{"id":7117,"date":"2013-03-15T17:09:13","date_gmt":"2013-03-15T21:09:13","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=7117"},"modified":"2026-04-30T13:21:01","modified_gmt":"2026-04-30T18:21:01","slug":"alkene-bromination-mechanism","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2013\/03\/15\/alkene-bromination-mechanism\/","title":{"rendered":"Halogenation of Alkenes and Halohydrin Formation"},"content":{"rendered":"<p><strong>Bromination, Chlorination, and Halohydrin Formation from Alkenes<\/strong><\/p>\n<ul>\n<li>Alkenes undergo <strong>halogenation<\/strong> when treated with Cl<sub>2<\/sub>, Br<sub>2<\/sub> and (less commonly) I<sub>2<\/sub> to give <strong>vicinal dihalides<\/strong><\/li>\n<li>These reactions are <strong>stereoselective<\/strong> and give\u00a0<em>anti<\/em>-addition products<\/li>\n<li>The mechanism proceeds through a cyclic\u00a0<strong>halonium\u00a0<\/strong>ion which undergoes backside attack at carbon by a nucleophile to give the <em>anti<\/em>-addition product.<\/li>\n<li>When water or alcohols are used as solvent,\u00a0<strong>halohydrins<\/strong> or\u00a0<strong>haloethers<\/strong> can form. These also give the products of\u00a0<strong>anti<\/strong> addition.<\/li>\n<li>With unsymmetrical halonium ions, the <strong>C-O bond tends to form at the most substituted carbon<\/strong> (&#8220;Markovnikov&#8221; regioselectivity).<\/li>\n<li>Dihalides can undergo elimination to give alkynes; halohydrins can be used to form epoxides.<\/li>\n<li>Overall, these reactions have similar mechanisms to those for\u00a0<strong>oxymercuration<\/strong> and for the opening of\u00a0<strong>epoxides\u00a0<\/strong>under acidic conditions [<span style=\"color: #993366;\"><em>See article: <a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/03\/20\/alkene-addition-pattern-2-the-three-membered-ring-pathway\/\">Alkene Addition Pattern #2 &#8211; The &#8220;Three Membered Ring&#8221; Pathway<\/a><\/em><\/span>]<\/li>\n<\/ul>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-35378\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2013\/03\/0-Summary-of-halogenation-reactions-bromination-chlorination-and-halohydrin-formation.gif\" alt=\"Summary of halogenation reactions bromination chlorination and halohydrin formation\" width=\"640\" height=\"733\" \/><\/a><\/p>\n<p><strong>Table of Contents<\/strong><\/p>\n<ol>\n<li style=\"list-style-type: none;\">\n<ol>\n<li><a href=\"#one\">Bromination and Chlorination of Alkenes With Br2 and Cl2<\/a><\/li>\n<li><a href=\"#two\">Halogenation is Stereoselective for Anti Addition Products<\/a><\/li>\n<li><a href=\"#three\">Halonium Ions<\/a><\/li>\n<li><a href=\"#four\">Halogenation of Alkenes &#8211; The Mechanism<\/a><\/li>\n<li><a href=\"#five\">Halohydrin Formation<\/a><\/li>\n<li><a href=\"#six\">Mechanism of Halohydrin Formation<\/a><\/li>\n<li><a href=\"#seven\">Haloethers<\/a><\/li>\n<li><a href=\"#eight\">Applications of Alkene Halogenation<\/a><\/li>\n<li><a href=\"#notes\">Notes<\/a><\/li>\n<li><a href=\"#quiz\">Quiz Yourself!<\/a><\/li>\n<li><a href=\"#references\">(Advanced) References and Further Reading<\/a><\/li>\n<\/ol>\n<\/li>\n<\/ol>\n<hr \/>\n<h2><a id=\"one\"><\/a>1. Halogenation of Alkenes<\/h2>\n<p>When <strong>alkenes\u00a0<\/strong>(<span style=\"color: #800080;\"><em>also known as olefins<\/em><\/span>) \u00a0are treated with bromine (Br<sub>2<\/sub>) or chlorine (Cl<sub>2<\/sub>) in an inert \u00a0solvent [<a href=\"#noteone\"><span style=\"color: #ff0000;\">Note 1<\/span><\/a>] such as carbon tetrachloride (CCl<sub>4<\/sub>) or dichloromethane (CH<sub>2<\/sub>Cl<sub>2<\/sub>), they are converted into dihalides (<span style=\"color: #993366;\"><em>specifically, &#8216;vicinal&#8217; dihalides since the C-halogen bonds are on adjacent carbons<\/em><\/span>).<\/p>\n<p>This reaction results in the formation of two new C-halogen bonds and breaks the C-C pi bond as well as a halogen-halogen bond. The two carbon-halogen bonds add to opposite faces of the alkene.<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-35379\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2013\/03\/1-scheme-describing-electrophilic-halogenation-of-alkenes-with-br2-or-cl2-anti-selective-.gif\" alt=\"scheme describing electrophilic halogenation of alkenes with br2 or cl2 - anti selective\" width=\"640\" height=\"390\" \/><\/a><\/p>\n<p><em><span style=\"color: #993366;\">It can also work for iodine (I<sub>2<\/sub>) but tends to be reversible, and the di-iodide products tend to be unstable towards light. Fluorine (F<sub>2<\/sub>)\u00a0 is such a ravenous beast that it requires special apparatus to work with, which means we won&#8217;t discuss it further here.\u00a0<\/span> \u00a0[<a href=\"#notetwo\"><span style=\"color: #ff0000;\">Note 2<\/span><\/a>]\u00a0<\/em><\/p>\n<p>This reaction is the basis of a common test used in teaching laboratories, the <strong>bromine water test. <\/strong><\/p>\n<p>Molecular bromine (Br<sub>2<\/sub>) has a characteristic orange color. In the presence of alkenes, the orange color disappears, indicating that a reaction has occurred.<\/p>\n<p><iframe class=\"giphy-embed\" src=\"https:\/\/giphy.com\/embed\/usR9yk0WNNYdXO7Btq\" width=\"480\" height=\"270\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<p><a href=\"https:\/\/giphy.com\/gifs\/chemistry-organicchemistry-bromination-usR9yk0WNNYdXO7Btq\">via GIPHY<\/a>. Original source: <a href=\"https:\/\/www.youtube.com\/watch?v=ZXcS3oY9wQo\">@FranklyChemistry (Youtube)<\/a><\/p>\n<p>Note that Br<sub>2<\/sub> does not ordinarily react with benzene (C<sub>6<\/sub>H<sub>6<\/sub>) or molecules containing an aromatic ring, even though these may appear at first glance to be similar to alkenes. More to say about that in a later chapter. [<span style=\"color: #993366;\"><em>See ar<span style=\"color: #993366;\">ticle: <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2018\/04\/18\/electrophilic-aromatic-substitutions-1-halogenation\/\">Halogenation of Benzene<\/a><\/span><\/em><\/span>]<\/p>\n<h2><a id=\"two\"><\/a>2. Halogenation Is Stereoselective For <em>anti-<\/em> Addition Products<\/h2>\n<p>Halogenation of alkenes is an example of a <strong>stereoselective<\/strong> reaction.<\/p>\n<p>The reaction of Br<sub>2<\/sub>, Cl<sub>2<\/sub> and other halogens with alkenes\u00a0 leads to products of <em>anti<\/em>&#8211; addition. A classic example is the bromination of cyclohexene (below), which gives <em>trans<\/em>-1,2-dibromocyclohexane as a <strong>racemic mixture<\/strong>. \u00a0No\u00a0<em>cis-<\/em>1,2-dibromocyclohexane is formed.<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-35355\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/3-halogenation-of-alkenes-with-br2-or-cl2-is-selective-for-anti-products-dihedral-angle-cyclohexene.gif\" alt=\"halogenation of alkenes with br2 or cl2 is selective for anti products - dihedral angle - cyclohexene\" width=\"640\" height=\"387\" \/><\/a><\/p>\n<p>The terms\u00a0<em>syn<\/em>&#8211; and\u00a0<em>anti<\/em>&#8211; refer to the dihedral angle observed between the two C-halogen bonds when we look directly along the C-C bond. We say this addition is\u00a0<em>anti<\/em>&#8211; because the dihedral angle between the two C-halogen bonds is 180\u00b0. [<span style=\"color: #993366;\"><em>See &#8211; <a href=\"https:\/\/www.masterorganicchemistry.com\/2020\/02\/28\/staggered-vs-eclipsed-conformations-of-ethane\/\">Staggered vs Eclipsed Conformations of Ethane<\/a><\/em><\/span>]<\/p>\n<p><span style=\"color: #800080;\"><em>While it&#8217;s understandable to use the word &#8220;trans&#8221; when referring to this orientation (and most people will know what you mean!)\u00a0 we usually reserve &#8220;trans&#8221; to refer to situations where two groups are on opposite faces of a multiple bond or small ring (i.e. in cis-trans isomerism). The term &#8220;anti&#8221;\u00a0is much more broad since it refers to the\u00a0relative\u00a0orientation between two groups, as defined by dihedral angle (180\u00b0 for anti, 0\u00b0 for syn).<\/em><\/span><\/p>\n<p>When drawing the products of\u00a0<em>anti<\/em>&#8211; addition on an alkene, it&#8217;s common to draw the two new C-halogen bonds in the plane of the page to clearly show the dihedral angle of 180\u00b0.<\/p>\n<p>Pro tip: make sure you keep the <strong>relative orientation<\/strong> of the groups on the alkene <strong>the same<\/strong> when drawing the product:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35375\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/4-some-tips-on-drawing-the-proudcts-of-anti-addition-in-addition-of-halogens-to-alkenes-to-give-dihalides-also-how-not-to-do-it-.gif\" alt=\"some tips on drawing the proudcts of anti addition in addition of halogens to alkenes to give dihalides - also how not to do it\" width=\"640\" height=\"366\" \/><\/a><\/p>\n<p>While the products are often drawn with the C-halogen bonds in the <strong>plane<\/strong>, be alert that this will not always be the case.<\/p>\n<p>After all, once the C-C pi bond has broken, free rotation may be possible about the C-C single bond. That means it&#8217;s possible to draw <strong>conformational isomers\u00a0<\/strong>of an <em>anti<\/em>&#8211; addition product where the two C-halogen bonds are on the same side of the C-C single bond.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35357\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/5-be-alert-that-anti-addition-products-can-undergo-bond-rotation-and-these-products-may-appear-syn.gif\" alt=\"be alert that anti addition products can undergo bond rotation and these products may appear syn\" width=\"640\" height=\"341\" \/><\/a><\/p>\n<p>This does\u00a0<em>not<\/em> suddenly make it a\u00a0<em>syn<\/em>-addition product, however, since <strong>rotating bonds does not change their<\/strong> <strong>configuration<\/strong>. No amount of bond rotation will convert a chiral center with an (<em>S<\/em>) configuration into (<em>R<\/em>) !<\/p>\n<p><span style=\"color: #993366;\"><em>You may have to work backwards sometimes (using bond rotations) from the product to figure out the alkene starting material.\u00a0 This may be a little confusing at first. When in doubt whether you have done a bond rotation correctly, it can be helpful to determine (R)\/(S) to make sure you didn&#8217;t accidentally flip any stereocenters.\u00a0<\/em><\/span><\/p>\n<p>A classic example showing the stereoselectivity of halogenation is\u00a0<em>cis<\/em>&#8211; and\u00a0<em>trans<\/em>-but-2-ene.<\/p>\n<ul>\n<li>When\u00a0<em>cis<\/em>-but-2-ene undergoes bromination, the product is a racemic mixture of (S,S)-2,3-dibromobutane and (R,R)-2,3-dibromobutane.<\/li>\n<li>When <em>trans<\/em>-but-2-ene undergoes bromination, the product is (2S, 3R)-2,3-dibromobutane. This molecule has two chiral centers but is an achiral molecule overall due to the presence of an internal mirror plane. This class of molecules are known as <strong>meso<\/strong> compounds. (<span style=\"color: #800080;\"><em>See article: <a style=\"color: #800080;\" href=\"https:\/\/www.masterorganicchemistry.com\/2011\/01\/12\/the-meso-trap\/\">The Meso Trap<\/a><\/em><\/span>)<\/li>\n<\/ul>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35358\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/6-halogenation-is-stereospecific-halogenation-of-cis-and-trane-2-butene-gives-products-that-are-stereoisomers.gif\" alt=\"halogenation is stereospecific - halogenation of cis and trane 2-butene gives products that are stereoisomers\" width=\"640\" height=\"584\" \/><\/a><\/p>\n<p>One stereoisomer of but-2-ene produces one set of stereoisomer products; the other stereoisomer of but-2-ene produces a product that is stereoisomeric. There is no crossover between these two reactions.<\/p>\n<p>This fits IUPAC&#8217;s definition of a <strong>stereospecific<\/strong> reaction. Several other examples of stereospecific reactions of alkenes include hydroboration, dihydroxylation, epoxidation and more.<\/p>\n<p>Now for some exercises. See if you can draw the products of the following reaction and determine the products:<\/p>\n<div class=\"wq-quiz-wrapper\" data-id=\"45635\"><style type=\"text\/css\" id=\"wq-flip-custom-css\">.wq-quiz-wrapper[data-id=\"45635\"] {\n--wq-question-width: 100%;\n--wq-question-color: #009cff;\n--wq-question-height: auto;\n--wq-font-color: #444;\n}\n\n\t\t\t.wq-quiz-wrapper[data-id=\"45635\"] {\n\t\t\t\t--wq-question-width: 600px;\n\t\t\t}\n\n\t\t\t@media screen and (max-width: 600px) {\n\t\t\t\t.wq-quiz-wrapper[data-id=\"45635\"] .wq_singleQuestionWrapper { width:100% !important; height:auto !important; }\n\t\t\t}\n\t\t<\/style><!-- wp quiz -->\n<div id=\"wp-quiz-45635\" class=\"wq_quizCtr single flip_quiz wq-quiz wq-quiz-45635 wq-quiz-flip wq-layout-single wq-skin-traditional wq-should-show-correct-answer\" data-quiz-id=\"45635\">\n<div class=\"wq-questions wq_questionsCtr\">\n\t<div class=\"wq-question wq_singleQuestionWrapper wq-question-517ud\" data-id=\"517ud\">\n\n\t\n\t<div class=\"item_top\">\n\t\t<div class=\"title_container\">\n\t\t\t<div class=\"wq_questionTextCtr\">\n\t\t\t\t<h4 class=\"wq-question-title\"><\/h4>\n\t\t\t<\/div>\n\t\t<\/div>\n\t<\/div>\n\n\t<div class=\"card \">\n\t\t<div class=\"front\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2549-Front.gif\" \/>\n\t\t\n\t\t\n\t\n\t\n\t\t\t<span class=\"top-desc\">Click to Flip<\/span>\n\t<\/div>\n\t\t<div class=\"back\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2549-Reverse.gif\" \/>\n\t\t\n\t\t\n\t\n\t<\/div>\n\t<\/div>\n\n\t\n<\/div>\n<\/div>\n<\/div>\n<!-- \/\/ wp quiz-->\n<\/div><!-- End .wq-quiz-wrapper -->\n<p>It&#8217;s also important to be able to work backwards! Can you identify the starting material of this halogenation reaction?<\/p>\n<div class=\"wq-quiz-wrapper\" data-id=\"45636\"><style type=\"text\/css\" id=\"wq-flip-custom-css\">.wq-quiz-wrapper[data-id=\"45636\"] {\n--wq-question-width: 100%;\n--wq-question-color: #009cff;\n--wq-question-height: auto;\n--wq-font-color: #444;\n}\n\n\t\t\t.wq-quiz-wrapper[data-id=\"45636\"] {\n\t\t\t\t--wq-question-width: 600px;\n\t\t\t}\n\n\t\t\t@media screen and (max-width: 600px) {\n\t\t\t\t.wq-quiz-wrapper[data-id=\"45636\"] .wq_singleQuestionWrapper { width:100% !important; height:auto !important; }\n\t\t\t}\n\t\t<\/style><!-- wp quiz -->\n<div id=\"wp-quiz-45636\" class=\"wq_quizCtr single flip_quiz wq-quiz wq-quiz-45636 wq-quiz-flip wq-layout-single wq-skin-traditional wq-should-show-correct-answer\" data-quiz-id=\"45636\">\n<div class=\"wq-questions wq_questionsCtr\">\n\t<div class=\"wq-question wq_singleQuestionWrapper wq-question-r548s\" data-id=\"r548s\">\n\n\t\n\t<div class=\"item_top\">\n\t\t<div class=\"title_container\">\n\t\t\t<div class=\"wq_questionTextCtr\">\n\t\t\t\t<h4 class=\"wq-question-title\"><\/h4>\n\t\t\t<\/div>\n\t\t<\/div>\n\t<\/div>\n\n\t<div class=\"card \">\n\t\t<div class=\"front\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2550-Front.gif\" \/>\n\t\t\n\t\t\n\t\n\t\n\t\t\t<span class=\"top-desc\">Click to Flip<\/span>\n\t<\/div>\n\t\t<div class=\"back\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2550-Reverse.gif\" \/>\n\t\t\n\t\t\n\t\n\t<\/div>\n\t<\/div>\n\n\t\n<\/div>\n<\/div>\n<\/div>\n<!-- \/\/ wp quiz-->\n<\/div><!-- End .wq-quiz-wrapper -->\n<h2><a id=\"three\"><\/a>3. Halonium Ions<\/h2>\n<p>OK. So how does halogenation of alkenes actually work?<\/p>\n<p>One initial idea was that they might proceed through a free carbocation intermediate, like the addition of HCl to alkenes.\u00a0 [<span style=\"color: #800080;\"><em>See article &#8211; <a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/02\/08\/markovnikovs-rule-1\/\">Markovnikov Addition of HCl to Alkenes<\/a><\/em><\/span>].<\/p>\n<p>An interesting test of this theory came from halogenation of the alkene below. <em>cis<\/em>-di-<em>t<\/em>-butylethylene has a lot of steric strain (9.3 kcal\/mol) due to those two bulky <em>t-<\/em>butyl groups jostling up against each other. <span style=\"color: #993366;\"><em>(This is technically called, &#8220;A-1,2 strain&#8221;)<\/em><\/span><\/p>\n<div class=\"wq-quiz-wrapper\" data-id=\"45637\"><style type=\"text\/css\" id=\"wq-flip-custom-css\">.wq-quiz-wrapper[data-id=\"45637\"] {\n--wq-question-width: 100%;\n--wq-question-color: #009cff;\n--wq-question-height: auto;\n--wq-font-color: #444;\n}\n\n\t\t\t.wq-quiz-wrapper[data-id=\"45637\"] {\n\t\t\t\t--wq-question-width: 600px;\n\t\t\t}\n\n\t\t\t@media screen and (max-width: 600px) {\n\t\t\t\t.wq-quiz-wrapper[data-id=\"45637\"] .wq_singleQuestionWrapper { width:100% !important; height:auto !important; }\n\t\t\t}\n\t\t<\/style><!-- wp quiz -->\n<div id=\"wp-quiz-45637\" class=\"wq_quizCtr single flip_quiz wq-quiz wq-quiz-45637 wq-quiz-flip wq-layout-single wq-skin-traditional wq-should-show-correct-answer\" data-quiz-id=\"45637\">\n<div class=\"wq-questions wq_questionsCtr\">\n\t<div class=\"wq-question wq_singleQuestionWrapper wq-question-2hw6l\" data-id=\"2hw6l\">\n\n\t\n\t<div class=\"item_top\">\n\t\t<div class=\"title_container\">\n\t\t\t<div class=\"wq_questionTextCtr\">\n\t\t\t\t<h4 class=\"wq-question-title\"><\/h4>\n\t\t\t<\/div>\n\t\t<\/div>\n\t<\/div>\n\n\t<div class=\"card \">\n\t\t<div class=\"front\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2551-Front.gif\" \/>\n\t\t\n\t\t\n\t\n\t\n\t\t\t<span class=\"top-desc\">Click to Flip<\/span>\n\t<\/div>\n\t\t<div class=\"back\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2551-Reverse.gif\" \/>\n\t\t\n\t\t\n\t\n\t<\/div>\n\t<\/div>\n\n\t\n<\/div>\n<\/div>\n<\/div>\n<!-- \/\/ wp quiz-->\n<\/div><!-- End .wq-quiz-wrapper -->\n<p>If halogenation went through a free carbocation, rotation would then be possible around the C-C bond. Consequently, we&#8217;d expect to see one of the <em>t<\/em>-butyl groups <strong>flip around<\/strong> to relieve the strain.<\/p>\n<p>What was found instead is that this reaction is also <strong>highly stereospecific<\/strong>, just like the reaction of <em>cis<\/em>&#8211; 2-butene. Furthermore, there were no alkyl or hydride shifts like we would expect with a free carbocation.<\/p>\n<p>Clearly,\u00a0<em>something\u00a0<\/em>is responsible for the stereoselectivity of this reaction. What is it?<\/p>\n<p>The best proposal we have for what is happening is formation of a cyclic <strong>halonium ion<\/strong>, which is a 3-membered ring bearing a positive formal charge on the halogen. <span style=\"color: #800080;\"><em>&#8220;Halonium ion&#8221; is the generic term for when a halogen serves as the bridge; we can also use &#8220;chloronium&#8221;, &#8220;bromonium&#8221; or &#8220;iodonium&#8221; when referring to specific halogens.\u00a0<\/em><\/span><\/p>\n<p>An example of an arrow-pushing mechanism for halonium formation that shows all the bonds being formed and broken looks like this:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35359\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/10-mechanism-for-the-formation-of-halonium-ion-bromonium-ion-from-cyclohexene-and-bromine.gif\" alt=\"mechanism for the formation of halonium ion bromonium ion from cyclohexene and bromine\" width=\"640\" height=\"238\" \/><\/a><\/p>\n<p><em><span style=\"color: #993366;\">Be aware that many textbooks will only show two arrows, like this <a href=\"\" class=\"custom-tooltip\" data-image=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/Supp-1-bromonium-ion-formation-contrast-how-its-drawn-in-textbooks.gif\" data-link=\"\" data-title=\"\" data-text=\"\">(hover here) <\/a> or click this<\/span> <a href=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/Supp-1-bromonium-ion-formation-contrast-how-its-drawn-in-textbooks.gif\">link<\/a>.<\/em><\/p>\n<p>Although a halonium ion has a positive formal charge on the halogen, it&#8217;s actually the <strong>carbons<\/strong> that are <strong>electrophilic<\/strong>. This is easier to recognize if you remember that halogens are more electronegative than carbon, so the carbon-halogen bonds will have partial positive character on carbon. That&#8217;s why it&#8217;s called, &#8220;formal&#8221; charge.<span style=\"color: #993366;\"><em> [See article:<span style=\"color: #993366;\"> <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2010\/09\/24\/how-to-calculate-formal-charge\/\">How to Calculate Formal Charge<\/a><\/span>]<\/em><\/span><\/p>\n<p>In a symmetrical halonium ion, the carbon-halogen bond lengths will be equal, as will be the partial positive charges on carbon.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35360\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/11-properties-of-halonium-ions-structure-of-bromonium-chloronium-iodonium-ions.gif\" alt=\"properties of halonium ions - structure of bromonium chloronium iodonium ions\" width=\"640\" height=\"380\" \/><\/a><\/p>\n<p><span style=\"color: #993366;\"><em>\u00a0X-ray crystal structures of stable bromonium and iodonium ions were determined by Brown &amp; co-workers a while back <\/em>[<a href=\"#notethree\"><span style=\"color: #ff0000;\">Note 3<\/span><\/a>]<\/span><\/p>\n<h2><a id=\"four\"><\/a>4. Halogenation of Alkenes &#8211; The Mechanism<\/h2>\n<p>So the first step in halogenation of alkenes is formation of a halonium ion.<\/p>\n<p>In the second step of halogenation, the halide ion attacks the carbon from the <strong>backside<\/strong> of the C-halogen bond, resulting in formation of C-halogen and breakage of C-halogen.<\/p>\n<p>This key step accounts for the high stereoselectivity of halogenation for the observed\u00a0<em>anti\u00a0<\/em>products.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35361\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/12-mechanism-of-alkene-halogenation-attack-of-halide-on-bromonium-ion-to-give-anti-addition.gif\" alt=\"mechanism of alkene halogenation - attack of halide on bromonium ion to give anti addition\" width=\"640\" height=\"281\" \/><\/a><\/p>\n<p><span style=\"color: #993366;\"><em>Note that the enantiomer (not shown) is also formed here.\u00a0<\/em><\/span><\/p>\n<p>This is similar to the backside attack in the SN2 mechanism [<span style=\"color: #993366;\"><em>See articl<span style=\"color: #993366;\">e &#8211; <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2012\/07\/04\/the-sn2-mechanism\/\">The S<sub>N<\/sub>2 mechanism<\/a><\/span>]<\/em><\/span> where a lone pair from the nucleophile overlaps with the sigma* orbital of the leaving group. However, this step does not have the same sensitivity to steric hindrance as the SN2 reaction, as we are about to see.<\/p>\n<h2><a id=\"five\"><\/a>5. Halohydrin Formation &#8211;\u00a0 with water and alcohols<\/h2>\n<p>When bromination of alkenes is carried out in the presence of water (often as a co-solvent), <strong>halohydrin<\/strong> products are formed.<\/p>\n<p>A halohydrin is a molecule containing\u00a0 C-OH and C-halogen bonds on adjacent carbons.<\/p>\n<p>The reaction also proceeds through a halonium ion intermediate. As with halogenation, <strong>anti-\u00a0<\/strong>addition products are formed exclusively.<\/p>\n<p>One difference between halogenation and halohydrin formation is that the two new sigma bonds are formed to non-identical atoms (oxygen and halogen). This gives rise to the possibility of forming a mixture of constitutional isomers. When one constitutional isomer dominates, we say that reaction is\u00a0<strong>regioselective.\u00a0<\/strong><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35362\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/13-halohydrin-formation-from-alkenes-with-halogen-and-water-h2o-gives-anti-product.gif\" alt=\"halohydrin formation from alkenes with halogen and water h2o gives anti product\" width=\"640\" height=\"372\" \/><\/a><\/p>\n<p><span style=\"color: #993366;\"><em>Halohydrins are useful for the formation of epoxides. For more, see this article &#8211; <a href=\"https:\/\/www.masterorganicchemistry.com\/2015\/01\/26\/epoxides-the-outlier-of-the-ether-family\/\">Formation of Epoxides<\/a><\/em><\/span><\/p>\n<p>It is found that halohydrin formation is\u00a0<strong>regio<\/strong><strong>selective<\/strong> for formation of the isomer where the C-OH bond forms on the <strong>more substituted<\/strong> carbon. This is similar to the pattern observed in addition reactions of HX to alkenes, which is known as &#8220;Markovnikov&#8221; regioselectivity. (<span style=\"color: #800080;\"><em>See pos<span style=\"color: #993366;\">t &#8211; <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2013\/02\/08\/markovnikovs-rule-1\/\">Markovnikov&#8217;s Rule<\/a><\/span><\/em><\/span>)<\/p>\n<p>Why might this be? After all, shouldn&#8217;t the more substituted carbon be more sensitive to steric hindrance?<\/p>\n<p>Experiments tell us that (in contrast to the S<sub>N<\/sub>2) nucleophiles can attack tertiary carbons of halonium ions perfectly well. In other words, steric hindrance is not a significant factor.<\/p>\n<p>Let&#8217;s take a closer look at the structure of a non-symmetrical halonium ion.<\/p>\n<p>As touched on earlier, although there is a formal charge of +1 on the halogen, it&#8217;s actually the\u00a0<strong>carbons<\/strong> that bear the positive charge density due to the greater electronegativity of halogens (&gt;3) versus carbon (2.5) . And to the extent that those carbons bear positive charge density, they are <strong>electrophilic<\/strong>.<\/p>\n<p>Which of the two carbons is best able to stabilize positive charge? The more substituted carbon, generally. (As well as any carbon capable of delocalizing a positive charge through resonance). [<span style=\"color: #993366;\"><em>See article &#8211;<a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2011\/03\/11\/3-factors-that-stabilize-carbocations\/\"> Carbocation Stability<\/a><\/em><\/span>]<\/p>\n<p>It&#8217;s not really proper to draw resonance forms where single bonds break, so hold your nose for a moment:<\/p>\n<p>It can help to imagine the halonium ion as a &#8220;resonance hybrid&#8221; of two different carbocations. The resonance form where the carbocation is on the more substituted carbon will make the greater contribution to our &#8220;hybrid&#8221;.<\/p>\n<p>Having greater positive charge density, it will be more electrophilic, and therefore will more readily undergo reaction with nucleophiles. <span style=\"color: #800080;\"><em>Note &#8211; calculations indicate a much weaker C-halogen bond on the more substituted carbon of the halonium ion. See Note\u00a0<\/em><\/span><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35363\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/14-rationalize-the-markovnikov-regioselectivity-of-halohydrin-formation-due-to-the-more-substituted-carbon-being-better-able-to-stabilize-positive-charge.gif\" alt=\"rationalize the markovnikov regioselectivity of halohydrin formation due to the more substituted carbon being better able to stabilize positive charge\" width=\"640\" height=\"439\" \/><\/a><\/p>\n<p>Another question that comes up is, &#8220;why does water attack the halonium ion instead of the halide ion? Isn&#8217;t the halide ion a better nucleophile?<\/p>\n<p>Yes, the halide ion is a better nucleophile than water. However, recall that the reaction rate will depend not just on nucleophilicity but also on <strong>concentration<\/strong>. Since water is generally used as the solvent (or co-solvent) here, it will have a <strong>vastly<\/strong> higher concentration.<\/p>\n<p>So in a sense we are &#8220;overwhelming&#8221; the halonium ion with a much higher concentration of a worse nucleophile<em> (<span style=\"color: #800080;\">the technical term for this is &#8220;<a style=\"color: #800080;\" href=\"https:\/\/en.wikipedia.org\/wiki\/Law_of_mass_action\">mass action<\/a>&#8220;<\/span>).\u00a0<\/em><\/p>\n<h2><a id=\"six\"><\/a>6. Mechanism of\u00a0 Halohydrin Formation<\/h2>\n<p>As with halogenation, the first step in halohydrin formation is creation of a halonium ion.<\/p>\n<p>In addition to Cl<sub>2<\/sub>, Br<sub>2<\/sub>, and I<sub>2<\/sub>, this can also be done with sources of electropositive halogen such as N-bromosuccinimide (NBS). [<span style=\"color: #993366;\"><em>See article &#8211; <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2011\/06\/10\/reagent-friday-nbs-n-bromo-succinimide\/\">N-Bromosuccinimide<\/a><\/em><\/span>]<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35364\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/15-formation-of-halonium-ions-with-ncs-nbs-nis-halosuccinimides.gif\" alt=\"formation of halonium ions with ncs nbs nis halosuccinimides\" width=\"640\" height=\"562\" \/><\/a><\/p>\n<p>After formation of the halonium ion, the next step is attack on the halonium ion by H<sub>2<\/sub>O, at the more substituted carbon.<\/p>\n<p><span style=\"color: #993366;\"><em>Why might H<sub>2<\/sub>O do this and not the counter-ion? If it&#8217;s present as solvent it just outnumbers the concentration of halide ion. [<span style=\"color: #ff0000;\"><a style=\"color: #ff0000;\" href=\"#notefour\">Note 4<\/a> <\/span>]<\/em><\/span><\/p>\n<p>This results in the anti product which can then be deprotonated (e.g. with solvent)\u00a0 to give the neutral halohydrin.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35365\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/16-mechanism-for-the-formation-of-halonium-ions-from-n-bromosuccinimide-and-others.gif\" alt=\"mechanism for the formation of halonium ions from n bromosuccinimide and others\" width=\"640\" height=\"425\" \/><\/a><\/p>\n<p>[<span style=\"color: #993366;\"><em>Note that, like a flat coin that can land on &#8220;heads&#8221; or &#8220;tails&#8221; with equal likelihood, the bromine can &#8220;land&#8221; on either face of the alkene &#8211; this will give rise to enantiomers in this case<\/em><\/span>].<\/p>\n<h2><a id=\"seven\"><\/a>7. Haloethers<\/h2>\n<p>Similarly, if alcohols are used as solvent, haloethers may form. The formation of haloethers passes through an identical mechanism to that of halohydrins.<\/p>\n<p>See if you can predict the product of the following reaction.<\/p>\n<div class=\"wq-quiz-wrapper\" data-id=\"45638\"><style type=\"text\/css\" id=\"wq-flip-custom-css\">.wq-quiz-wrapper[data-id=\"45638\"] {\n--wq-question-width: 100%;\n--wq-question-color: #009cff;\n--wq-question-height: auto;\n--wq-font-color: #444;\n}\n\n\t\t\t.wq-quiz-wrapper[data-id=\"45638\"] {\n\t\t\t\t--wq-question-width: 600px;\n\t\t\t}\n\n\t\t\t@media screen and (max-width: 600px) {\n\t\t\t\t.wq-quiz-wrapper[data-id=\"45638\"] .wq_singleQuestionWrapper { width:100% !important; height:auto !important; }\n\t\t\t}\n\t\t<\/style><!-- wp quiz -->\n<div id=\"wp-quiz-45638\" class=\"wq_quizCtr single flip_quiz wq-quiz wq-quiz-45638 wq-quiz-flip wq-layout-single wq-skin-traditional wq-should-show-correct-answer\" data-quiz-id=\"45638\">\n<div class=\"wq-questions wq_questionsCtr\">\n\t<div class=\"wq-question wq_singleQuestionWrapper wq-question-8stx7\" data-id=\"8stx7\">\n\n\t\n\t<div class=\"item_top\">\n\t\t<div class=\"title_container\">\n\t\t\t<div class=\"wq_questionTextCtr\">\n\t\t\t\t<h4 class=\"wq-question-title\"><\/h4>\n\t\t\t<\/div>\n\t\t<\/div>\n\t<\/div>\n\n\t<div class=\"card \">\n\t\t<div class=\"front\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2552-Front.gif\" \/>\n\t\t\n\t\t\n\t\n\t\n\t\t\t<span class=\"top-desc\">Click to Flip<\/span>\n\t<\/div>\n\t\t<div class=\"back\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2552-Reverse.gif\" \/>\n\t\t\n\t\t\n\t\n\t<\/div>\n\t<\/div>\n\n\t\n<\/div>\n<\/div>\n<\/div>\n<!-- \/\/ wp quiz-->\n<\/div><!-- End .wq-quiz-wrapper -->\n<p>An interesting wrinkle in haloether formation is the possibility for <strong>intramolecular<\/strong> reactions.<\/p>\n<p>For example, treating this alkene with NBS (a source of Br+) results in formation of a new ring.<\/p>\n<p>See if you can figure out the mechanism.<\/p>\n<div class=\"wq-quiz-wrapper\" data-id=\"45639\"><style type=\"text\/css\" id=\"wq-flip-custom-css\">.wq-quiz-wrapper[data-id=\"45639\"] {\n--wq-question-width: 100%;\n--wq-question-color: #009cff;\n--wq-question-height: auto;\n--wq-font-color: #444;\n}\n\n\t\t\t.wq-quiz-wrapper[data-id=\"45639\"] {\n\t\t\t\t--wq-question-width: 600px;\n\t\t\t}\n\n\t\t\t@media screen and (max-width: 600px) {\n\t\t\t\t.wq-quiz-wrapper[data-id=\"45639\"] .wq_singleQuestionWrapper { width:100% !important; height:auto !important; }\n\t\t\t}\n\t\t<\/style><!-- wp quiz -->\n<div id=\"wp-quiz-45639\" class=\"wq_quizCtr single flip_quiz wq-quiz wq-quiz-45639 wq-quiz-flip wq-layout-single wq-skin-traditional wq-should-show-correct-answer\" data-quiz-id=\"45639\">\n<div class=\"wq-questions wq_questionsCtr\">\n\t<div class=\"wq-question wq_singleQuestionWrapper wq-question-739l0\" data-id=\"739l0\">\n\n\t\n\t<div class=\"item_top\">\n\t\t<div class=\"title_container\">\n\t\t\t<div class=\"wq_questionTextCtr\">\n\t\t\t\t<h4 class=\"wq-question-title\"><\/h4>\n\t\t\t<\/div>\n\t\t<\/div>\n\t<\/div>\n\n\t<div class=\"card \">\n\t\t<div class=\"front\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2553-Front.gif\" \/>\n\t\t\n\t\t\n\t\n\t\n\t\t\t<span class=\"top-desc\">Click to Flip<\/span>\n\t<\/div>\n\t\t<div class=\"back\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2553-Reverse.gif\" \/>\n\t\t\n\t\t\n\t\n\t<\/div>\n\t<\/div>\n\n\t\n<\/div>\n<\/div>\n<\/div>\n<!-- \/\/ wp quiz-->\n<\/div><!-- End .wq-quiz-wrapper -->\n<p>As always, be on the lookout for intramolecular reactions as they make for great exam questions.<\/p>\n<h2><a id=\"eight\"><\/a>8. Some Applications of Halogenation Reactions<\/h2>\n<p>It&#8217;s worth knowing how some of these reactions can be applied later on in the course.<\/p>\n<p>Alkynes can be converted into\u00a0<em>cis<\/em>-dihaloalkenes through treatment with halogens (<span style=\"color: #993366;\"><em>See article: <span style=\"color: #993366;\"><a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2013\/05\/29\/alkyne-halogenation-bromination-chlorination\/\">Halogenation of Alkynes<\/a><\/span><\/em><\/span>)<\/p>\n<p>There is no <strong>direct<\/strong> way to convert an alkene to an alkyne, but they can be indirectly converted to alkynes through formation of a dihalide followed by double elimination to give the alkyne. (<span style=\"color: #993366;\"><em>See article: <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2013\/06\/11\/alkynes-via-elimination-reactions\/\">Alkenes to Alkynes via Elimination Reactions<\/a><\/em><\/span>)<\/p>\n<p><em>[<span style=\"color: #993366;\">Worth noting: bromination of alkenes is technically an oxidation reaction, because each carbon goes from being bound to another carbon (0) to bromine (\u20131). The oxidation state of each carbon in ethene is +2; the oxidation state of each carbon in dibromoethane is +1.<\/span> ]<\/em><\/p>\n<p>Epoxides can be formed from alkenes in two steps via formation of a halohydrin followed by deprotonation with a strong base (such as NaH). The intermediate alkoxide performs an intramolecular S<sub>N<\/sub>2 reaction to give a new 3-membered ring. (<span style=\"color: #993366;\"><em>See article: <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/reaction-guide\/formation-of-epoxides-from-bromohydrins\/\">Formation of Epoxides<\/a><\/em><\/span>)<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35376\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/19-applications-of-dihalide-formation-elimination-alkynes-and-halohydrins-1.gif\" alt=\"applications of dihalide formation - elimination alkynes and halohydrins\" width=\"640\" height=\"389\" \/><\/a><\/p>\n<h2><a id=\"nine\"><\/a>9. Summary<\/h2>\n<p>Remember this reaction and remember this mechanism! It bears a lot of similarity to other mechanisms you will encounter in the chapter on alkenes, such as:<\/p>\n<ul>\n<li>Oxymercuration (Markovnikov-selective, also goes through a cyclic, 3-membered &#8220;mercurinium ion&#8221; intermediate)<\/li>\n<li>Opening of epoxides under acidic conditions (also undergoes addition at the most substituted carbon)<\/li>\n<\/ul>\n<p>Furthermore, the reaction can also be applied to alkynes.<\/p>\n<p>The <strong>stereochemistry<\/strong> of this reaction is very frequently tested. Make sure you can properly draw the products of anti addition and additionally, can work backwards from the dihalide products to the alkene starting materials.<\/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\/03\/20\/alkene-addition-pattern-2-the-three-membered-ring-pathway\/\" class=\"\"><span>Alkene Addition Pattern #2: The \u201cThree-Membered Ring\u201d Pathway<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/02\/08\/markovnikovs-rule-1\/\" class=\"\"><span>Markovnikov Addition Of HCl To Alkenes<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/reaction-guide\/bromination-of-alkenes-with-br2-to-give-dibromides\/\" class=\"\"><span>Bromination of alkenes with Br2 to give dibromides (MOC Membership)<\/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\/reaction-guide\/formation-of-alcohols-from-alkenes-using-hgoac2-and-water\/\" class=\"\"><span>Oxymercuration: Alcohols from alkenes using Hg(OAc)2 and Water (MOC Membership)<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/organic-chemistry-practice-problems\/alkene-reactions-practice-problems\/\" class=\"\"><span>Alkene Reactions Practice Problems (MOC Membership)<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2015\/02\/02\/opening-of-epoxides-with-acid\/\" class=\"\"><span>Opening of Epoxides With Acid<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2023\/08\/31\/oxymercuration-demercuration\/\" class=\"\"><span>Oxymercuration Demercuration of Alkenes<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2010\/07\/02\/stereoselective-stereospecific\/\" class=\"\"><span>Stereoselective and Stereospecific Reactions<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/01\/22\/alkene-addition-regioselectivity-syn-anti\/\" class=\"\"><span>Alkene Addition Reactions: \u201cRegioselectivity\u201d and \u201cStereoselectivity\u201d (Syn\/Anti)<\/span><\/a><\/li><\/ul><\/div>\n<p><strong><a id=\"noteone\"><\/a>Note 1<\/strong>. &#8220;<strong>Inert<\/strong> solvent&#8221; here just means a solvent that won&#8217;t react with the halonium ion intermediate. CCl<sub>4<\/sub> and CH<sub>2<\/sub>Cl<sub>2<\/sub> leave halonium ions alone; H<sub>2<\/sub>O, alcohols, and carboxylic acids can potentially undergo reactions with them.<\/p>\n<p><strong><a id=\"notetwo\"><\/a>Note 2.\u00a0<\/strong>Here are some specific examples of iodination and fluorination:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35367\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/F1-specific-examples-of-iodination-of-2-pentene-and-fluorination-of-alkenes-with-f2.gif\" alt=\"specific examples of iodination of 2-pentene and fluorination of alkenes with f2\" width=\"640\" height=\"373\" \/><\/a><\/p>\n<p><strong><a id=\"notethree\"><\/a>Note 3. <\/strong>Halonium ions are not fictional chemical entities. The halonium ion from the reaction below is particularly stable, since it is very difficult for nucleophiles to attack the very sterically hindered carbon atoms:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15005\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/4-halonium-ion-from-iodination-of-di-adamantyl-alkene-is-particularly-stable.gif\" alt=\"halonium ion from iodination of di-adamantyl alkene is particularly stable\" width=\"600\" height=\"141\" \/><\/p>\n<p><a href=\"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/ja00085a027\">R.S. Brown and co-workers <\/a>succeeded in using X-ray diffraction to obtain a crystal structure of this molecule. Here&#8217;s the iodonium ion portion of the structure:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15006\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/5-From-JACS-1994-116-2448.png\" alt=\"From JACS 1994 116 2448\" width=\"600\" height=\"371\" srcset=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/5-From-JACS-1994-116-2448.png 774w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/5-From-JACS-1994-116-2448-300x185.png 300w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/5-From-JACS-1994-116-2448-768x474.png 768w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/5-From-JACS-1994-116-2448-320x198.png 320w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/5-From-JACS-1994-116-2448-640x395.png 640w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/5-From-JACS-1994-116-2448-360x222.png 360w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/5-From-JACS-1994-116-2448-720x445.png 720w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/5-From-JACS-1994-116-2448-760x469.png 760w\" sizes=\"(max-width: 600px) 100vw, 600px\" \/><\/p>\n<p>The iodine-carbon bond length is 2.48 Angstrom, significantly longer than the value of 2.13 Angstrom for a typical C-I bond.<\/p>\n<p>The C-C bond length is 1.49 A, which is typical for a C-C single bond.<\/p>\n<p><strong><a id=\"notefour\"><\/a>Note 4.\u00a0<\/strong>It&#8217;s possible to influence the product distribution by adding various salts (e.g. NaCl, sodium acetate, lithium bromide, etc.) that may intercept the halonium ion.\u00a0 See here [<a href=\"#refnine\">Ref<\/a>] for a study.<\/p>\n<p><strong>Note 5.\u00a0<\/strong>In some situations the high selectivity for the\u00a0<em>anti<\/em>&#8211; product can break down. See [<a href=\"#refnine\">Ref<\/a>] for more.<\/p>\n<p><strong>Note 6.<\/strong> How might we resolve the role of &#8220;nucleophiles&#8221; and &#8220;electrophiles&#8221;in formation of the bromonium ion? This is not easy, because it entails making further assumptions about the reaction mechanism that might not be based on solid evidence. With that hedge out of the way, here&#8217;s a proposal. : instead of thinking of C-C \u03c0 and Br monolithically,\u00a0\u00a0break down each component into molecular orbitals. The C-C \u03c0 orbital could act as a nucleophile while the C-C \u03c0* acts as an electrophile; the Br-Br \u03c3* orbital could act as an electrophile while the Br lone pair could act as a nucleophile.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15047\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-in-bromination-define-nucleophile-as-a-particular-orbital-and-electrophile-as-a-particular-orbital.gif\" alt=\"in bromination define nucleophile as a particular orbital and electrophile as a particular orbital\" width=\"600\" height=\"224\" \/><\/p>\n<p>As it turns out, calculations indicate the bromination of alkenes with Br<sub>2<\/sub> to be more complex than we might initially suppose. The initial step is coordination of Br<sub>2<\/sub> to the alkene in a loosely bonded structure known as a &#8220;\u03c0 complex&#8221;. The\u00a0\u03c0 complex then breaks down to give the bromonium ion. A proper treatment of the orbitals would therefore not strictly be of the alkene and Br<sub>2<\/sub>, but of the orbitals in the \u03c0\u00a0complex itself.<\/p>\n<p>It gets more complicated. In some solvents it turns out to be energetically favorable for a\u00a0<em>second<\/em> molecule of Br<sub>2<\/sub> to be involved in bonding to the Br- that is expelled in the process (yes, a &#8220;termolecular&#8221; mechanism). <a href=\"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/jp075674b\">For more details see here<\/a>\u00a0(J. Phys Chem A, 2007, 111, 13218)<\/p>\n<p><strong>Note 7.\u00a0<\/strong>Calculations on several bromonium ions indicate a much weaker C-Br bond on the more substituted carbon of the halonium ion [<a href=\"https:\/\/pubs.rsc.org\/en\/content\/articlelanding\/1990\/C3\/c39900000898\">Ref<\/a>]<br \/>\n<img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35898\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2013\/03\/F3-Bond-length-geometries-in-the-bromonium-ion.gif\" alt=\"Bond length geometries in the bromonium ion\" width=\"640\" height=\"339\" \/><\/a><\/p>\n<p>Here&#8217;s an interactive model, courtesy of <a href=\"https:\/\/www.rowansci.com\/\">Rowan<\/a><\/p>\n<p><iframe title=\"Bromonium ion of 2-methylpropene\" src=\"https:\/\/labs.rowansci.com\/iframe\/calculations\/a5d6aa9a-10e0-47ea-9840-d7647e5838dc\" width=\"640\" height=\"640\"><\/iframe><\/p>\n<hr \/>\n<h2><strong><a id=\"quiz\"><\/a>Quiz Yourself!<\/strong><\/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\/3582-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\/3584-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\/2554-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\/0615-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. <br \/>\n<\/p>\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/0631-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. <br \/>\n<\/p>\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/0628-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. <br \/>\n<\/p>\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/0630-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. <br \/>\n<\/p>\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/0625-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. <br \/>\n<\/p>\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/2403-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\/2555-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><strong><a id=\"references\"><\/a>(Advanced) References and Further Reading<\/strong><\/h2>\n<ol>\n<li><strong>THE RELATIVE RATES OF BROMINATION OF THE OLEFINS<br \/>\n<\/strong>Harold S. Davis<strong><br \/>\n<\/strong><em>Journal of the American Chemical Society<\/em><strong> 1928<\/strong>, <em>50<\/em> (10), 2769-2780<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/abs\/10.1021\/ja01397a031\">1021\/ja01397a031<\/a><br \/>\nAn early paper studying the kinetics of alkene bromination under a variety of conditions.<\/li>\n<li><strong>The Halogenation of Ethylenes<br \/>\n<\/strong>Irving Roberts and George E. Kimball<br \/>\n<em>Journal of the American Chemical Society<\/em><strong> 1937, <\/strong><em>59<\/em> (5), 947-948<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/10.1021\/ja01284a507\">1021\/ja01284a507<\/a><br \/>\nOne of the earliest descriptions in the literature of a three-membered bromonium ion, accounting for the <em>anti<\/em> stereochemistry of this reaction.<\/li>\n<li><strong>The question of reversible formation of bromonium ions during the course of electrophilic bromination of olefins. 2. The crystal and molecular structure of the bromonium ion of adamantylideneadamantane<br \/>\n<\/strong>H. Slebocka-Tilk, R. G. Ball, and R. Stan Brown<strong><br \/>\n<\/strong><em>Journal of the American Chemical Society<\/em><strong> 1985, <\/strong><em>107<\/em> (15), 4504-4508<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/abs\/10.1021\/ja00301a021\">10.1021\/ja00301a021<\/a><strong><br \/>\n<\/strong>This paper describes the X-ray crystal structure of an isolated, stable bromonium ion. This is significant because it proves the intermediacy of these three-membered cyclic bromonium ions in the electrophilic addition of bromine to alkenes.<\/li>\n<li><strong>Stable Bromonium and Iodonium Ions of the Hindered Olefins Adamantylideneadamantane and Bicyclo[3.3.1]nonylidenebicyclo[3.3.1]nonane. X-Ray Structure, Transfer of Positive Halogens to Acceptor Olefins, and ab Initio Studies<br \/>\n<\/strong>R. S. Brown, R. W. Nagorski, A. J. Bennet, R. E. D. McClung, G. H. M. Aarts, M. Klobukowski, R. McDonald, and B. D. Santarsiero<strong><br \/>\n<\/strong><em>Journal of the American Chemical Society<\/em><strong> 1994, <\/strong><em>116<\/em> (6), 2448-2456<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/10.1021\/ja00085a027\">10.1021\/ja00085a027<\/a><br \/>\nCyclic iodonium ions, analogous to the bromonium ions, can also be isolated and characterized. The parent alkene in these studies, adamantylideneadamantane, is prepared using a McMurry reaction with 2-adamantanone.<\/li>\n<li><strong>ALKYNE via SOLID-LIQUID PHASE-TRANSFER CATALYZED DEHYDROHALOGENATION: ACETYLENE DICARBOXALDEHYDE TETRAMETHYL ACETAL AND ACETYLENE DICARBOXALDEHYDE DIMETHYL ACETAL<br \/>\n<\/strong>Rufine Aku\u00e9-G\u00e9du and Beno\u00eet Rigo<strong><br \/>\n<\/strong><em>Org. Synth.<\/em><strong> 2005, <\/strong><em>82<\/em>, 179<strong><br \/>\nDOI: <\/strong><a href=\"http:\/\/www.orgsyn.org\/demo.aspx?prep=v82p0179\">10.15227\/orgsyn.082.0179<\/a><br \/>\nFormation of alkynes via double elimination of a dihalide.<\/li>\n<li><strong>Principles of an Electronic Theory of Organic Reactions.<br \/>\n<\/strong>Christopher K. Ingold<strong><br \/>\n<\/strong><em>Chemical Reviews<\/em><strong> 1934, <\/strong><em>15<\/em> (2), 225-274<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/10.1021\/cr60051a003\">1021\/cr60051a003<\/a><\/li>\n<li><strong>\u2014The relative directive powers of groups of the forms RO and RR\u2032N in aromatic substitution. Part IV. A discussion of the observations recorded in parts I, II, and III<br \/>\n<\/strong>James Allan, Albert Edward Oxford, Robert Robinson, and John Charles Smith<strong><br \/>\n<\/strong><em>J. Chem. Soc. (Res.)<\/em><strong> 1926, <\/strong><em>129<\/em>, 401-411<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.rsc.org\/en\/content\/articlelanding\/1926\/JR\/JR9262900401#!divAbstract\">10.1039\/JR9262900401<\/a><br \/>\nNobel Prize winner Robert Robinson was wrong about the mechanism of halogenation reactions.<\/li>\n<li><strong>Polar Additions to Olefins. II. The Chlorination of Di-t-butylethylene<\/strong><br \/>\nRobert C. Fahey<br \/>\n<em>Journal of the American Chemical Society<\/em> <strong>1966<\/strong> 88 (20), 4681-4684<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/pubs.acs.org\/doi\/10.1021\/ja00972a030\">10.1021\/ja00972a030<\/a><br \/>\nStudy on the halogenation of\u00a0<em>cis<\/em>&#8211; and\u00a0<em>trans-\u00a0<\/em>di-t<em>&#8211;<\/em>butylethylene, showing the stereospecific nature of the halogenation reaction.<\/li>\n<li><strong><a id=\"refnine\"><\/a>Polar additions to the styrene and 2-butene systems. II. Medium dependence of bromination products<\/strong><br \/>\nJohn H. Rolston and Keith Yates<br \/>\n<em>Journal of the American Chemical Society<\/em> <strong>1969<\/strong> 91 (6), 1477-1483<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/pubs.acs.org\/doi\/10.1021\/ja01034a034\">10.1021\/ja01034a034<\/a><br \/>\nIn some situations the stereoselectivity of halogenation reactions can be significantly eroded by using highly polar solvents or electron-releasing substituents on the alkene that will result in more stabilized carbocation intermediates.<\/li>\n<\/ol>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Bromination, Chlorination, and Halohydrin Formation from Alkenes Alkenes undergo halogenation when treated with Cl2, Br2 and (less commonly) I2 to give vicinal dihalides These reactions <\/p>\n","protected":false},"author":1,"featured_media":35378,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1418],"tags":[169,880,796,903,310,382,902,887,233,528],"post_folder":[],"class_list":["post-7117","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-alkene-reactions","tag-alkenes","tag-anti","tag-bromination","tag-bromonium","tag-chlorination","tag-halogenation","tag-halonium","tag-markovnikov","tag-mechanisms-2","tag-syn"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.7 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Bromination of Alkenes - The Mechanism &#8211; Master Organic Chemistry<\/title>\n<meta name=\"description\" content=\"Halogenation of alkenes with Cl2 and Br2 goes through a halonium ion intermediate to give anti addition products. 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