguish whether or not the allylic rearrangement occurred during the free-radical reaction itself or during the workup procedure. See <a href=\"#refsix\">this reference<\/a> for a great example.<\/li>\n<\/ul>\n<p><strong>Table of Contents<\/strong><\/p>\n<ol>\n<li><a href=\"#one\">An Allylic\u00a0 Bromination\u00a0 That Gives\u00a0 Two Products<\/a><\/li>\n<li><a href=\"#two\">&#8220;Product B&#8221; Is An Example Of Allylic Bromination Accompanied By Allylic Rearrangement<\/a><\/li>\n<li><a href=\"#three\">Why Might &#8220;Product B&#8221; Be Favored Here? Remember\u00a0 Zaitsev&#8217;s Rule!\u00a0\u00a0<\/a><\/li>\n<li><a href=\"#notes\">Notes<\/a><\/li>\n<li><a href=\"#quizzes\">Quiz Yourself!<\/a><\/li>\n<li><a href=\"#references\">(Advanced) References and Further Reading<\/a><\/li>\n<\/ol>\n<hr \/>\n<h2><a id=\"one\"><\/a>1. An Allylic Bromination That Gives Two Products<\/h2>\n<p>Today&#8217;s topic flows right from the subject of the last post, on <a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/11\/25\/allylic-bromination\/\">allylic bromination<\/a>. It&#8217;s on allylic bromination reactions\u00a0 that happen with a rearrangements.<\/p>\n<p>In the last post on allylic bromination, the examples used were actually quite simple.<\/p>\n<p>For example, if you take cyclopentene and treat with NBS and light (hv) in carbon tetrachloride solvent (CCl<sub>4<\/sub>), you get this product:<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-45767\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2026\/04\/1-allylic-bromination-of-cyclopentene-with-nbs.gif\" alt=\"allylic bromination of cyclopentene with nbs\" width=\"640\" height=\"301\" \/><\/a><\/p>\n<p>There&#8217;s no other\u00a0 possibility.<\/p>\n<p>Now let&#8217;s extend the complexity of the substrate a little bit. Just replace one of the C\u2013H bonds with a C\u2013CH<sub>3<\/sub> bond, and do the exact same reaction.<\/p>\n<p>Here, we get not one product, but <strong>two<\/strong>. And note how our major product is different!<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-45744\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2026\/04\/2-allylic-bromination-of-3-methylcyclopentene-with-nbs-allylic-rearrangement-.gif\" alt=\"allylic bromination of 3-methylcyclopentene with nbs allylic rearrangement\" width=\"640\" height=\"428\" \/><\/a><\/p>\n<p>What&#8217;s going on here?<\/p>\n<h2><a id=\"two\"><\/a>2. &#8220;Product B&#8221; Is An Example Of Allylic Bromination Accompanied By Allylic Rearrangement<\/h2>\n<p>Let&#8217;s think about the mechanism of this reaction. What&#8217;s the first thing to happen (after initiation, of course)?<\/p>\n<p>Removal of the weakest C-H bond by the bromine radical!\u00a0This leaves us with an allylic radical, which can then react with Br<sub>2<\/sub> to give us product A.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-45751\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2026\/04\/3-typical-reaction-product-from-allylic-bromination-of-3-methylcyclopentene.gif\" alt=\"typical reaction product from allylic bromination of 3 methylcyclopentene\" width=\"640\" height=\"263\" \/><\/a><\/p>\n<p>Simple enough. But how do we explain the formation of product B?<\/p>\n<p>Look again at the free radical that is produced. Notice anything special about it?<\/p>\n<p>We can draw a resonance form!<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-45755\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2026\/04\/4-resonance-forms-of-allylic-radical-from-3-methylcyclopentene.gif\" alt=\"resonance forms of allylic radical from 3 methylcyclopentene\" width=\"640\" height=\"343\" \/><\/a><\/p>\n<p>This means that there are\u00a0<strong>two<\/strong> carbons on this molecule which can potentially participate in free radical reactions. Therefore we can also draw a reaction mechanism which shows Br<sub>2<\/sub> reacting at this bottom (tertiary) carbon:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-45756\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2026\/04\/5-allylic-bromination-product-with-allylic-rearrangement-of-3-methylcyclopentene.gif\" alt=\"allylic bromination product with allylic rearrangement of 3 methylcyclopentene\" width=\"640\" height=\"268\" \/><\/a><\/p>\n<p>See how the tertiary radical forms a new\u00a0\u03c0 bond while the other\u00a0\u03c0 bond breaks? This leaves one of the electrons of the &#8220;top&#8221; alkene to form a new bond with bromine, giving us a new C-Br bond.<\/p>\n<p>If you analyze the bonds that form and break in this reaction (always a useful exercise), note that this reaction has an extra pair of events &#8211; one C-C\u00a0\u03c0 bond breaks, and one C-C\u00a0\u00a0\u03c0 bond forms.<\/p>\n<p>The net effect is that it looks like the\u00a0\u00a0\u03c0 bond has moved. This phenomenon is called &#8220;<strong>allylic rearrangement<\/strong>&#8220;.<\/p>\n<p><span style=\"color: #993366;\"><em>Note: as commenter Keith helpfully points out, remember that resonance forms are &#8220;hybrids&#8221;. When drawing the mechanism, it&#8217;s best to show it all happening in one step (as in the middle drawing, above) rather than to draw the resonance form and\u00a0then\u00a0draw bromination.<\/em><\/span><\/p>\n<h2><a id=\"three\"><\/a>3. Why Might &#8220;Product B&#8221; Be Favored Here? Remember\u00a0 Zaitsev&#8217;s Rule!<\/h2>\n<p>Last question. Can you think of a reason why product B might be more favoured, especially under conditions of high temperature?<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-45762\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2026\/04\/6-with-allylic-bromination-and-allylic-rearrangement-the-more-substituted-double-bond-is-more-favored.gif\" alt=\"with allylic bromination and allylic rearrangement the more substituted double bond is more favored\" width=\"640\" height=\"348\" \/><\/a><\/p>\n<p>Think back to <a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/08\/31\/elimination-reactions-2-zaitsevs-rule\/\">Zaitsev&#8217;s rule<\/a> (if you&#8217;ve covered this) : <strong>the more substituted an alkene is, the more stable it is.<em>\u00a0<\/em><\/strong><em>(<span style=\"color: #800080;\">See article &#8211; <a style=\"color: #800080;\" href=\"https:\/\/www.masterorganicchemistry.com\/2020\/04\/30\/alkene-stability\/\">Stability of Alkenes<\/a><\/span>)\u00a0<\/em><\/p>\n<p><span style=\"color: #993366;\"><em>(why? the reason is complex and usually not covered in introductory textbooks &#8211; it has to do with a phenomenon called &#8220;hyperconjugation&#8221;) .<\/em><\/span><\/p>\n<p>The alkene in product A is what we&#8217;d call &#8220;di-substituted&#8221; &#8211; it is directly attached to two carbon atoms and two hydrogen atoms. The alkene in product B is &#8220;trisubstituted&#8221; &#8211; it is directly attached to three carbon atoms and one hydrogen atom. Therefore there is good reason to expect that product B will be a significant product in this case.<\/p>\n<p><span style=\"color: #993366;\"><em>[I&#8217;m hedging on the exact ratio because I don&#8217;t have a literature reference. You shouldn&#8217;t completely believe me without firm data from a literature reference; I&#8217;ll try to dig one up].<\/em><\/span><\/p>\n<p><strong>Next Post: <a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/04\/12\/a-fourth-alkene-addition-pattern-free-radical-addition\/\">Free Radical Addition Of HBr To Alkenes<\/a><\/strong><\/p>\n<hr \/>\n<h2><strong><a id=\"notes\"><\/a>Notes<\/strong><\/h2>\n<div class=\"related-articles\"><p><strong>Related Articles<\/strong><\/p><ul><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/11\/25\/allylic-bromination\/\" class=\"\"><span>Allylic Bromination<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/glossary\/sn1\/\" class=\"\"><span>Allylic bromination of alkanes using NBS (Reaction Guide)<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/08\/31\/elimination-reactions-2-zaitsevs-rule\/\" class=\"\"><span>Elimination Reactions (2): The Zaitsev Rule<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2018\/06\/13\/reactions-on-the-benzylic-carbon-bromination-and-oxidation\/\" class=\"\"><span>Reactions on the \u201cBenzylic\u201d Carbon: Bromination And Oxidation<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/reaction-guide\/free-radical-addition-of-hbr-to-alkenes\/\" class=\"\"><span>Free Radical Addition of HBr To Alkenes<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2020\/04\/30\/alkene-stability\/\" class=\"\"><span>Alkene Stability<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/08\/31\/elimination-reactions-2-zaitsevs-rule\/\" class=\"\"><span>Elimination Reactions (2): The Zaitsev Rule<\/span><\/a><\/li><\/ul><\/div>\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\/3644-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<p>&nbsp;<\/p>\n<hr \/>\n<h2><a id=\"references\"><\/a>(Advanced) References and Further Reading<\/h2>\n<ol>\n<li><strong>Brominations with N-Bromosuccinimide and Related Compounds. The Wohl-Ziegler Reaction.<br \/>\n<\/strong>Carl Djerassi<strong><br \/>\n<\/strong><em>Chemical Reviews<\/em><strong> 1948, <\/strong><em>43<\/em> (2), 271-317<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/10.1021\/cr60135a004\">1021\/cr60135a004<\/a><strong><br \/>\n<\/strong>An old review by noted chemist Carl Djerassi, whose major contribution to global health was the development of norethindrone \u2013 the first female contraceptive. This review contains several examples of allylic systems that rearrange under free-radical conditions (bromination with NBS).<\/li>\n<li><strong>Substitution And Rearrangement Reactions Of Allylic Compounds<br \/>\n<\/strong>R. H. DeWolfe and W. G. Young<strong><br \/>\n<\/strong><em>Chemical Reviews<\/em><strong> 1956<\/strong>, <em>56<\/em> (4), 753-901<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/abs\/10.1021\/cr50010a002\">10.1021\/cr50010a002<\/a><br \/>\nThis early review mentions free-radical allylic rearrangements briefly \u2013 see pg. 761. Prof. W. G. Young helped build up the UCLA department of chemistry to what it is today, and there is a building named after him on the UCLA campus.<\/li>\n<li><strong>Cyclic Polyolefins. II. Synthesis of Cyclo\u00f6ctatetraene from Chloroprene<br \/>\n<\/strong>Arthur C. Cope and William J. Bailey<strong><br \/>\n<\/strong><em>Journal of the American Chemical Society<\/em><strong> 1948, <\/strong><em>70<\/em> (7), 2305-2309<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/abs\/10.1021\/ja01187a002\">1021\/ja01187a002<\/a><br \/>\nAn early paper by Prof. Arthur C. Cope (of Cope rearrangement fame, and now the eponymous ACS Cope Scholar Award) on the synthesis of cyclooctatetraene. One of the steps involves bromination of 1,5-cyclooctadiene with NBS, which gives a monobromo compound. Cope mentions that they did not ascertain the exact location of the Br in the product and that 2 possible compounds can be formed (IIIa and IIIb), due to rearrangement.<\/li>\n<li><strong>THE PEROXIDE EFFECT IN THE ADDITION OF REAGENTS TO UNSATURATED COMPOUNDS. XIII. THE ADDITION OF HYDROGEN BROMIDE TO BUTADIENE<br \/>\n<\/strong>M. S. KHARASCH, ELLY T. MARGOLIS, and FRANK R. MAYO<br \/>\n<em>The Journal of Organic Chemistry<\/em><strong> 1936, <\/strong><em>01<\/em> (4), 393-404<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/abs\/10.1021\/jo01233a008\">10.1021\/jo01233a008<\/a><strong><br \/>\n<\/strong>Another early paper by M. S. Kharasch in which he studies the addition of HBr to butadiene, in which he attempts to rigorously separate the two modes of addition \u2013 electrophilic vs. radical. There is an example of an \u2018allylic rearrangement\u2019 here \u2013 Prof. Kharasch shows that reaction of 3-bromo-1-butene with HBr under free-radical conditions (peroxides) can give 1,3-dibromobutane. This would be obtained via the intermediate allyl radical, which can add a bromine radical at either the 1- or 3- position.<\/li>\n<li><strong>Cyclobutane Derivatives. IV. Ziegler Bromination of Methylenecyclobutane<br \/>\n<\/strong>Edwin R. Buchman and David R. Howton<strong><br \/>\n<\/strong><em>Journal of the American Chemical Society<\/em><strong> 1948, <\/strong><em>70<\/em> (7), 2517-2520<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/abs\/10.1021\/ja01187a063\">1021\/ja01187a063<\/a><br \/>\nZiegler reaction of methylenecyclobutane gives a mixture of &#8220;normal&#8221; and rearranged allylic bromination products (normal dominates).<\/li>\n<li><strong><a id=\"refsix\"><\/a>CYCLOHEXENE-3,3,6,6-d4 A USEFUL COMPOUND FOR THE STUDY OF MECHANISM AND STRUCTURE<br \/>\n<\/strong>Saul Wolfe and P. G. C. Campbell<strong><br \/>\n<\/strong><em>Can. J. Chem.<\/em><strong> 1965<\/strong>, <em>43<\/em> (5), 1184-1198<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/cdnsciencepub.com\/doi\/abs\/10.1139\/v65-156\">10.1139\/v65-156<\/a><br \/>\nAllylic bromides can easily undergo rearrangement \u2013 while the initial \u201cnormal\u201d product might be formed in the reaction mixture, workup can often lead to scrambling. This study found that allylic bromination occurred &#8220;normally&#8221; but upon workup, rearrangement occurred to give the isomeric allyl bromide.<\/li>\n<li><strong>The Reaction of N-Bromosuccinimide with 2-Heptene<\/strong><br \/>\nFred L. Greenwood and Morton D. Kellert<br \/>\n<em>Journal of the American Chemical Society<\/em> <strong>1953<\/strong> 75 (19), 4842-4843<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/pubs.acs.org\/doi\/10.1021\/ja01115a514\">10.1021\/ja01115a514<\/a><br \/>\nThis study set out to test if allylic bromination would result in bromination of the methyl or the methylene position of 2-heptene. The result was a 58-64% yield of 4-bromo-2-heptene as the major product (See\u00a0<a href=\"https:\/\/www.orgsyn.org\/demo.aspx?prep=CV4P0108#Ref108N3\"><em>Org. Syn.\u00a0<\/em><strong>1958<\/strong>, <em>38<\/em>, 8)<\/a> with no observed bromination of the methyl position. It should be noted that this study predates NMR &#8211; a more modern study would probably show bromination of the methyl group as a minor product.<br \/>\nIndeed, a recent (2024) investigation at <a href=\"https:\/\/openriver.winona.edu\/rca\/2024\/schedule\/18\/\">Winona State University in Minnesota by Devann J. Harris and Bryanna L. Wichner<\/a> found that bromination of <em>trans<\/em>-2-hexene gave 4-bromo-2-hexene (50% by GC-MS) followed by the rearrangement product 2-bromo-3-hexene (32%) and several minor products.<\/li>\n<\/ol>\n","protected":false},"excerpt":{"rendered":"<p>Allylic Rearrangements &#8211; Allylic Bromination With Rearrangement In allylic bromination reactions, the C-H bond of an allylic carbon breaks, and a new C-Br bond is <\/p>\n","protected":false},"author":1,"featured_media":38591,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1411],"tags":[170,951,796,241],"post_folder":[],"class_list":["post-7771","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-free-radical-reactions","tag-allylic-bromination","tag-allylic-rearrangement","tag-bromination","tag-nbs"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.7 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Allylic Bromination With Allylic Rearrangement &#8211; Master Organic Chemistry<\/title>\n<meta name=\"description\" content=\"Sometimes when conducting an allylic bromination reaction the double bond appears to &quot;move&quot; during the course of bromination! 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