{"id":7392,"date":"2013-06-11T08:15:47","date_gmt":"2013-06-11T12:15:47","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=7392"},"modified":"2025-09-24T11:56:22","modified_gmt":"2025-09-24T16:56:22","slug":"alkynes-via-elimination-reactions","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2013\/06\/11\/alkynes-via-elimination-reactions\/","title":{"rendered":"Alkenes To Alkynes Via Halogenation And Elimination Reactions"},"content":{"rendered":"<p><strong>Formation of Alkynes Via Double Elimination Of Halides<\/strong><\/p>\n<ul>\n<li>Alkynes can be produced from vicinal or geminal dihalides through double elimination reactions.<\/li>\n<li>The usual choice of base for these reactions is sodium amide (NaNH<sub>2<\/sub>)<\/li>\n<li>The first equivalent of strong base forms an alkenyl halide. The second equivalent forms the alkyne.<\/li>\n<li>If a terminal alkyne is formed, a third equivalent of base will be consumed, since the resulting alkyne proton is relatively acidic (pK<sub>a<\/sub> 25)<\/li>\n<li>Overall, this process can be used for the synthesis of alkynes from alkenes, through 1) halogenation of alkene 2) double elimination of dihalide<\/li>\n<\/ul>\n<p><strong>Table of Contents<\/strong><\/p>\n<ol>\n<li class=\"p1\"><a href=\"#one\"><span class=\"s1\">Elimination Reactions: Form C\u2013C (pi), Break C\u2013H And C\u2013LG<\/span><\/a><\/li>\n<li class=\"p1\"><a href=\"#two\">Elimination Of A Vinyl Halide To Give An Alkyne<\/a><\/li>\n<li class=\"p1\"><a href=\"#three\">Formation of Alkynes From Double Elimination Of Vicinal Dihalides<\/a><\/li>\n<li class=\"p1\"><a href=\"#four\">Formation Of Alkynes From Double Elimination Of Geminal Dihalides<\/a><\/li>\n<li class=\"p1\"><a href=\"#five\">Why Is This Useful? Going From An Alkene To An Alkyne<\/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. Alkenes Via Elimination Reactions Of Alkyl Halides: Form C\u2013C (pi), Break C\u2013H And C\u2013LG<\/h2>\n<p>We&#8217;ve gone through elimination reactions before &#8211; treatment of alkyl halides with base gives alkenes. Today we&#8217;ll discuss a pathway to get to alkynes from alkyl dibromides (which in turn can be made from alkenes) via double elimination.<\/p>\n<p>Some time ago we discussed elimination reactions (<span style=\"color: #993366;\"><em>See post: <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2012\/09\/12\/two-types-of-elimination-reactions\/\">Elimination reactions<\/a><\/em><\/span>) follow the general pattern below, where two adjacent bonds to carbon are broken &#8211; usually C-H and C-X, where X is a leaving group &#8211; and in place we form a new \u03c0 bond.<\/p>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-41557\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-elimination-of-alkyl-halide-with-base-form-c-c-pi-break-c-x-and-c-h-giving-new-pi-bond.gif\" alt=\"elimination of alkyl halide with base form c c pi break c x and c h giving new pi bond\" width=\"640\" height=\"264\" \/><\/a><\/p>\n<h2><strong><a id=\"two\"><\/a>2. Alkynes From Alkenyl Halides: Elimination Of A Vinyl Halide To Give An Alkyne<\/strong><\/h2>\n<p>If we can form alkenes through elimination reactions of alkyl halides, it&#8217;s natural to ask: can we also use elimination reactions to form alkynes? Perhaps through a reaction like this elimination of an alkenyl (aka vinyl) halide?<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-41558\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-elimination-of-alkenyl-halide-with-base-giving-alkyne-forms-c-c-pi-breaks-c-h-c-br-requires-more-harsh-conditions-alkoxide-amide.gif\" alt=\"elimination of alkenyl halide with base giving alkyne forms c c pi breaks c h c br requires more harsh conditions alkoxide amide\" width=\"640\" height=\"423\" \/><\/a><\/p>\n<p>The answer is yes! Although to be fair, these types of molecules [alkenyl halides]\u00a0 are perhaps not the most familiar to us so far, We&#8217;ve only seen them once, and that was a post about how they can be synthesized from alkynes. (<em>See post: <a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/05\/24\/hydrohalogenation-of-alkynes\/\">Hydrohalogenation\u00a0of Alkynes<\/a><\/em>)<\/p>\n<p>For our purposes, in order to synthesize them it&#8217;s common practice to start with an alkyl <i>di<\/i> halide.<\/p>\n<p>In other words, we start with an alkyl di-halide, and then do <b>two<\/b> elimination reactions &#8211; one to form the alkenyl halide, and a second to form the alkyne.<\/p>\n<p>Let&#8217;s have a look.<\/p>\n<h2><a id=\"three\"><\/a>3. Alkynes From Double Elimination Of Vicinal Dihalides<\/h2>\n<p>There are two types of alkyl dihalides we&#8217;ve met so far. <strong>Vicinal <\/strong>dihalides have halogens on adjacent carbons &#8211; &#8220;in the <b>vic<\/b>inity&#8221;, if you will. Treatment of vicinal dihalides with strong base can lead to an elimination reaction [<em>through the <a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/09\/27\/the-e2-mechanism\/\">E2 mechanism<\/a><\/em>] giving an alkenyl halide.<\/p>\n<p>Treatment of this alkenyl halide with a second equivalent of base then gives the alkyne.<\/p>\n<p>In previous posts we saw that a common base used for elimination reactions are alkoxide bases such as sodium ethoxide. But here, we typically go for a more powerful base, sodium amide (<em>See post: <a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/07\/29\/reagent-friday-sodium-amide-nanh2-2\/\">Sodium Amide NaNH<sub>2<\/sub><\/a><\/em>)<\/p>\n<p>Here&#8217;s an illustration of how it works.<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-41559\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-elimination-of-vicinal-dihalides-with-nanh2-gives-alkenyl-halides-further-elimination-gives-alkynes-3-equiv-required-alkyne-acidic.gif\" alt=\"-elimination of vicinal dihalides with nanh2 gives alkenyl halides further elimination gives alkynes 3 equiv required alkyne acidic\" width=\"640\" height=\"545\" \/><\/a><\/p>\n<p>In the first step, NaNH<sub>2<\/sub> is the base in an elimination reaction [E2] to give the alkenyl bromide.\u00a0In the second reaction, likewise a second equivalent of\u00a0 NaNH<sub>2<\/sub> performs a second elimination reaction to form the alkyne.<\/p>\n<p>This is one example &#8211; a rare example, I may add &#8211; of an elimination reaction that works on an <em>sp<\/em><sup>2<\/sup> hybridized carbon. You might recall seeing at some point that reactions of the SN1\/SN2\/E1\/E2 types typically don&#8217;t occur on carbons other than sp<sup>3<\/sup> hybridized systems. <strong>This is a rare exception!<\/strong><\/p>\n<p>Finally, what&#8217;s often not mentioned in this reaction is that the product alkyne, if terminal [i.e. has a C-H on the end] is acidic [pK<sub>a<\/sub> 25] &#8211; any excess NaNH<sub>2<\/sub> will thus remove the alkyne C-H and give the alkynyl anion. So if a terminal alkyne is formed,<strong> three equivalents<\/strong> of NaNH<sub>2<\/sub> will be consumed; the alkyne is protonated upon workup, usually by adding water.<\/p>\n<h2><a id=\"four\"><\/a>4. Alkynes From Double Elimination Of Geminal Dihalides<\/h2>\n<p><strong>Geminal dihalides<\/strong> contain two halogen atoms attached to the same carbon.<\/p>\n<p>Treatment of geminal dihalides with NaNH<sub>2<\/sub> likewise gives alkynes through two successive elimination reactions. <span style=\"color: #993366;\"><em>[We haven&#8217;t really covered any reactions that form geminal dihalides, except for the di-addition of HX to alkynes. So if you happened to start off with an alkyne and made a geminal dihalide with it, this would be a way of getting the alkyne back]<\/em><\/span>.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-41563\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2013\/06\/4-both-vicinal-and-geminal-dihalides-work-well.gif\" alt=\"both vicinal and geminal dihalides work well\" width=\"640\" height=\"460\" \/><\/a><\/p>\n<p>In total, the elimination reactions described above represent a second way of making alkynes, other than through S<sub>N<\/sub>2 of acetylides with alkyl halides (<span style=\"color: #993366;\"><em>See post: <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2013\/05\/01\/the-2-most-important-reactions-of-alkynes\/\">SN2 of Acetylides<\/a><\/em><\/span>)<\/p>\n<p>Let&#8217;s bring it\u00a0 back.\u00a0 What does it matter that we can do this?<\/p>\n<p>OK. Let&#8217;s start putting some reactions together that show why this might be important.<\/p>\n<h2><a id=\"five\"><\/a>5. Alkenes To Alkynes, Via Halogenation And Double Elimination<\/h2>\n<p>Let&#8217;s say we wanted to make an alkyne. But all we have to start with is an alkene. How might we get there?<\/p>\n<ul>\n<li>We&#8217;ve just learned how to make alkynes from vicinal dihalides.<\/li>\n<li>How might we make a vicinal dihalide from an alkene?<\/li>\n<li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/03\/15\/alkene-bromination-mechanism\/\"><em>Bromination!\u00a0<\/em><\/a><\/li>\n<\/ul>\n<p>So we can start off with an alkene\u2026 and brominate\u2026 and then add two equivalents of strong base to give ourselves the alkyne.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-41561\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/5-how-to-convert-alkene-to-alkyne-start-with-halogenation-br2-and-double-elimination-with-nanh2-to-give-alkyne-synthesis-problem.gif\" alt=\"how to convert alkene to alkyne start with halogenation br2 and double elimination with nanh2 to give alkyne synthesis problem\" width=\"640\" height=\"457\" \/><\/a><\/p>\n<p>This is a quick example of multi-step organic synthesis. We will soon see much more of this in the context of alkynes!<\/p>\n<p><b>Next Post: <a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/06\/24\/alkynes-are-a-blank-canvas\/\">Alkynes Are A Blank Canvas<\/a><\/b><\/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\/06\/24\/alkynes-are-a-blank-canvas\/\" class=\"\"><span>Alkynes Are A Blank Canvas<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2014\/01\/29\/synthesis-5-reactions-of-alkynes\/\" class=\"\"><span>Synthesis (5) \u2013 Reactions of Alkynes<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/05\/29\/alkyne-halogenation-bromination-chlorination\/\" class=\"\"><span>Alkyne Halogenation: Bromination, Chlorination, and Iodination of Alkynes<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/07\/29\/reagent-friday-sodium-amide-nanh2-2\/\" class=\"\"><span>Reagent Friday: Sodium Amide (NaNH2)<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/05\/01\/the-2-most-important-reactions-of-alkynes\/\" class=\"\"><span>Acetylides from Alkynes, And Substitution Reactions of Acetylides<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/09\/27\/the-e2-mechanism\/\" class=\"\"><span>The E2 Mechanism<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/organic-chemistry-practice-problems\/alkyne-reactions-practice-problems-with-answers\/\" class=\"\"><span>Alkyne Reactions Practice Problems With Answers (MOC Membership)<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/06\/04\/oxidation-of-alkynes\/\" class=\"\"><span>Oxidation of Alkynes With O3 and KMnO4<\/span><\/a><\/li><\/ul><\/div>\n<p>[Alkoxide bases]<\/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\/1659-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\/1660-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\/1661-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\/1892-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\/2143-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><strong><a id=\"references\"><\/a>(Advanced) References and Further Reading<\/strong><\/h2>\n<ol>\n<li><strong>Eliminations from Olefins<br \/>\n<\/strong> DR. G. KOBRICH.<br \/>\n<em>Angew. Chem. Int. Ed. <\/em><strong>1965<\/strong> <em>4<\/em> (1), 49<br \/>\n<strong>DOI:<\/strong> <a href=\"https:\/\/onlinelibrary.wiley.com\/doi\/abs\/10.1002\/anie.196500491\">10.1002\/anie196500491<\/a><br \/>\nA review describing various types of a- and b-elimination reactions of alkenes to give alkynes.<\/li>\n<li><strong>Elimination Strategy for Aromatic Acetylenes<br \/>\n<\/strong>Orita, H.; Otera, J.<br \/>\n<em>Chem. <\/em><em>Rev.<\/em> <strong>2006<\/strong>, <em>106<\/em>, 5387<br \/>\n<strong>DOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/10.1021\/cr050560m\">10.1021\/cr050560m<\/a><br \/>\nSection 3.3 in this review covers the synthesis of alkynes by double dehydrobromination reactions from <em>vic<\/em>-dibromoalkanes.<\/li>\n<li><strong>Unsaturated eight-membered ring compounds. XI. Synthesis of sym-dibenzo-1,5-cyclooctadiene-3,7-diyne and sym-dibenzo-1,3,5-cyclooctatrien-7-yne, presumably planar conjugated eight-membered ring compounds<br \/>\n<\/strong>Henry N. C. Wong, Peter J. Garratt, and Franz Sondheimer<strong><br \/>\n<\/strong><em>Journal of the American Chemical Society<\/em> <strong>1974<\/strong> <em>96<\/em> (17), 5604-5605<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/abs\/10.1021\/ja00824a066\">10.1021\/ja00824a066<\/a><br \/>\nThe reaction used to synthesize the strained cyclooctyne is the double dehydrobromination reaction, using fairly standard conditions (KOtBu in THF).<\/li>\n<li><strong>Proton NMR study of two tetradehydrocyclodecabiphenylenes<br \/>\n<\/strong>Charles F. Wilcox Jr. and Karl A. Weber<br \/>\n<em>The Journal of Organic Chemistry<\/em> <strong>1986<\/strong> <em>51<\/em> (7), 1088-1094<br \/>\n<strong>DOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/abs\/10.1021\/jo00357a028\">10.1021\/jo00357a028<\/a><br \/>\nThe authors also use a double dehydrobromination reaction to obtain cyclic dialkynes, also using standard conditions (KOtBu in THF). The experimental section has detailed procedures.<\/li>\n<\/ol>\n","protected":false},"excerpt":{"rendered":"<p>Formation of Alkynes Via Double Elimination Of Halides Alkynes can be produced from vicinal or geminal dihalides through double elimination reactions. The usual choice of <\/p>\n","protected":false},"author":1,"featured_media":15092,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1419],"tags":[930,931,473,201,543,352],"post_folder":[],"class_list":["post-7392","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-alkyne-reactions","tag-alkyne","tag-dihalides","tag-e2","tag-elimination","tag-nanh2","tag-synthesis"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.7 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Alkenes To Alkynes Via Halogenation And Elimination Reactions<\/title>\n<meta name=\"description\" content=\"Converting an alkene to an alkyne is an oxidation. Alkenes can be converted to alkynes in 2 steps. 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