{"id":3433,"date":"2026-05-13T07:33:03","date_gmt":"2026-05-13T06:33:03","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/?page_id=3433"},"modified":"2026-10-19T20:48:51","modified_gmt":"2026-10-19T19:48:51","slug":"hydrogen-halide","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/reactions-of-alkenes\/hydrogen-halide\/","title":{"rendered":"Hydrogen Halide"},"content":{"rendered":"\n<section class=\"ols-revision-page ols-alkene-hydrogen-halide-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, 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.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    if (!sidebar) return false;\r\n\r\n    var currentPath = normalisePath(window.location.pathname);\r\n    var links = sidebar.querySelectorAll('a[href]');\r\n    var matched = null;\r\n    var matchedLength = 0;\r\n\r\n    sidebar.querySelectorAll('.active, .active-main, .parent-active').forEach(function(item) {\r\n      item.classList.remove('active', 'active-main', 'parent-active');\r\n    });\r\n\r\n    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of Alkenes with Hydrogen Halides<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to the addition reaction of alkenes with hydrogen halides, including HBr and HCl, the electrophilic addition mechanism, carbocation formation and why one product may form in greater amount with unsymmetrical alkenes.<\/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 2: Hydrogen Halide<\/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 covalent bonding, acids and the addition reactions of alkenes.<\/p>\n<ul class=\"ols-list\">\n<li>A covalent bond is a shared pair of electrons between two atoms.<\/li>\n<li>Acids release <strong>hydrogen ions<\/strong>, H<sup>+<\/sup>, in solution; hydrogen bromide and hydrogen chloride dissolve in water to form acids.<\/li>\n<li>Alkenes undergo <strong>addition<\/strong> reactions: 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: Covalent Molecules and Acids<\/h2>\n<p>Four quick GCSE questions on covalent bonds, acids and addition before you meet the hydrogen halides.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"1270\"><\/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 hydrogen halides such as <strong>hydrogen bromide, HBr<\/strong>, or <strong>hydrogen chloride, HCl<\/strong>, in an <strong>addition reaction<\/strong>.<\/p>\n        <p>The C=C double bond opens. The hydrogen atom adds to one carbon atom from the original double bond, and the halogen atom adds to the other carbon atom.<\/p>\n        <p>The functional group changes from an <strong>alkene<\/strong> to a <strong>halogenoalkane<\/strong>.<\/p>\n\n        <div class=\"ols-reaction-summary\">\n          <div class=\"ols-summary-item\"><span>Functional group change<\/span><strong>alkene \u2192 halogenoalkane<\/strong><\/div>\n          <div class=\"ols-summary-item\"><span>Reagent<\/span><strong>HBr or HCl<\/strong><\/div>\n          <div class=\"ols-summary-item\"><span>Conditions<\/span><strong>Room temperature<\/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> This is addition because two atoms are added across the C=C double bond and the alkene becomes a saturated halogenoalkane.<\/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\/t5-cd-03-but2enehbr.svg\" alt=\"But-2-ene and HBr giving 2-bromobutane, with the curly-arrow mechanism through a secondary carbocation\" width=\"1394\" height=\"732\">\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: Formula and Reaction Type<\/h2>\n<p>Work out the product of an addition that this page does not show.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-30\" class=\"h5p-iframe\" data-content-id=\"30\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hydrogen Halide Addition MCQ: Hex-1-ene and Hydrogen Iodide\"><\/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 HBr and HCl Act as Electrophiles<\/h2>\n        <\/div>\n\n        <p>In hydrogen bromide, bromine is more electronegative than hydrogen. This means the H-Br bond is <strong>polar<\/strong>, with an uneven distribution of electron density.<\/p>\n        <p>The hydrogen atom carries a partial positive charge, <strong>H\u03b4<sup>+<\/sup><\/strong>, while the bromine atom carries a partial negative charge, <strong>Br\u03b4<sup>&#8211;<\/sup><\/strong>.<\/p>\n        <p>The electron-rich \u03c0 bond in the alkene is attracted to <strong>H\u03b4<sup>+<\/sup><\/strong>. The hydrogen atom acts as the <strong>electrophile<\/strong> because it accepts electron density from the alkene \u03c0 bond.<\/p>\n\n        <div class=\"ols-definition-box\">\n          <div class=\"ols-definition-circle\">H\u03b4<sup>+<\/sup><\/div>\n          <p><strong>Electrophile:<\/strong> An electron-pair acceptor. In this reaction, H\u03b4<sup>+<\/sup> is attracted to the electron-rich \u03c0 bond and begins the electrophilic addition mechanism.<\/p>\n        <\/div>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Exam focus:<\/strong> The electrophile is the H\u03b4<sup>+<\/sup> atom of HBr, not Br<sup>&#8211;<\/sup>. The \u03c0 bond attacks the hydrogen atom first.<\/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-25-hbrelectrophile.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-25-hbrelectrophile.jpg\" alt=\"Tug-of-war picture of the polar H\u2013Br bond with H\u03b4+ and Br\u03b4\u2212, the H end attracted to the \u03c0 cloud, and heterolytic fission to Br\u207b\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-img-25-hbrelectrophile.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>Bromine pulls the bonding pair towards itself, so the H\u03b4+ end of H\u2013Br is attracted to the \u03c0 bond 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: Which End Attacks?<\/h2>\n<p>Decide whether the statement about hydrogen iodide is true or false.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-745\" class=\"h5p-iframe\" data-content-id=\"745\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hydrogen Halide Addition True or False: The Slightly Positive End of H-I\"><\/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>The Electrophilic Addition Mechanism<\/h2>\n        <\/div>\n\n        <p>The \u03c0 bond donates electron density to <strong>H\u03b4<sup>+<\/sup><\/strong>, forming a new C-H bond. At the same time, the H-Br bond breaks by <strong>heterolytic fission<\/strong>, producing <strong>Br<sup>&#8211;<\/sup><\/strong>.<\/p>\n        <p>After H<sup>+<\/sup> has added, one carbon atom from the original C=C double bond becomes positively charged. This produces a <strong>carbocation intermediate<\/strong>.<\/p>\n        <p>The bromide ion, <strong>Br<sup>&#8211;<\/sup><\/strong>, then donates a lone pair to the carbocation, forming the final <strong>bromoalkane<\/strong> product.<\/p>\n\n        <div class=\"ols-mechanism-steps\">\n          <div class=\"ols-step-card\">\n            <h3>Step 1: The \u03c0 bond attacks H\u03b4<sup>+<\/sup><\/h3>\n            <p>The alkene \u03c0 bond is electron-rich, so it is attracted to the partially positive hydrogen atom in HBr.<\/p>\n          <\/div>\n\n          <div class=\"ols-step-card\">\n            <h3>Step 2: H-Br breaks heterolytically<\/h3>\n            <p>Both electrons from the H-Br bond move to bromine, forming Br<sup>&#8211;<\/sup> and leaving a carbocation on the organic molecule.<\/p>\n          <\/div>\n\n          <div class=\"ols-step-card\">\n            <h3>Step 3: Br<sup>&#8211;<\/sup> attacks the carbocation<\/h3>\n            <p>The bromide ion donates a lone pair to the positively charged carbon, forming a new C-Br bond.<\/p>\n          <\/div>\n        <\/div>\n\n        <div class=\"ols-figure-card\">\n<div class=\"ols-figure-image\"><img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-cd-03b-but2enehbrmech-v2.svg\" alt=\"But-2-ene and HBr giving 2-bromobutane, with the curly-arrow mechanism through a secondary carbocation\" width=\"1529\" height=\"1311\"><\/div>\n<div class=\"ols-figure-caption\"><p>The \u03c0 bond reacts with H\u03b4+ first, then Br- attacks the carbocation intermediate to form the halogenoalkane.<\/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: Run the Mechanism<\/h2>\n<p>Follow hydrogen iodide and oct-4-ene through the mechanism, one step at a time.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-31\" class=\"h5p-iframe\" data-content-id=\"31\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hydrogen Halide Addition Quick-Fire: Hydrogen Iodide with Oct-4-ene\"><\/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>Unsymmetrical Alkenes Can Form More Than One Product<\/h2>\n        <\/div>\n\n        <p>When an alkene is <strong>unsymmetrical<\/strong>, HBr or HCl can add in two different ways. This is because H<sup>+<\/sup> can add to either carbon atom of the C=C double bond.<\/p>\n        <p>Each possible route forms a different <strong>carbocation intermediate<\/strong>. These carbocations can have different stabilities, so the products are not always formed in equal amounts.<\/p>\n\n        <div class=\"ols-pathway-grid\">\n          <div class=\"ols-pathway-card\">\n            <h3>Route 1<\/h3>\n            <p>H<sup>+<\/sup> adds to one carbon atom of the C=C bond, forming one possible carbocation intermediate.<\/p>\n          <\/div>\n\n          <div class=\"ols-pathway-card\">\n            <h3>Route 2<\/h3>\n            <p>H<sup>+<\/sup> adds to the other carbon atom of the C=C bond, forming a different carbocation intermediate.<\/p>\n          <\/div>\n        <\/div>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Key idea:<\/strong> The major product usually forms from the pathway that produces the more stable carbocation intermediate.<\/p>\n        <\/div>\n\n        \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: Compare the Two Routes<\/h2>\n<p>Drag the words and numbers into place to compare the two carbocations.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-29\" class=\"h5p-iframe\" data-content-id=\"29\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hydrogen Halide Addition Drag: The Two Routes Open to 2-Methylbut-1-ene\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">6<\/div>\n          <h2>Why One Product Forms in Greater Amount<\/h2>\n        <\/div>\n\n        <p>A <strong>carbocation<\/strong> is an organic ion with a positively charged carbon atom. Carbocations are stabilised when alkyl groups are attached to the positively charged carbon.<\/p>\n        <p>Alkyl groups are electron-releasing. They help spread out and reduce the positive charge, making the carbocation less reactive and more stable.<\/p>\n        <p>Because the more stable carbocation is formed more readily, the reaction proceeds mainly through that pathway. This leads to a higher yield of the corresponding product.<\/p>\n\n        <div class=\"ols-table-wrap\">\n          <table class=\"ols-table\">\n            <thead>\n              <tr>\n                <th>Carbocation type<\/th>\n                <th>Alkyl groups attached to C<sup>+<\/sup><\/th>\n                <th>Relative stability<\/th>\n              <\/tr>\n            <\/thead>\n            <tbody>\n              <tr>\n                <td><strong>Primary<\/strong><\/td>\n                <td>One alkyl group<\/td>\n                <td>Less stable<\/td>\n              <\/tr>\n              <tr>\n                <td><strong>Secondary<\/strong><\/td>\n                <td>Two alkyl groups<\/td>\n                <td>More stable<\/td>\n              <\/tr>\n              <tr>\n                <td><strong>Tertiary<\/strong><\/td>\n                <td>Three alkyl groups<\/td>\n                <td>Most stable<\/td>\n              <\/tr>\n            <\/tbody>\n          <\/table>\n        <\/div>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Exam focus:<\/strong> Explain the major product by comparing carbocation stability, not just by stating which product forms.<\/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\/t5-cd-05-carbocationstability.svg\" alt=\"Primary, secondary and tertiary carbocations with arrowheads showing electron release towards C\u207a; stability increases\" width=\"1520\" height=\"1033\">\n          <\/div>\n          <div class=\"ols-figure-caption\">\n            <p>Alkyl groups stabilise carbocations by releasing electron density towards the positively charged carbon.<\/p>\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-24-majorproduct.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-24-majorproduct.jpg\" alt=\"Decision path for an unsymmetrical alkene plus HX: two possible carbocations, count alkyl groups, the more stable one gives the major product\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-img-24-majorproduct.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>The major product comes from the more stable carbocation: the one with more alkyl groups attached to the positive carbon.<\/p><\/div>\n<\/div>\n<\/article>\n<!-- Copyright (c) 2026 Dr. Mohammed Al-Fatah, onlinelearningsystem.net. 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text-align:center; font-size:11px; font-style:italic; color:#aab0c0;\n}\n.ols-cc-hbrp-001 a{color:#aab0c0; text-decoration:none;}\n.ols-cc-hbrp-001 a:hover{text-decoration:underline;}\n.ols-hbrp-001 .hbrp-seg button{display:inline-flex; align-items:center;}\n.ols-hbrp-001 .hbrp-seg button .hbrp-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-hbrp-001 .hbrp-seg button[aria-pressed=\"true\"] .hbrp-n{background:#ffffff; color:#2563eb;}\n<\/style>\n\n<div class=\"hbrp-head\">\n  <h2 class=\"hbrp-title\">HBr and Propene: Major and Minor Products<\/h2>\n  <p class=\"hbrp-sub\">Choose a step to watch HBr add to propene by two routes, compare the two carbocations and see why 2-bromopropane is the major product.<\/p>\n<\/div>\n\n<div class=\"hbrp-stage\" id=\"hbrpStage\">\n  <canvas class=\"hbrp-canvas\" id=\"hbrpCanvas\"><\/canvas>\n  <div class=\"hbrp-overlay\" id=\"hbrpOverlay\"><\/div>\n  <div class=\"hbrp-caption\" id=\"hbrpCaption\"><\/div>\n  <div class=\"hbrp-hint\" id=\"hbrpHint\">Drag to rotate<\/div>\n  <div class=\"hbrp-badges\" id=\"hbrpBadges\"><\/div>\n<\/div>\n\n<div class=\"hbrp-controls\">\n  <div class=\"hbrp-row\">\n    <span class=\"hbrp-rowlab\">Step<\/span>\n    <div class=\"hbrp-seg\" role=\"group\" aria-label=\"Step\">\n      <button type=\"button\" data-step=\"1\" aria-pressed=\"true\"><span class=\"hbrp-n\">1<\/span>Propene meets HBr<\/button>\n      <button type=\"button\" data-step=\"2\" aria-pressed=\"false\"><span class=\"hbrp-n\">2<\/span>Two possible carbocations<\/button>\n      <button type=\"button\" data-step=\"3\" aria-pressed=\"false\"><span class=\"hbrp-n\">3<\/span>Carbocation stability<\/button>\n      <button type=\"button\" data-step=\"4\" aria-pressed=\"false\"><span class=\"hbrp-n\">4<\/span>Major product<\/button>\n      <button type=\"button\" data-step=\"5\" aria-pressed=\"false\"><span class=\"hbrp-n\">5<\/span>Minor product<\/button>\n    <\/div>\n  <\/div>\n\n  <div class=\"hbrp-row\">\n    <span class=\"hbrp-rowlab\">View<\/span>\n    <button type=\"button\" class=\"hbrp-pill\" id=\"hbrpResetV\">Reset view<\/button>\n  <\/div>\n<\/div>\n\n<div class=\"hbrp-info\">\n  <h3 id=\"hbrpInfoTitle\"><\/h3>\n  <p id=\"hbrpInfoText\"><\/p>\n  <div class=\"hbrp-facts\" id=\"hbrpFacts\"><\/div>\n  <div class=\"hbrp-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 molecular orbital (\u03c0 bond)<\/span>\n    <span><i class=\"k-ball\" style=\"background:#f3e2b4\"><\/i>Empty p orbital (C\u207a)<\/span>\n    <span><i class=\"\" style=\"background:#2f5f9a\"><\/i>Electron density pushed by an alkyl group<\/span>\n    <span><i class=\"k-ball\" style=\"background:rgba(232,96,60,0.7)\"><\/i>Positive charge<\/span>\n  <\/div>\n<\/div>\n\n<script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/hbr-propene-carbocations.js?v=20261019g\"><\/script>\n<\/section>\n\n<p class=\"ols-cc-hbrp-001\">&copy; Dr. Mohammed Al-Fatah &#8211; <a href=\"https:\/\/www.onlinelearningsystem.net\" target=\"_blank\" rel=\"noopener\">onlinelearningsystem.net<\/a><\/p>\n\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: Explain the Major Product<\/h2>\n<p>Write a short explanation, then compare it with the mark points and the model answer.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-32\" class=\"h5p-iframe\" data-content-id=\"32\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hydrogen Halide Addition Explain: The Major Product from 4-Methylpent-1-ene\"><\/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: Pick the Accurate Statement<\/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-746\" class=\"h5p-iframe\" data-content-id=\"746\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hydrogen Halide Addition Summary: Major, Minor and What Decides Them\"><\/iframe><\/div><\/div>\n<\/section>\n\n       \n\n      <section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>These questions secure the electrophile and major-product reasoning that HX questions reward.<\/p>\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Why does the hydrogen atom, not the bromine atom, react with the \u03c0 bond first?<\/h3>\n<p>Bromine is more electronegative than hydrogen, so H-Br is polar with H\u03b4<sup>+<\/sup>. The electron-rich \u03c0 bond is attracted to H\u03b4<sup>+<\/sup> and donates its electron pair to it. Br<sup>&#8211;<\/sup> only attacks once the carbocation has formed.<\/p>\n<\/div>\n<div class=\"ols-faq-item\">\n<h3>Does but-2-ene give a major and a minor product with HBr?<\/h3>\n<p>No. But-2-ene is symmetrical, so adding H<sup>+<\/sup> to either carbon of the C=C bond gives the same secondary carbocation. Only one product forms: 2-bromobutane.<\/p>\n<\/div>\n<div class=\"ols-faq-item\">\n<h3>Why is a tertiary carbocation more stable than a primary one?<\/h3>\n<p>A tertiary carbocation has three alkyl groups on the positive carbon, and each releases electron density towards it. This spreads out and reduces the positive charge more than the single alkyl group of a primary carbocation.<\/p>\n<\/div>\n<div class=\"ols-faq-item\">\n<h3>Is the minor product formed at all?<\/h3>\n<p>Yes, in a small amount. The less stable carbocation still forms some of the time, so a mixture results. For an unsymmetrical alkene, name both the major and the minor product.<\/p>\n<\/div>\n<div class=\"ols-faq-item\">\n<h3>Does HCl add to alkenes in the same way as HBr?<\/h3>\n<p>Yes. H-Cl is polar with H\u03b4<sup>+<\/sup>, so it adds by the same electrophilic addition mechanism through a carbocation. Ethene and HCl give chloroethane, a halogenoalkane.<\/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 hydrogen halide addition to halogen addition, bromine water and the general electrophilic addition mechanism.<\/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\/\">Halogens<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/\">Bromine Water<\/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  \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            \"publisher\": 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