{"id":3150,"date":"2026-05-04T17:49:26","date_gmt":"2026-05-04T16:49:26","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/?page_id=3150"},"modified":"2026-10-19T20:43:26","modified_gmt":"2026-10-19T19:43:26","slug":"halogens","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/reactions-of-alkenes\/halogens\/","title":{"rendered":"Halogens"},"content":{"rendered":"\n<section class=\"ols-revision-page ols-alkene-halogens-page\">\n  <style>\n    .ols-revision-page {\n      --navy: #1C244B;\n      --blue: #2563eb;\n      --soft-blue: #eef4ff;\n      --soft-red: #fff7f7;\n      --soft-purple: #f7f0ff;\n      --soft-green: #f0f7f1;\n      --green: #3f8f46;\n      --red: #d94b2b;\n      --gold: #c9973a;\n      --grey-text: #667085;\n      --body-text: #1f2937;\n      --border: rgba(28, 36, 75, 0.14);\n      --shadow: 0 18px 45px rgba(28, 36, 75, 0.10);\n      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strong {\n      font-weight: 700;\n      color: var(--navy);\n    }\n.ols-faq-list {\n      display: grid;\n      gap: 14px;\n      margin-top: 18px;\n    }\n.ols-faq-item {\n      background: #ffffff;\n      border: 1px solid var(--border);\n      border-radius: 18px;\n      padding: 18px 20px;\n      box-shadow: var(--inner-shadow);\n    }\n.ols-faq-item h3 {\n      margin: 0 0 8px;\n      font-size: 20px;\n      line-height: 1.3;\n      color: var(--navy);\n    }\n.ols-faq-item p {\n      margin: 0;\n      font-size: 16px;\n      line-height: 1.65;\n      color: var(--body-text);\n    }\n.ols-faq-card,\n      .ols-quicksnap-card,\n      .ols-attribution-card {\n        padding: 24px 18px;\n        border-radius: 22px;\n      }\n<\/style>\n\n  <aside class=\"ols-sidebar\">\r\n  <div class=\"ols-sidebar-header\">\r\n    <h3>Revision Notes<\/h3>\r\n    <p>A Level Chemistry<\/p>\r\n  <\/div>\r\n\r\n  <div class=\"ols-topic-group\">\r\n    <h4>Topic 5 Alkenes<\/h4>\r\n\r\n    <ul class=\"ols-topic-list\">\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/alkene-bonding\/\">Alkene Bonding<\/a>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/geometric-isomerism\/\">Geometric Isomerism<\/a>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/electrophilic-addition-mechanism\/\">Electrophilic Addition Mechanism<\/a>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/reactions-of-alkenes\/\">Reactions of Alkenes<\/a>\r\n\r\n        <ul class=\"ols-subtopic-list\">\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/reactions-of-alkenes\/halogens\/\">Halogens<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/reactions-of-alkenes\/hydrogen-halide\/\">Hydrogen Halide<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/reactions-of-alkenes\/bromine-water\/\">Bromine Water<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/reactions-of-alkenes\/hydrogen\/\">Hydrogen<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/reactions-of-alkenes\/steam\/\">Steam<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/reactions-of-alkenes\/kmno4\/\">KMnO4<\/a>\r\n          <\/li>\r\n        <\/ul>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/polymerisation-reactions\/\">Polymerisation Reactions<\/a>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/polymer-waste\/\">Polymer Waste<\/a>\r\n      <\/li>\r\n    <\/ul>\r\n  <\/div>\r\n\r\n  <div class=\"ols-topic-group\">\r\n    <h4>Next Topics<\/h4>\r\n\r\n    <ul class=\"ols-topic-list\">\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-6-energetics\/\">Energetics<\/a>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-7-intermolecular-forces\/\">Intermolecular Forces<\/a>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8A-redox-chemistry\/\">Redox Chemistry<\/a>\r\n      <\/li>\r\n    <\/ul>\r\n  <\/div>\r\n<\/aside>\r\n\r\n<script>\r\n(function() {\r\n  function normalisePath(path) {\r\n    return String(path || '')\r\n      .split('?')[0]\r\n      .split('#')[0]\r\n      .replace(\/\\\/+$\/, '')\r\n      .toLowerCase();\r\n  }\r\n\r\n  function highlightActive() {\r\n    var sidebar = document.querySelector('.ols-sidebar');\r\n    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href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/\">Edexcel International<\/a> \/\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/\">Topic 5 Alkenes<\/a> \/\n        <span>Reactions of Alkenes: Halogens<\/span>\n      <\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Reactions of Alkenes with Halogens<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to the addition reaction of alkenes with bromine or chlorine, including the functional group change, reaction conditions and the electrophilic addition mechanism.<\/p>\n\n        <div class=\"ols-badges\">\n          <div class=\"ols-badge\">Exam board: Edexcel International<\/div>\n          <div class=\"ols-badge\">Topic 5: Alkenes<\/div>\n          <div class=\"ols-badge\">Reaction 1: Halogens<\/div>\n        <\/div>\n      <\/header>\n\n      <article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">1<\/div>\n<h2>Before You Start<\/h2>\n<\/div>\n<p>This page builds on GCSE work on halogen molecules, the bromine water test and addition reactions of alkenes.<\/p>\n<ul class=\"ols-list\">\n<li>Chlorine and bromine exist as <strong>diatomic molecules<\/strong>, Cl<sub>2<\/sub> and Br<sub>2<\/sub>.<\/li>\n<li>A covalent bond is a shared pair of electrons between two atoms.<\/li>\n<li>Alkenes are unsaturated and decolourise orange bromine water; alkanes do not.<\/li>\n<li>In an <strong>addition reaction<\/strong>, atoms add across the C=C double bond to give one product.<\/li>\n<\/ul>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline ols-h5p-recap\">\n<span class=\"ols-h5p-kicker\">Before you start<\/span>\n<h2>GCSE Recap: Halogens and the Bromine Test<\/h2>\n<p>Four quick GCSE questions on halogen molecules and testing for alkenes before you look at the reaction in detail.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"1268\"><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">2<\/div>\n          <h2>The Overall Reaction<\/h2>\n        <\/div>\n\n        <p>Alkenes react with halogens such as <strong>bromine, Br<sub>2<\/sub><\/strong>, or <strong>chlorine, Cl<sub>2<\/sub><\/strong>, in an <strong>addition reaction<\/strong>. The C=C double bond opens and one halogen atom adds to each carbon from the original double bond.<\/p>\n        <p>The functional group changes from an <strong>alkene<\/strong> to a <strong>dihalogenoalkane<\/strong>. For example, ethene reacts with bromine to form <strong>1,2-dibromoethane<\/strong>.<\/p>\n\n        <div class=\"ols-reaction-summary\">\n          <div class=\"ols-summary-item\"><span>Functional group change<\/span><strong>alkene \u2192 dihalogenoalkane<\/strong><\/div>\n          <div class=\"ols-summary-item\"><span>Reagent<\/span><strong>Bromine in an organic solvent<\/strong><\/div>\n          <div class=\"ols-summary-item\"><span>Conditions<\/span><strong>Room temperature, not in UV light<\/strong><\/div>\n          <div class=\"ols-summary-item\"><span>Mechanism<\/span><strong>Electrophilic addition<\/strong><\/div>\n        <\/div>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Key idea:<\/strong> The reaction is addition because atoms are added across the C=C double bond and only one organic product is formed from the alkene.<\/p>\n        <\/div>\n\n        <div class=\"ols-figure-card\">\n          <div class=\"ols-figure-image\">\n            <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t4-cd-17-ethenebr2.svg\" alt=\"Ethene plus bromine giving 1,2-dibromoethane at room temperature, displayed formulae\" width=\"1452\" height=\"696\">\n          <\/div>\n        <\/div>\n      <div class=\"ols-figure-card ols-zoom-pop\">\n<div class=\"ols-figure-image\">\n<a class=\"ols-image-fullscreen-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-img-23-halogencard.jpg\" target=\"_blank\" rel=\"noopener\" aria-label=\"Open image fullscreen\">\n<img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-img-23-halogencard.jpg\" alt=\"Summary card for alkene plus bromine or chlorine: reagent, conditions, product, test tubes orange-brown to colourless, and the alkane contrast\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-img-23-halogencard.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>Bromine or chlorine adds across the C=C at room temperature, without UV light, and the orange-brown colour of bromine disappears.<\/p><\/div>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: Predict the Product<\/h2>\n<p>Work out what chlorine does to an alkene that is not shown on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-26\" class=\"h5p-iframe\" data-content-id=\"26\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Halogen Addition MCQ: Predicting the Product from Oct-1-ene and Chlorine\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">3<\/div>\n          <h2>Why Bromine Acts as an Electrophile<\/h2>\n        <\/div>\n\n        <p>The \u03c0 bond in an alkene is a region of <strong>high electron density<\/strong>. As a Br<sub>2<\/sub> molecule approaches the alkene, the electron-rich \u03c0 bond repels the shared electron pair in the Br-Br bond.<\/p>\n        <p>This repulsion <strong>induces a dipole<\/strong> in the bromine molecule. One bromine atom becomes slightly positive, <strong>Br\u03b4<sup>+<\/sup><\/strong>, while the other becomes slightly negative, <strong>Br\u03b4<sup>&#8211;<\/sup><\/strong>.<\/p>\n        <p>The slightly positive bromine atom acts as the <strong>electrophile<\/strong> because it is attracted to the electron-rich \u03c0 bond.<\/p>\n\n        <div class=\"ols-definition-box\">\n          <div class=\"ols-definition-circle\">Br\u03b4<sup>+<\/sup><\/div>\n          <p><strong>Electrophile:<\/strong> An electron-pair acceptor. In this reaction, the polarised bromine molecule provides Br\u03b4<sup>+<\/sup>, which accepts electron density from the alkene \u03c0 bond.<\/p>\n        <\/div>\n      <div class=\"ols-figure-card ols-zoom-pop\">\n<div class=\"ols-figure-image\">\n<a class=\"ols-image-fullscreen-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-img-22-br2dipole.jpg\" target=\"_blank\" rel=\"noopener\" aria-label=\"Open image fullscreen\">\n<img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-img-22-br2dipole.jpg\" alt=\"Three frames: Br\u2082 with an even electron cloud, then polarised near the \u03c0 bond with \u03b4+ nearer and \u03b4\u2212 further, then heterolytic fission to Br\u207b\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-img-22-br2dipole.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>The \u03c0 electrons repel the Br\u2013Br bonding pair, inducing a dipole: the nearer bromine atom becomes Br\u03b4+ and acts as the electrophile.<\/p><\/div>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: The Induced Dipole<\/h2>\n<p>In each round, choose the one statement that is accurate.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-27\" class=\"h5p-iframe\" data-content-id=\"27\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Halogen Addition Summary: What the Induced Dipole Really Means\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card purple\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">4<\/div>\n          <h2>Bond Fission and Carbocation Formation<\/h2>\n        <\/div>\n\n        <p>The Br-Br bond breaks by <strong>heterolytic fission<\/strong>. This means both electrons from the covalent bond go to one bromine atom, forming a bromide ion, <strong>Br<sup>&#8211;<\/sup><\/strong>.<\/p>\n        <p>At the same time, the alkene \u03c0 bond forms a new bond to Br\u03b4<sup>+<\/sup>. One of the carbon atoms from the original double bond becomes positively charged, producing a <strong>carbocation intermediate<\/strong>.<\/p>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Remember:<\/strong> A carbocation is an organic ion with a positively charged carbon atom.<\/p>\n        <\/div>\n\n        <div class=\"ols-figure-card\">\n          <div class=\"ols-figure-image\">\n            <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t4-cd-18-br2mechanism-v2.svg\" alt=\"Curly-arrow mechanism for bromine adding to ethene via a carbocation, with an inset of the induced dipole in Br\u2082\" width=\"1321\" height=\"1238\">\n          <\/div>\n          <div class=\"ols-figure-caption\">\n            <p>The \u03c0 bond polarises Br<sub>2<\/sub>, Br\u03b4<sup>+<\/sup> adds first, then Br<sup>&#8211;<\/sup> attacks the carbocation to give the dibromo product.<\/p>\n          <\/div>\n        <\/div>\n      <\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: Charges, Atoms and Amounts<\/h2>\n<p>Answer as quickly as you can, using new alkenes each time.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-28\" class=\"h5p-iframe\" data-content-id=\"28\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Halogen Addition Quick-Fire: Intermediates, Counting and Amounts\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">5<\/div>\n          <h2>The Mechanism in Three Steps<\/h2>\n        <\/div>\n\n        <div class=\"ols-mechanism-steps\">\n          <div class=\"ols-step-card\">\n            <h3>Step 1: The \u03c0 bond polarises Br<sub>2<\/sub><\/h3>\n            <p>The high electron density in the C=C bond repels the Br-Br bonding pair, making the nearer bromine atom Br\u03b4<sup>+<\/sup>.<\/p>\n          <\/div>\n\n          <div class=\"ols-step-card\">\n            <h3>Step 2: The electrophile adds<\/h3>\n            <p>The alkene \u03c0 bond donates electron density to Br\u03b4<sup>+<\/sup>, forming a C-Br bond and a carbocation intermediate.<\/p>\n          <\/div>\n\n          <div class=\"ols-step-card\">\n            <h3>Step 3: Bromide attacks<\/h3>\n            <p>The Br<sup>&#8211;<\/sup> ion donates a lone pair to the carbocation, forming a second C-Br bond and producing the dihalogenoalkane.<\/p>\n          <\/div>\n        <\/div>\n      <\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: Chlorine with Cyclopentene<\/h2>\n<p>Drag the words and numbers into place to build the account of the reaction.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-25\" class=\"h5p-iframe\" data-content-id=\"25\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Halogen Addition Drag: Chlorine with Cyclopentene\"><\/iframe><\/div><\/div>\n<\/section>\n<!-- 3D card: electrophilic-addition-ethene (5 Oct 2026, Topics 1-5 upgrade) -->\n<!-- Copyright (c) 2026 Dr. Mohammed Al-Fatah, onlinelearningsystem.net. 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border-radius:22px;}\n  .ols-eae-001 h2.eae-title{font-size:20.5px;}\n  .ols-eae-001 p.eae-sub{font-size:13.5px;}\n  .ols-eae-001 .eae-stage{height:400px;}\n  .ols-eae-001 .eae-rowlab{min-width:100%;}\n  .ols-eae-001 .eae-seg button{padding:9px 13px; font-size:13px;}\n  .ols-eae-001 .eae-lab{font-size:10.5px; padding:4px 8px;}\n  .ols-eae-001 .eae-lab.l-part{font-size:9.5px; padding:3px 7px;}\n  .ols-eae-001 .eae-lab.l-read{font-size:12px; padding:5px 9px;}\n  .ols-eae-001 .eae-caption{font-size:10.5px; padding:5px 9px; max-width:64%;}\n  .ols-eae-001 .eae-badge{font-size:10.5px; padding:4px 9px;}\n  .ols-eae-001 .eae-badge.b-eq{font-size:11px;}\n  .ols-eae-001 .eae-badges{max-width:calc(100% - 28px);}\n  .ols-eae-001 .eae-info{padding:18px;}\n}\n\/* on a phone the caption needs the full top row, so the drag hint goes *\/\n@media (max-width:480px){\n  .ols-eae-001 .eae-hint{display:none;}\n  .ols-eae-001 .eae-caption{max-width:calc(100% - 28px);}\n}\n@media (prefers-reduced-motion:reduce){\n  .ols-eae-001 .eae-seg button,.ols-eae-001 .eae-pill{transition:none;}\n}\n\n.ols-cc-eae-001{\n  font-family:'Poppins',system-ui,-apple-system,'Segoe UI',Roboto,Helvetica,Arial,sans-serif;\n  margin:10px auto 0; text-align:center; font-size:11px; font-style:italic; color:#aab0c0;\n}\n.ols-cc-eae-001 a{color:#aab0c0; text-decoration:none;}\n.ols-cc-eae-001 a:hover{text-decoration:underline;}\n.ols-eae-001 .eae-seg button{display:inline-flex; align-items:center;}\n.ols-eae-001 .eae-seg button .eae-n{display:inline-flex; align-items:center; justify-content:center; width:20px; height:20px; margin-right:9px; border-radius:50%; font-size:11px; font-weight:600; background:rgba(37,99,235,0.10); color:#2563eb; flex:0 0 auto;}\n.ols-eae-001 .eae-seg button[aria-pressed=\"true\"] .eae-n{background:#ffffff; color:#2563eb;}\n<\/style>\n\n<div class=\"eae-head\">\n  <h2 class=\"eae-title\">Electrophilic Addition in Motion<\/h2>\n  <p class=\"eae-sub\">Choose a step to watch HBr and then Br\u2082 add across the \u03c0 bond of ethene, one curly arrow at a time.<\/p>\n<\/div>\n\n<div class=\"eae-stage\" id=\"eaeStage\">\n  <canvas class=\"eae-canvas\" id=\"eaeCanvas\"><\/canvas>\n  <div class=\"eae-overlay\" id=\"eaeOverlay\"><\/div>\n  <div class=\"eae-caption\" id=\"eaeCaption\"><\/div>\n  <div class=\"eae-hint\" id=\"eaeHint\">Drag to rotate<\/div>\n  <div class=\"eae-badges\" id=\"eaeBadges\"><\/div>\n<\/div>\n\n<div class=\"eae-controls\">\n  <div class=\"eae-row\">\n    <span class=\"eae-rowlab\">Step<\/span>\n    <div class=\"eae-seg\" role=\"group\" aria-label=\"Step\">\n      <button type=\"button\" data-step=\"1\" aria-pressed=\"true\"><span class=\"eae-n\">1<\/span>HBr meets the \u03c0 bond<\/button>\n      <button type=\"button\" data-step=\"2\" aria-pressed=\"false\"><span class=\"eae-n\">2<\/span>The \u03c0 bond attacks H\u03b4+<\/button>\n      <button type=\"button\" data-step=\"3\" aria-pressed=\"false\"><span class=\"eae-n\">3<\/span>Br\u207b attacks the carbocation<\/button>\n      <button type=\"button\" data-step=\"4\" aria-pressed=\"false\"><span class=\"eae-n\">4<\/span>Bromine is polarised<\/button>\n      <button type=\"button\" data-step=\"5\" aria-pressed=\"false\"><span class=\"eae-n\">5<\/span>Bromine adds<\/button>\n    <\/div>\n  <\/div>\n\n  <div class=\"eae-row\">\n    <span class=\"eae-rowlab\">View<\/span>\n    <button type=\"button\" class=\"eae-pill\" id=\"eaeResetV\">Reset view<\/button>\n  <\/div>\n<\/div>\n\n<div class=\"eae-info\">\n  <h3 id=\"eaeInfoTitle\"><\/h3>\n  <p id=\"eaeInfoText\"><\/p>\n  <div class=\"eae-facts\" id=\"eaeFacts\"><\/div>\n  <div class=\"eae-key\">\n    <span><i class=\"k-ball\" style=\"background:#464a54\"><\/i>Carbon<\/span>\n    <span><i class=\"k-ball\" style=\"background:#ececf0\"><\/i>Hydrogen<\/span>\n    <span><i class=\"k-ball\" style=\"background:#8c3222\"><\/i>Bromine<\/span>\n    <span><i class=\"k-ball\" style=\"background:#f6c426\"><\/i>Electron<\/span>\n    <span><i class=\"k-ball\" style=\"background:#b4cbe6\"><\/i>\u03c0 bond<\/span>\n    <span><i class=\"k-ball\" style=\"background:#f3e2b4\"><\/i>Empty p orbital<\/span>\n  <\/div>\n<\/div>\n\n<script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/electrophilic-addition-ethene.js?v=20261019f\"><\/script>\n<\/section>\n\n<p class=\"ols-cc-eae-001\">&copy; Dr. Mohammed Al-Fatah &#8211; <a href=\"https:\/\/www.onlinelearningsystem.net\" target=\"_blank\" rel=\"noopener\">onlinelearningsystem.net<\/a><\/p>\n\n      <article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">6<\/div>\n          <h2>Bromine and Chlorine Give Similar Products<\/h2>\n        <\/div>\n\n        <p>The same addition pattern applies to chlorine. With Cl<sub>2<\/sub>, the product is a <strong>dichloroalkane<\/strong>. With Br<sub>2<\/sub>, the product is a <strong>dibromoalkane<\/strong>.<\/p>\n\n        <div class=\"ols-table-wrap\">\n          <table class=\"ols-table\">\n            <thead>\n              <tr>\n                <th>Halogen<\/th>\n                <th>Product type<\/th>\n                <th>Example from ethene<\/th>\n              <\/tr>\n            <\/thead>\n            <tbody>\n              <tr>\n                <td><strong>Br<sub>2<\/sub><\/strong><\/td>\n                <td>Dibromoalkane<\/td>\n                <td>1,2-dibromoethane<\/td>\n              <\/tr>\n              <tr>\n                <td><strong>Cl<sub>2<\/sub><\/strong><\/td>\n                <td>Dichloroalkane<\/td>\n                <td>1,2-dichloroethane<\/td>\n              <\/tr>\n            <\/tbody>\n          <\/table>\n        <\/div>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Exam focus:<\/strong> Do not describe this as substitution. The C=C double bond breaks and two atoms are added, so this is an addition reaction.<\/p>\n        <\/div>\n      <\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: Work It Out, Then Flip<\/h2>\n<p>Work each answer out on paper first, then turn the card over to check your reasoning.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-744\" class=\"h5p-iframe\" data-content-id=\"744\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Halogen Addition Flip Cards: Work Out the Halogenation Product\"><\/iframe><\/div><\/div>\n<\/section>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: Halogen Addition from Start to Finish<\/h2>\n<p>Each round covers one part of the page, from the product to the mechanism. Choose the one accurate statement.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-1269\" class=\"h5p-iframe\" data-content-id=\"1269\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Halogen Addition Summary: Bromine and 2-Methylbut-2-ene\"><\/iframe><\/div><\/div>\n<\/section>\n\n      \n\n      <section class=\"ols-infographic-full\" aria-label=\"Reactions of alkenes with halogens visual summary\">\n        <div class=\"ols-infographic-full-frame\">\n          <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/skel-29-halogen-addition-summary-a.svg\" alt=\"Summary of halogen addition to alkenes: functional group change, reagent, conditions and the equation for ethene with bromine\" width=\"1520\" height=\"743\">\n          <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/skel-29-halogen-addition-summary-b.svg\" alt=\"Why bromine reacts with alkenes: the \u03c0 electrons induce a dipole in Br\u2082, with key terms\" width=\"1520\" height=\"1101\">\n          <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/skel-29-halogen-addition-summary-c-v2.svg\" alt=\"Mechanism of ethene with bromine to give 1,2-dibromoethane, with curly arrows and lone pairs\" width=\"1321\" height=\"1238\">\n        <\/div>\n      <\/section>\n\n      <section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>These questions secure the electrophile, conditions and naming points examiners test on halogen addition.<\/p>\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Why does a non-polar Br<sub>2<\/sub> molecule act as an electrophile?<\/h3>\n<p>As Br<sub>2<\/sub> approaches, the \u03c0 electrons repel the electron pair in the Br-Br bond. This induces a dipole, so the nearer bromine atom becomes Br\u03b4<sup>+<\/sup> and accepts an electron pair from the \u03c0 bond.<\/p>\n<\/div>\n<div class=\"ols-faq-item\">\n<h3>Why should the reaction be carried out away from UV light?<\/h3>\n<p>In UV light, bromine splits into radicals that can substitute hydrogen atoms in the molecule. At room temperature in the dark, the reaction stays as electrophilic addition across the C=C bond.<\/p>\n<\/div>\n<div class=\"ols-faq-item\">\n<h3>What forms when bromine adds to propene?<\/h3>\n<p>1,2-dibromopropane, CH<sub>3<\/sub>CHBrCH<sub>2<\/sub>Br. One bromine atom adds to each carbon of the original C=C bond. Both added atoms are the same, so, unlike HBr, there is only one possible product.<\/p>\n<\/div>\n<div class=\"ols-faq-item\">\n<h3>Can I write &#8220;dibromoethane&#8221; for the product from ethene?<\/h3>\n<p>Give the full name, 1,2-dibromoethane, because the two bromine atoms sit on different carbon atoms. 1,1-dibromoethane is a different compound and is not formed by this addition.<\/p>\n<\/div>\n<div class=\"ols-faq-item\">\n<h3>Does chlorine react with alkenes in the same way as bromine?<\/h3>\n<p>Yes. Cl<sub>2<\/sub> is polarised by the \u03c0 bond in the same way and adds across the C=C bond by electrophilic addition. Ethene gives 1,2-dichloroethane, another dihalogenoalkane.<\/p>\n<\/div>\n<\/div>\n<\/section>\n\n      <!-- author card (moved to the end, 10 Oct 2026) -->\n      <div class=\"ols-author\">\n\n    <img decoding=\"async\"\n      class=\"ols-author-avatar-img\"\n      src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/Author-Profile.jpeg\"\n      alt=\"Dr. Mohammed Al-Fatah\"\n    >\n\n    <div class=\"ols-author-content\">\n\n      <h2 class=\"ols-author-title\">\n        Written by: Dr. Mohammed Al-Fatah\n      <\/h2>\n\n      <p class=\"ols-author-description\">\n        Chemistry specialist revision notes for A Level Chemistry.\n      <\/p>\n\n      <a\n        class=\"ols-linkedin-pill\"\n        href=\"https:\/\/www.linkedin.com\/in\/doctormohammedfatah\/\"\n        target=\"_blank\"\n        rel=\"noopener noreferrer\"\n      >\n        <svg class=\"ols-linkedin-icon\" viewBox=\"0 0 24 24\" fill=\"currentColor\" aria-hidden=\"true\">\n          <path d=\"M4.98 3.5C4.98 4.88 3.86 6 2.48 6S0 4.88 0 3.5 1.12 1 2.48 1s2.5 1.12 2.5 2.5zM.5 8h4V24h-4V8zm7 0h3.8v2.2h.1c.5-.9 1.8-2.2 3.9-2.2 4.2 0 5 2.8 5 6.4V24h-4v-7.6c0-1.8 0-4.2-2.6-4.2s-3 2-3 4v7.8h-4V8z\"\/>\n        <\/svg>\n        View LinkedIn Profile\n      <\/a>\n\n    <\/div>\n\n  <\/div>\n\n\n      <section class=\"ols-related-card\">\n        <h2>Related Alkene Revision Notes<\/h2>\n        <p>Continue through Topic 5 by linking alkene reactions to electrophilic addition, hydrogen halide addition and polymerisation.<\/p>\n\n        <div class=\"ols-related-grid\">\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/electrophilic-addition-mechanism\/\">Electrophilic Addition Mechanism<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/\">Hydrogen Halide<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/polymerisation-reactions\/\">Polymerisation Reactions<\/a>\n        <\/div>\n      <\/section>\n\n      <section class=\"ols-attribution-card ols-attr-std\" aria-label=\"Copyright notice\">\n<style id=\"ols-attribution-card-css\">.ols-attribution-card.ols-attr-std{background:linear-gradient(135deg,#ffffff,#f8fbff);border:1px solid rgba(28,36,75,0.14);border-radius:28px;box-shadow:0 18px 45px rgba(28,36,75,0.10);padding:34px;margin:24px 0;overflow:hidden;font-family:Poppins,Arial,sans-serif;box-sizing:border-box}.ols-attribution-card.ols-attr-std *{box-sizing:border-box}.ols-attribution-card.ols-attr-std p{margin:0;font-size:14px;line-height:1.65;font-weight:300;color:#667085}.ols-attribution-card.ols-attr-std strong{font-weight:700;color:#1C244B}@media (max-width:760px){.ols-attribution-card.ols-attr-std{padding:24px 18px;border-radius:22px}}<\/style>\n<p><strong>Copyright notice:<\/strong> This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.<\/p>\n<\/section>\n    <\/main>\n<\/section>\n<script type=\"application\/ld+json\">\n{\n    \"@context\": \"https:\/\/schema.org\",\n    \"@graph\": [\n        {\n            \"@type\": \"Organization\",\n            \"@id\": \"https:\/\/www.onlinelearningsystem.net\/#organization\",\n            \"name\": \"Online Learning System\",\n            \"url\": \"https:\/\/www.onlinelearningsystem.net\/\",\n            \"logo\": {\n                \"@type\": \"ImageObject\",\n                \"url\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2025\/07\/a-level-chemistry-virtual-lesson-tutor.webp\"\n            }\n        },\n        {\n            \"@type\": \"WebSite\",\n            \"@id\": \"https:\/\/www.onlinelearningsystem.net\/#website\",\n            \"url\": \"https:\/\/www.onlinelearningsystem.net\/\",\n            \"name\": \"Online Learning System\",\n            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\"acceptedAnswer\": {\n                        \"@type\": \"Answer\",\n                        \"text\": \"As Br2 approaches, the \u03c0 electrons repel the electron pair in the Br-Br bond. This induces a dipole, so the nearer bromine atom becomes Br\u03b4+ and accepts an electron pair from the \u03c0 bond.\"\n                    }\n                },\n                {\n                    \"@type\": \"Question\",\n                    \"name\": \"Why should the reaction be carried out away from UV light?\",\n                    \"acceptedAnswer\": {\n                        \"@type\": \"Answer\",\n                        \"text\": \"In UV light, bromine splits into radicals that can substitute hydrogen atoms in the molecule. At room temperature in the dark, the reaction stays as electrophilic addition across the C=C bond.\"\n                    }\n                },\n                {\n                    \"@type\": \"Question\",\n                    \"name\": \"What forms when bromine adds to propene?\",\n                    \"acceptedAnswer\": {\n                        \"@type\": \"Answer\",\n                        \"text\": \"1,2-dibromopropane, CH3CHBrCH2Br. One bromine atom adds to each carbon of the original C=C bond. Both added atoms are the same, so, unlike HBr, there is only one possible product.\"\n                    }\n                },\n                {\n                    \"@type\": \"Question\",\n                    \"name\": \"Can I write \"dibromoethane\" for the product from ethene?\",\n                    \"acceptedAnswer\": {\n                        \"@type\": \"Answer\",\n                        \"text\": \"Give the full name, 1,2-dibromoethane, because the two bromine atoms sit on different carbon atoms. 1,1-dibromoethane is a different compound and is not formed by this addition.\"\n                    }\n                },\n                {\n                    \"@type\": \"Question\",\n                    \"name\": \"Does chlorine react with alkenes in the same way as bromine?\",\n                    \"acceptedAnswer\": {\n                        \"@type\": \"Answer\",\n                        \"text\": \"Yes. Cl2 is polarised by the \u03c0 bond in the same way and adds across the C=C bond by electrophilic addition. Ethene gives 1,2-dichloroethane, another dihalogenoalkane.\"\n                    }\n                }\n            ]\n        }\n    ]\n}\n<\/script>\n\n\n\n<p class=\"wp-block-paragraph\"><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Revision Notes \/ A Level Chemistry \/ Edexcel International \/ Topic 5 Alkenes \/ Reactions of Alkenes: Halogens Reactions of Alkenes with Halogens A concise revision guide to the addition reaction of alkenes with bromine or chlorine, including the functional group change, reaction conditions and the electrophilic addition mechanism. 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