{"id":5522,"date":"2012-08-22T08:49:33","date_gmt":"2012-08-22T08:49:33","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=5522"},"modified":"2026-04-18T06:17:12","modified_gmt":"2026-04-18T11:17:12","slug":"rearrangement-reactions-2-alkyl-shifts","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2012\/08\/22\/rearrangement-reactions-2-alkyl-shifts\/","title":{"rendered":"Carbocation Rearrangement Reactions (2) &#8211; Alkyl Shifts"},"content":{"rendered":"<p><strong>Alkyl Shifts In Carbocation Rearrangement Reactions, Including Ring Expansion<\/strong><\/p>\n<ul>\n<li>Hydride shifts can sometimes occur when a more stable carbocation can be formed through migration of a C-H bond.<\/li>\n<li>If no hydride shift is possible that will result in a more stable carbocation, then it is possible that an\u00a0<strong>alkyl\u00a0<\/strong>shift can occur, where a C-C bond breaks and a C-C bond breaks. The net effect is the shifting over of the carbocation to the adjacent carbon.<\/li>\n<li>Alkyl shifts are most likely to occur when a\u00a0<strong>quaternary\u00a0<\/strong>carbon is adjacent to a\u00a0<strong>secondary\u00a0<\/strong>(or primary)\u00a0<strong>carbocation<\/strong><\/li>\n<li>Alkyl shifts adjacent to strained rings (e.g. cyclobutane) can result in\u00a0<strong>ring expansion<\/strong><\/li>\n<\/ul>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-38629\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/12\/0-Summary-Rearrangement-Reactions-Alkyl-Shifts-ring-expansions-more-likely-with-cyclobutane-to-cyclopentane-relief-of-ring-strain.gif\" alt=\"Summary Rearrangement Reactions Alkyl Shifts ring expansions more likely with cyclobutane to cyclopentane relief of ring strain\" width=\"640\" height=\"654\" \/><\/a><\/p>\n<p><strong>Table of Contents<\/strong><\/p>\n<ol>\n<li><a href=\"#one\">Alkyl Shifts Can Lead To More Stable Carbocations, Too<\/a><\/li>\n<li><a href=\"#two\">The Mechanism Of Alkyl Shift Reactions<\/a><\/li>\n<li><a href=\"#three\">Example of An S<span class=\"s2\"><sub>N<\/sub><\/span><span class=\"s1\"><span class=\"s1\">1 With Alkyl Shift<\/span><\/span><\/a><\/li>\n<li><a href=\"#four\">Ring-Expansion Reactions Also Involve Alkyl Shifts<\/a><\/li>\n<li><a href=\"#notes\">Notes<\/a><\/li>\n<li><a href=\"#quizzes\">Quiz Yourself!\u00a0<\/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. Alkyl Shifts Can Lead To More Stable Carbocations, Too<\/h2>\n<p>In the previous post we saw how certain carbocations can sometimes rearrange (through hydride shifts) to give more stable carbocations (<span style=\"color: #993366;\"><em>See post: <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2012\/08\/15\/rearrangement-reactions-1-hydride-shifts\/\">Hydride Shifts<\/a><\/em><\/span>)<\/p>\n<p>However, sometimes there are situations where a hydride shift would not lead to a more stable carbocation, but an <strong>alkyl shift<\/strong> would!<\/p>\n<p>Take a look at this carbocation. If a hydride shift occurred\u00a0here, we&#8217;d be going to a less stable (primary) carbocation! Not going to happen.<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-14907\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-secondary-carbocation-rearrangement-to-tertiary-carbocation-via-alkyl-shift.gif\" alt=\"secondary carbocation rearrangement to tertiary carbocation via alkyl shift\" width=\"600\" height=\"161\" \/><\/p>\n<p>You might note something with this example, however: it <em>is<\/em> possible for a more stable tertiary carbocation to be formed.<\/p>\n<p>How? If an <em>alkyl\u00a0<\/em> group migrates instead! (Look at that red methyl group! )<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-14908\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/4-stepwise-mechanism-secondary-carbocation-to-tertiary-carbocation-substitution-with-alkyl-shift.gif\" alt=\"stepwise mechanism secondary carbocation to tertiary carbocation substitution with alkyl shift\" width=\"600\" height=\"174\" \/><\/p>\n<p>The most common situation where alkyl shifts can occur is when a <strong>quaternary carbon<\/strong> (that&#8217;s a carbon attached to 4 carbons) is adjacent to a <strong>secondary carbocation<\/strong>. (<em>See post: <a href=\"https:\/\/www.masterorganicchemistry.com\/2010\/06\/16\/1-2-3-4\/\">Primary, Secondary, Tertiary, Quaternary<\/a><\/em>)<\/p>\n<h2><a id=\"two\"><\/a>2. The Mechanism Of Alkyl Shift Reactions In Carbocation Rearrangements<\/h2>\n<p>How does this work? First, the pair of electrons from the C-C bond must align with the empty p orbital on the carbocation (<span style=\"color: #993366;\"><em>side note: this means they have to be aligned in the same plane in order for orbital overlap to occur<\/em><\/span>).<\/p>\n<p>Then, as the pair of electrons from the C\u2013C bond is donated into the empty p-orbital, one C\u2013C bond begins to <strong>break<\/strong> and the new C\u2013C bond begins to <strong>form<\/strong>.<\/p>\n<p>In the <strong>transition state<\/strong>, there are <strong>partial<\/strong> bonds between the carbon being transferred and each of the two adjacent carbon atoms. Then, as one bond shortens and the other lengthens, we end up with a (more stable) tertiary carbocation. (<em>See post: <a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/03\/11\/3-factors-that-stabilize-carbocations\/\">3 Factors That Stabilize Carbocations<\/a><\/em>)<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-14909\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/5-substitution-reaction-with-alkyl-shift-ring-expansion-mechanism.gif\" alt=\"substitution reaction with alkyl shift ring expansion mechanism\" width=\"600\" height=\"198\" \/><a href=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2012\/08\/trans-state1.png\"><br \/>\n<\/a><a href=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2012\/08\/trans-state.png\"><br \/>\n<\/a><\/p>\n<p><strong>Rearrangements can potentially occur any time a carbocation is formed<\/strong>. That includes S<sub>N<\/sub>1 reactions <span style=\"color: #993366;\"><em>(and as we&#8217;ll later see, elimination and addition reactions).<\/em><\/span><\/p>\n<h2><strong><a id=\"three\"><\/a>3. Example of An S<sub>N<\/sub>1 With Alkyl Shift<\/strong><\/h2>\n<div>Here&#8217;s an example of an <strong>S<sub>N<\/sub>1<\/strong> with an <strong>alkyl<\/strong> shift (note that the CH<sub>3<\/sub> groups here are just shown as lines).<\/div>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-14905\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-example-of-secondary-carbocation-rearrangment-to-tertiary-carbocation-via-hydride-shift.gif\" alt=\"example of secondary carbocation rearrangment to tertiary carbocation via hydride shift\" width=\"600\" height=\"467\" \/><\/p>\n<h2><strong><a id=\"four\"><\/a>4. Ring-Expansion Reactions Also Involve Migration of Carbon<\/strong><\/h2>\n<p>It doesn&#8217;t always have to be a methyl group that moves!\u00a0 One interesting example is when a carbocation is formed adjacent to a strained ring, such as a <strong>cyclobutane<\/strong>. (<em>See: <a href=\"https:\/\/www.masterorganicchemistry.com\/2014\/04\/03\/cycloalkanes-ring-strain-in-cyclopropane-and-cyclobutane\/\">Ring Strain in Cyclopropane and Cyclobutane<\/a><\/em>)<\/p>\n<p>Even though the CH<sub>3<\/sub> <em>could<\/em> potentially migrate in this case, it&#8217;s more favorable to shift one of the alkyl groups in the ring, which leads to <strong>ring expansion<\/strong> and the formation of a less strained, five-membered ring.<\/p>\n<p>Here&#8217;s an example of an S<sub>N<\/sub>1 where migration of a carbon-carbon bond leads to ring expansion.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-14906\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-rearrangement-of-secondary-carbocation-to-tertiary-carbocation-through-alkyl-shift.gif\" alt=\"rearrangement of secondary carbocation to tertiary carbocation through alkyl shift\" width=\"600\" height=\"390\" \/><\/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\/2015\/04\/21\/carbocations-and-the-sn1-e1-and-alkene-addition-reactions\/\" class=\"\"><span>The SN1, E1, and Alkene Addition Reactions All Pass Through A Carbocation Intermediate<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/08\/15\/rearrangement-reactions-1-hydride-shifts\/\" class=\"\"><span>Rearrangement Reactions (1) \u2013 Hydride Shifts<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/02\/26\/rearrangements-in-alkene-addition-reactions\/\" class=\"\"><span>Rearrangements in Alkene Addition Reactions<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/11\/09\/e1-reaction-rearrangement\/\" class=\"\"><span>Elimination (E1) Reactions With Rearrangements<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/03\/11\/3-factors-that-stabilize-carbocations\/\" class=\"\"><span>3 Factors That Stabilize Carbocations<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/reaction-guide\/pinacol-rearrangement\/\" class=\"\"><span>Pinacol Rearrangement<\/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\/1792-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\/2502-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\/1793-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\/0175-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\/0179-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\/0180-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>(Advanced) References and Further Reading<\/h2>\n<p>[references]&#8217;,&#8217;Carbocation Rearrangement Reactions (2) &#8211; Alkyl Shifts<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Alkyl Shifts In Carbocation Rearrangement Reactions, Including Ring Expansion Hydride shifts can sometimes occur when a more stable carbocation can be formed through migration of <\/p>\n","protected":false},"author":1,"featured_media":38629,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1416],"tags":[364,862,864,397,861,866,293,865,502,279],"post_folder":[],"class_list":["post-5522","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-rearrangements","tag-364","tag-2-shift","tag-alkyl-shift","tag-carbocations","tag-hydride-shift","tag-quaternary-carbons","tag-rearrangements","tag-ring-expansion","tag-sn1","tag-substitution"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.7 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Rearrangements: Alkyl Shifts and Ring-Expansion Reactions<\/title>\n<meta name=\"description\" content=\"Sometimes carbocation rearrangements involve alkyl shifts. 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