{"id":3148,"date":"2026-05-04T17:48:38","date_gmt":"2026-05-04T16:48:38","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/?page_id=3148"},"modified":"2026-10-19T20:43:23","modified_gmt":"2026-10-19T19:43:23","slug":"electrophilic-addition-mechanism","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/electrophilic-addition-mechanism\/","title":{"rendered":"Electrophilic Addition Mechanism"},"content":{"rendered":"\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     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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  <div class=\"ols-revision-layout\">\n    <main class=\"ols-main\">\n      <nav class=\"ols-breadcrumbs\" aria-label=\"Breadcrumb\">\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/\">Revision Notes<\/a> \/\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/\">A Level Chemistry<\/a> \/\n        <a 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>Electrophilic Addition Mechanism<\/span>\n      <\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Electrophilic Addition Mechanism<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to addition reactions of alkenes, electrophiles, electron-rich \u03c0 bonds and why the carbon-carbon double bond is the reactive centre in alkene chemistry.<\/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\">Level: A Level Chemistry<\/div>\n        <\/div>\n      <\/header>\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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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\n\n\n      \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: What Alkenes Do<\/h2>\n<p>Three quick questions on the reactions of alkenes that you met at GCSE.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"735\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">1<\/div>\n          <h2>Addition Reactions of Alkenes<\/h2>\n        <\/div>\n\n        <p>An <strong>addition reaction<\/strong> is a reaction in which two reactant molecules combine to form a single product molecule.<\/p>\n        <p>Alkenes commonly undergo addition reactions because their <strong>carbon-carbon double bond<\/strong> can be broken and replaced by new single bonds.<\/p>\n        <p>This is why the C=C bond is the functional group that controls the characteristic reactions of alkenes.<\/p>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Key idea:<\/strong> In an alkene addition reaction, atoms add across the C=C double bond and only one organic product molecule is formed.<\/p>\n        <\/div>\n\n        <div class=\"ols-figure-card\">\n          <div class=\"ols-figure-image\" role=\"img\" aria-label=\"Addition reaction diagram showing an alkene double bond opening to form a saturated product\">\n            <svg viewBox=\"0 0 900 360\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\">\n              <rect width=\"900\" height=\"360\" rx=\"28\" fill=\"#ffffff\"\/>\n              <rect x=\"38\" y=\"42\" width=\"260\" height=\"236\" rx=\"24\" fill=\"#eef4ff\" stroke=\"#d8e5ff\"\/>\n              <rect x=\"600\" y=\"42\" width=\"260\" height=\"236\" rx=\"24\" fill=\"#fff7f7\" stroke=\"#f4dada\"\/>\n              <text x=\"168\" y=\"88\" text-anchor=\"middle\" font-family=\"Poppins, Arial\" font-size=\"26\" font-weight=\"800\" fill=\"#1C244B\">Alkene<\/text>\n              <text x=\"730\" y=\"88\" text-anchor=\"middle\" font-family=\"Poppins, Arial\" font-size=\"26\" font-weight=\"800\" fill=\"#1C244B\">Addition product<\/text>\n              <line x1=\"118\" y1=\"166\" x2=\"218\" y2=\"166\" stroke=\"#1C244B\" stroke-width=\"10\" stroke-linecap=\"round\"\/>\n              <line x1=\"118\" y1=\"188\" x2=\"218\" y2=\"188\" stroke=\"#2563eb\" stroke-width=\"10\" stroke-linecap=\"round\"\/>\n              <text x=\"168\" y=\"235\" text-anchor=\"middle\" font-family=\"Poppins, Arial\" font-size=\"28\" font-weight=\"700\" fill=\"#1C244B\">C=C<\/text>\n              <line x1=\"365\" y1=\"176\" x2=\"535\" y2=\"176\" stroke=\"#1C244B\" stroke-width=\"6\" stroke-linecap=\"round\"\/>\n              <polygon points=\"535,176 510,160 510,192\" fill=\"#1C244B\"\/>\n              <text x=\"450\" y=\"138\" text-anchor=\"middle\" font-family=\"Poppins, Arial\" font-size=\"23\" font-weight=\"700\" fill=\"#2563eb\">two reactants<\/text>\n              <text x=\"450\" y=\"225\" text-anchor=\"middle\" font-family=\"Poppins, Arial\" font-size=\"23\" font-weight=\"700\" fill=\"#2563eb\">one product<\/text>\n              <line x1=\"680\" y1=\"176\" x2=\"780\" y2=\"176\" stroke=\"#1C244B\" stroke-width=\"10\" stroke-linecap=\"round\"\/>\n              <circle cx=\"660\" cy=\"176\" r=\"22\" fill=\"#2563eb\"\/>\n              <circle cx=\"800\" cy=\"176\" r=\"22\" fill=\"#2563eb\"\/>\n              <text x=\"660\" y=\"184\" text-anchor=\"middle\" font-family=\"Poppins, Arial\" font-size=\"18\" font-weight=\"800\" fill=\"#ffffff\">X<\/text>\n              <text x=\"800\" y=\"184\" text-anchor=\"middle\" font-family=\"Poppins, Arial\" font-size=\"18\" font-weight=\"800\" fill=\"#ffffff\">Y<\/text>\n              <text x=\"730\" y=\"235\" text-anchor=\"middle\" font-family=\"Poppins, Arial\" font-size=\"25\" font-weight=\"700\" fill=\"#1C244B\">new single bonds<\/text>\n            <\/svg>\n          <\/div>\n          <div class=\"ols-figure-caption\">\n            <h3>Addition across a double bond<\/h3>\n            <p>The double bond opens up, allowing new atoms or groups to attach to the two carbon atoms from the original C=C bond.<\/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: Addition or Not?<\/h2>\n<p>Decide quickly whether each equation shows an addition reaction.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-24\" class=\"h5p-iframe\" data-content-id=\"24\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Electrophilic Addition Quick-Fire: Addition or Not Addition\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">2<\/div>\n          <h2>Why the \u03c0 Bond Reacts<\/h2>\n        <\/div>\n\n        <p>In an alkene, the <strong>\u03c0 bond is weaker than the \u03c3 bond<\/strong>, so it requires less energy to break during a reaction.<\/p>\n        <p>The \u03c0 bond also contains a region of <strong>high electron density<\/strong>. This exposed electron density is especially attractive to electron-seeking species.<\/p>\n        <p>Because the \u03c0 bond is both weaker and more exposed than a \u03c3 bond, it is particularly susceptible to attack by electrophiles.<\/p>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Exam focus:<\/strong> Link alkene reactivity to the exposed, electron-rich \u03c0 bond rather than simply saying that alkenes have a double bond.<\/p>\n        <\/div>\n\n        <div class=\"ols-figure-card\">\n          <div class=\"ols-figure-image\" role=\"img\" aria-label=\"Pi bond electron density diagram showing electron-rich regions above and below the carbon-carbon bond\">\n            <svg viewBox=\"0 0 900 390\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\">\n              <rect width=\"900\" height=\"390\" rx=\"28\" fill=\"#ffffff\"\/>\n              <ellipse cx=\"450\" cy=\"124\" rx=\"250\" ry=\"62\" fill=\"#2563eb\" opacity=\"0.18\"\/>\n              <ellipse cx=\"450\" cy=\"266\" rx=\"250\" ry=\"62\" fill=\"#2563eb\" opacity=\"0.18\"\/>\n              <ellipse cx=\"450\" cy=\"124\" rx=\"172\" ry=\"42\" fill=\"#2563eb\" opacity=\"0.26\"\/>\n              <ellipse cx=\"450\" cy=\"266\" rx=\"172\" ry=\"42\" fill=\"#2563eb\" opacity=\"0.26\"\/>\n              <circle cx=\"355\" cy=\"195\" r=\"40\" fill=\"#1C244B\"\/>\n              <circle cx=\"545\" cy=\"195\" r=\"40\" fill=\"#1C244B\"\/>\n              <text x=\"355\" y=\"205\" text-anchor=\"middle\" font-family=\"Poppins, Arial\" font-size=\"30\" font-weight=\"800\" fill=\"#ffffff\">C<\/text>\n              <text x=\"545\" y=\"205\" text-anchor=\"middle\" font-family=\"Poppins, Arial\" font-size=\"30\" font-weight=\"800\" fill=\"#ffffff\">C<\/text>\n              <line x1=\"395\" y1=\"183\" x2=\"505\" y2=\"183\" stroke=\"#1C244B\" stroke-width=\"10\" stroke-linecap=\"round\"\/>\n              <line x1=\"395\" y1=\"207\" x2=\"505\" y2=\"207\" stroke=\"#2563eb\" stroke-width=\"10\" stroke-linecap=\"round\"\/>\n              <text x=\"450\" y=\"60\" text-anchor=\"middle\" font-family=\"Poppins, Arial\" font-size=\"28\" font-weight=\"800\" fill=\"#1C244B\">exposed \u03c0 electron density<\/text>\n              <text x=\"450\" y=\"350\" text-anchor=\"middle\" font-family=\"Poppins, Arial\" font-size=\"24\" font-weight=\"700\" fill=\"#667085\">weaker than the \u03c3 bond and easier to break<\/text>\n              <path d=\"M712 112 C660 126, 632 154, 602 180\" fill=\"none\" stroke=\"#c81e1e\" stroke-width=\"6\" stroke-linecap=\"round\"\/>\n              <polygon points=\"602,180 612,154 626,177\" fill=\"#c81e1e\"\/>\n              <text x=\"744\" y=\"105\" font-family=\"Poppins, Arial\" font-size=\"22\" font-weight=\"800\" fill=\"#c81e1e\">E\u207a<\/text>\n            <\/svg>\n          <\/div>\n          <div class=\"ols-figure-caption\">\n            <h3>Electron-rich \u03c0 bond<\/h3>\n            <p>The \u03c0 bond forms exposed electron density above and below the molecular plane, making the alkene attractive to electrophiles.<\/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-mv-02-displayed.svg\" 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-mv-02-displayed.svg\" alt=\"Displayed formulae of ethene, ethane\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-mv-02-displayed.svg\">\n<\/a>\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: A New Electrophile<\/h2>\n<p>Explain why a different reagent, iodine monochloride, attacks the \u03c0 bond of but-1-ene.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-1061\" class=\"h5p-iframe\" data-content-id=\"1061\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Electrophilic Addition Drag: Iodine Monochloride Meets But-1-ene\"><\/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\">3<\/div>\n          <h2>Electrophiles<\/h2>\n        <\/div>\n\n        <p>An <strong>electrophile<\/strong> is an <strong>electron pair acceptor<\/strong>.<\/p>\n        <p>Electrophiles are attracted to regions of high electron density. In alkenes, this means the electrophile is drawn towards the electron-rich \u03c0 bond.<\/p>\n        <p>This attraction begins the electrophilic addition mechanism.<\/p>\n\n        <div class=\"ols-definition-box\">\n          <div class=\"ols-definition-circle\">E\u207a<\/div>\n          <p><strong>Electrophile:<\/strong> A species that accepts an electron pair from an electron-rich region during a chemical reaction.<\/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-20-electrophile.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-20-electrophile.jpg\" alt=\"Electron-rich \u03c0 cloud attracting three electron-pair acceptors: H\u03b4+ in H\u2013Br, Br\u03b4+ in Br\u2013Br and H\u207a from an acid, with the definition\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-img-20-electrophile.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>An electrophile is an electron-pair acceptor: a positive or \u03b4+ species attracted to the electron-rich \u03c0 bond.<\/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: Find the Electrophile<\/h2>\n<p>Identify the electrophile in a reagent that this page has not used.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-23\" class=\"h5p-iframe\" data-content-id=\"23\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Electrophilic Addition MCQ: The Electrophile in Hydrogen Chloride\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card\">\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>In electrophilic addition, the \u03c0 bond breaks and new single bonds are formed. The electrophile accepts an electron pair from the alkene.<\/p>\n        <p>At this stage, students must be careful with curly arrows. A curly arrow shows the movement of an <strong>electron pair<\/strong>, so the arrow should start from a bond or lone pair, not from a positive charge.<\/p>\n\n        <div class=\"ols-mechanism-grid\">\n          <div class=\"ols-mech-step\">\n            <h3>1. Attraction<\/h3>\n            <p>The electrophile is attracted to the high electron density in the \u03c0 bond.<\/p>\n          <\/div>\n          <div class=\"ols-mech-step\">\n            <h3>2. Bond breaking<\/h3>\n            <p>The \u03c0 bond breaks because it is weaker than the \u03c3 bond.<\/p>\n          <\/div>\n          <div class=\"ols-mech-step\">\n            <h3>3. New bonds form<\/h3>\n            <p>New single bonds form to give an addition product.<\/p>\n          <\/div>\n        <\/div>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Remember:<\/strong> Electrophilic addition happens because the \u03c0 bond is electron-rich, exposed and relatively easy to break.<\/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-21-threemoves.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-21-threemoves.jpg\" alt=\"Storyboard of electrophilic addition: attraction, heterolytic bond breaking to a carbocation, then the negative ion forms the second bond\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-img-21-threemoves.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>The electrophile is attracted to the \u03c0 bond, two bonds break heterolytically to give a carbocation, then the negative ion forms the second new bond.<\/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: Order the Mechanism<\/h2>\n<p>Put the events of an electrophilic addition you have not seen into the correct order.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-21\" class=\"h5p-iframe\" data-content-id=\"21\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Electrophilic Addition Sort: Hydrogen Chloride with But-2-ene\"><\/iframe><\/div><\/div>\n<\/section>\n\n\n      <article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">5<\/div>\n          <h2>Exam Link: Why Alkenes React This Way<\/h2>\n        <\/div>\n\n        <p>Exam answers should connect three ideas: the alkene contains a C=C bond, the \u03c0 bond has high electron density, and electrophiles are attracted to this electron-rich region.<\/p>\n        <p>It is not enough to write that alkenes are reactive because they have a double bond. The explanation needs to identify the role of the \u03c0 bond and the electrophile.<\/p>\n\n        <div class=\"ols-table-wrap\">\n          <table class=\"ols-table\">\n            <thead>\n              <tr>\n                <th>Exam phrase<\/th>\n                <th>Why it matters<\/th>\n                <th>Common mistake to avoid<\/th>\n              <\/tr>\n            <\/thead>\n            <tbody>\n              <tr>\n                <td><strong>\u03c0 bond has high electron density<\/strong><\/td>\n                <td>Explains why an electrophile is attracted to the alkene.<\/td>\n                <td>Only saying that the double bond is reactive.<\/td>\n              <\/tr>\n              <tr>\n                <td><strong>Electrophile accepts an electron pair<\/strong><\/td>\n                <td>Links the definition of electrophile to the mechanism.<\/td>\n                <td>Calling the electrophile an electron donor.<\/td>\n              <\/tr>\n              <tr>\n                <td><strong>\u03c0 bond breaks and new single bonds form<\/strong><\/td>\n                <td>Explains why the reaction is an addition reaction.<\/td>\n                <td>Describing substitution instead of addition.<\/td>\n              <\/tr>\n            <\/tbody>\n          <\/table>\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: Explain Why the Alkene Reacts<\/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-740\" class=\"h5p-iframe\" data-content-id=\"740\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Electrophilic Addition Explain: Hex-3-ene Reacts but Hexane Does Not\"><\/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: Hydrogen Iodide and Cyclohexene<\/h2>\n<p>In each round, pick the one accurate statement about this addition.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-736\" class=\"h5p-iframe\" data-content-id=\"736\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Electrophilic Addition Summary: Hydrogen Iodide with Cyclohexene\"><\/iframe><\/div><\/div>\n<\/section>\n\n      \n\n      <section class=\"ols-infographic-full\" aria-label=\"Electrophilic addition mechanism visual summary\">\n        <div class=\"ols-infographic-full-frame\">\n          <img decoding=\"async\"\n            src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-cd-02-electrophilicaddition-v2.svg\"\n            alt=\"General electrophilic addition: \u03c0 bond attacks E\u207a, carbocation forms, then Nu\u207b adds; inset of \u03c0 electrons attracting the \u03b4+ end\" width=\"1515\" height=\"1061\"\n          >\n          <img decoding=\"async\"\n            src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/skel-21-electrophilic-addition-b-v2.svg\"\n            alt=\"General mechanism of electrophilic addition to ethene, through a carbocation, with a summary\" width=\"1514\" height=\"1275\"\n          >\n        <\/div>\n      <\/section>\n      <section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>These questions secure the curly-arrow and electrophile ideas that every alkene mechanism uses.<\/p>\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Does the first curly arrow start at the electrophile or at the C=C bond?<\/h3>\n<p>At the C=C bond. A curly arrow shows an electron pair moving, so it starts at the \u03c0 bond and points to the electrophile. An arrow drawn from a positive atom shows nothing moving and loses the mark.<\/p>\n<\/div>\n<div class=\"ols-faq-item\">\n<h3>Is an electrophile always a positive ion?<\/h3>\n<p>No. An electrophile is an electron pair acceptor. It can be a positive ion such as H<sup>+<\/sup>, or the \u03b4<sup>+<\/sup> atom of a polar or polarised molecule, such as H\u03b4<sup>+<\/sup> in HBr or Br\u03b4<sup>+<\/sup> in Br<sub>2<\/sub>.<\/p>\n<\/div>\n<div class=\"ols-faq-item\">\n<h3>Why is the carbocation called an intermediate and not a product?<\/h3>\n<p>It forms part-way through the mechanism and is used up straight away when the negative ion attacks it. It is never isolated, so it does not appear in the overall equation.<\/p>\n<\/div>\n<div class=\"ols-faq-item\">\n<h3>Why do alkenes react with electrophiles when alkanes do not?<\/h3>\n<p>Alkanes contain only \u03c3 bonds, with the electron density held between the nuclei. Alkenes have an exposed \u03c0 bond with high electron density above and below the plane. This attracts electrophiles and breaks more easily than a \u03c3 bond.<\/p>\n<\/div>\n<div class=\"ols-faq-item\">\n<h3>If a bond breaks, why is the reaction called addition?<\/h3>\n<p>Only the \u03c0 bond breaks. The \u03c3 bond stays, two new \u03c3 bonds form and all the atoms of the reagent end up in one product. Nothing is removed from the alkene, so it is addition, not substitution.<\/p>\n<\/div>\n<\/div>\n<\/section>\n\n      <!-- author card (moved to the end, 10 Oct 2026) -->\n      <style id=\"ols-author-card-css\">\n.ols-author {\n      display: flex;\n      align-items: center;\n      gap: 20px;\n      padding: 28px;\n      border-radius: 28px;\n      background: linear-gradient(135deg, #ffffff 0%, #f8fbff 100%);\n      border: 1px solid rgba(28, 36, 75, 0.12);\n      box-shadow: 0 18px 45px rgba(28, 36, 75, 0.10);\n      margin-top: 22px;\n    }\n.ols-author-avatar-img {\n      width: 92px;\n      height: 92px;\n      border-radius: 50%;\n      object-fit: cover;\n      object-position: center;\n      flex-shrink: 0;\n      border: 4px solid #ffffff;\n      box-shadow: 0 14px 32px rgba(28, 36, 75, 0.18), 0 0 0 1px rgba(28, 36, 75, 0.10);\n      transition: transform 0.25s ease, box-shadow 0.25s ease;\n    }\n.ols-author:hover .ols-author-avatar-img {\n      transform: scale(1.04);\n      box-shadow: 0 20px 42px rgba(28, 36, 75, 0.22), 0 0 0 1px rgba(28, 36, 75, 0.10);\n    }\n.ols-author-content { min-width: 0; }\n.ols-author-title {\n      margin: 0 0 4px;\n      font-size: clamp(24px, 3vw, 34px);\n      line-height: 1.15;\n      font-weight: 700;\n      color: var(--navy);\n      letter-spacing: -0.03em;\n    }\n.ols-author-description {\n      margin: 0;\n      font-size: 17px;\n      line-height: 1.7;\n      font-weight: 300;\n      color: var(--grey-text);\n    }\n.ols-linkedin-pill {\n      display: inline-flex;\n      align-items: center;\n      justify-content: center;\n      gap: 10px;\n      margin-top: 16px;\n      padding: 11px 18px;\n      border-radius: 999px;\n      background: linear-gradient(135deg, #0A66C2, #004182);\n      color: #ffffff;\n      text-decoration: none;\n      font-size: 14px;\n      line-height: 1;\n      font-weight: 700;\n      letter-spacing: 0.01em;\n      box-shadow: 0 10px 22px rgba(10, 102, 194, 0.24);\n      position: relative;\n      overflow: hidden;\n      transition: transform 0.22s ease, box-shadow 0.22s ease;\n    }\n.ols-linkedin-pill:hover {\n      transform: translateY(-3px) scale(1.03);\n      box-shadow: 0 16px 34px rgba(10, 102, 194, 0.34), 0 0 0 6px rgba(10, 102, 194, 0.10);\n    }\n.ols-linkedin-icon { width: 18px; height: 18px; display: block; flex-shrink: 0; }\n@media (max-width: 640px) { .ols-author { flex-direction: column; align-items: flex-start; } }\n<\/style>\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:<br><span>Dr. Mohammed Al-Fatah<\/span>\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 electrophilic addition to alkene bonding, geometric isomerism and reactions of alkenes.<\/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\/alkene-bonding\/\">Alkene Bonding<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/geometric-isomerism\/\">Geometric Isomerism<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/reactions-of-alkenes\/\">Reactions of Alkenes<\/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  <\/div>\n<\/section>\n\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 \/ Electrophilic Addition Mechanism Electrophilic Addition Mechanism A concise revision guide to addition reactions of alkenes, electrophiles, electron-rich \u03c0 bonds and why the carbon-carbon double bond is the reactive centre in alkene chemistry. Exam board: Edexcel International Topic 5: Alkenes Level: A [&hellip;]<\/p>\n","protected":false},"author":12,"featured_media":0,"parent":3113,"menu_order":2,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-3148","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-json\/wp\/v2\/pages\/3148","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-json\/wp\/v2\/users\/12"}],"replies":[{"embeddable":true,"href":"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-json\/wp\/v2\/comments?post=3148"}],"version-history":[{"count":0,"href":"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-json\/wp\/v2\/pages\/3148\/revisions"}],"up":[{"embeddable":true,"href":"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-json\/wp\/v2\/pages\/3113"}],"wp:attachment":[{"href":"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-json\/wp\/v2\/media?parent=3148"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}