{"id":10990,"date":"2026-09-26T22:01:26","date_gmt":"2026-09-26T21:01:26","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-15-halogen-compounds\/nucleophilic-substitution-reactions\/"},"modified":"2026-09-27T10:01:58","modified_gmt":"2026-09-27T09:01:58","slug":"nucleophilic-substitution-reactions","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-15-halogen-compounds\/nucleophilic-substitution-reactions\/","title":{"rendered":"Nucleophilic Substitution Reactions"},"content":{"rendered":"<script src=\"https:\/\/cdnjs.cloudflare.com\/ajax\/libs\/three.js\/r128\/three.min.js\"><\/script>\n\n<section class=\"ols-revision-page ols-nature-of-covalent-bonding-9ch0-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      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760px) {\n      .ols-title-card, .ols-note-card, .ols-h5p-card, .ols-related-card, .ols-faq-card, .ols-attribution-card { padding: 24px 18px; border-radius: 22px; }\n      .ols-note-title { align-items: flex-start; }\n      .ols-related-grid, .ols-course-cta-features { grid-template-columns: 1fr; }\n      .ols-figure-card { border-radius: 22px; }\n      .ols-figure-image { min-height: 210px; padding: 14px; }\n      .ols-figure-caption { padding: 16px; }\n      .ols-table-wrap { border: none; background: transparent; }\n      .ols-table, .ols-table thead, .ols-table tbody, .ols-table th, .ols-table td, .ols-table tr { display: block; width: 100%; }\n      .ols-table { border-collapse: separate; border-spacing: 0; }\n      .ols-table thead { display: none; }\n      .ols-table tr { margin-bottom: 14px; border: 1px solid var(--border); border-radius: 18px; overflow: hidden; background: #ffffff; box-shadow: var(--inner-shadow); }\n      .ols-table td { border-bottom: 1px solid 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background: linear-gradient(135deg, #ffffff 0%, #fff8e8 100%); }\n    .ols-h5p-recap .ols-h5p-kicker { background: #fdf0d2; color: #8a5a00; }\n    @media (max-width: 760px) { .ols-h5p-card.ols-h5p-inline { padding: 20px 16px; } }\n<\/style>\n\n  <aside class=\"ols-sidebar\">\n  <div class=\"ols-sidebar-header\">\n    <h3>Revision Notes<\/h3>\n    <p>A Level Chemistry<\/p>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Topic 15 Halogen Compounds<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-15-halogen-compounds\/\">Topic 15 Overview<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-15-halogen-compounds\/reaction-types-mechanisms-and-nucleophiles\/\">Reaction Types, Mechanisms and Nucleophiles<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-15-halogen-compounds\/halogenoalkanes-naming-classification-and-bonding\/\">Halogenoalkanes: Naming, Classification and the C\u2013X Bond<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-15-halogen-compounds\/nucleophilic-substitution-reactions\/\">Nucleophilic Substitution Reactions<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-15-halogen-compounds\/elimination-reactions-of-halogenoalkanes\/\">Elimination Reactions of Halogenoalkanes<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-15-halogen-compounds\/rates-of-hydrolysis-and-bond-enthalpy\/\">Rates of Hydrolysis and Bond Enthalpy<\/a>\n      <\/li>\n    <\/ul>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Other Sections<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-16-hydroxy-compounds\/\">Topic 16 Hydroxy Compounds<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-22-analytical-techniques\/\">Topic 22 Analytical Techniques<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-14-hydrocarbons\/\">Topic 14 Hydrocarbons<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-13-an-introduction-to-as-level-organic-chemistry\/\">Topic 13 Introduction to AS Organic Chemistry<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-20-polymerisation\/\">Topic 20 Polymerisation<\/a>\n      <\/li>\n    <\/ul>\n  <\/div>\n<\/aside>\n\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    sidebar.querySelectorAll('a[data-ols-disabled-parent=\"1\"], 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Substitution Reactions<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to the nucleophilic substitution reactions of halogenoalkanes with aqueous hydroxide, water with silver nitrate, ammonia and cyanide ions, their conditions and products, and the mechanism drawn with curly arrows, including the S\u20991 and S\u20992 mechanisms.<\/p>\n\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">AS Level<\/div>\n<div class=\"ols-badge\">Topic 15: Halogen Compounds<\/div>\n<div class=\"ols-badge\">9701 Papers 1 and 2<\/div>\n<\/div>\n\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 class=\"ols-linkedin-pill\" href=\"https:\/\/www.linkedin.com\/in\/doctormohammedfatah\/\" target=\"_blank\" rel=\"noopener noreferrer\">\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      <\/header>\n\n      <article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">1<\/div>\n<h2>The Mechanism with Hydroxide Ions<\/h2>\n<\/div>\n<p>When a halogenoalkane is heated under reflux with <strong>aqueous potassium hydroxide<\/strong> (or sodium hydroxide), the halogen is replaced by an OH group and an alcohol forms: CH\u2083CH\u2082CH\u2082Br + OH\u207b \u2192 CH\u2083CH\u2082CH\u2082OH + Br\u207b. The hydroxide ion acts as a <strong>nucleophile<\/strong>: it donates its lone pair to the \u03b4+ carbon of the C\u2013Br bond. The reaction is a <strong>nucleophilic substitution<\/strong>, and because water and hydroxide ions split the molecule it is also a hydrolysis.<\/p>\n<p>The mechanism for a primary halogenoalkane (the S\u20992 mechanism) is a single step. Two curly arrows are drawn: one from the lone pair on the oxygen of HO\u207b to the \u03b4+ carbon, showing the new C\u2013O bond forming, and one from the middle of the C\u2013Br bond to the bromine, showing the bond breaking heterolytically so that bromine leaves as a bromide ion. The \u03b4+ and \u03b4\u2212 on the C\u2013Br bond, the lone pair on the nucleophile and the negative charge on the leaving bromide ion must all be shown. The hydroxide attacks from the side opposite the halogen, and at the moment of substitution the carbon is briefly bonded to both groups in a transition state.<\/p>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/alcohols-halogenoalkanes-spectra-nucsub.jpg\" aria-label=\"Open image full screen\">\n<img decoding=\"async\" class=\"ols-zoomable-img ols-lightbox-target\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/alcohols-halogenoalkanes-spectra-nucsub.jpg\" alt=\"Nucleophilic substitution mechanism of 1-bromopropane with hydroxide ions, and a table of four nucleophiles\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>The nucleophilic substitution of 1-bromopropane by hydroxide ions with its curly arrows, and the four nucleophiles with their conditions and products.<\/p><\/div>\n<\/div>\n<!-- 3D card: sn2-bromoethane (26 Sep 2026) -->\n<section class=\"ols-sn2-001\" id=\"olsSn2001\">\n<style>\n@import url('https:\/\/fonts.googleapis.com\/css2?family=Poppins:wght@300;400;500;600&display=swap');\n\n.ols-sn2-001{\n  font-family:'Poppins',system-ui,-apple-system,'Segoe UI',Roboto,Helvetica,Arial,sans-serif;\n  color:#1C244B; 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bottom:auto; top:48px; max-width:72%;}\n  .ols-sn2-001 .r-line{font-size:10.5px;}\n  .ols-sn2-001 .r-line{white-space:normal;}\n  .ols-sn2-001 .sn2-lab.l-dp,.ols-sn2-001 .sn2-lab.l-dm{font-size:12px;}\n  .ols-sn2-001 .r-bars{font-size:9.5px;}\n  .ols-sn2-001 .sn2-lab.l-ion,.ols-sn2-001 .sn2-lab.l-prod{font-size:10.5px;}\n  .ols-sn2-001 .sn2-lab.l-rul{font-size:9.5px; padding:1px 5px;}\n  .ols-sn2-001 .sn2-badge.b-eq{font-size:11px;}\n  .ols-sn2-001 .sn2-read{padding:5px 9px;}\n  .ols-sn2-001 .sn2-panel{left:auto; right:14px; transform:none; font-size:11px; padding:6px 10px;}\n}\n@media (prefers-reduced-motion:reduce){\n  .ols-sn2-001 .sn2-seg button,.ols-sn2-001 .sn2-pill{transition:none;}\n}\n\n.ols-cc-sn2-001{\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-sn2-001 a{color:#aab0c0; text-decoration:none;}\n.ols-cc-sn2-001 a:hover{text-decoration:underline;}\n<\/style>\n\n<div class=\"sn2-head\">\n  <h2 class=\"sn2-title\">Nucleophilic Substitution of Bromoethane by Hydroxide<\/h2>\n  <p class=\"sn2-sub\">Watch a hydroxide ion attack the \u03b4+ carbon of bromoethane from behind, pass through a single transition state and push out the bromide ion, turning the carbon inside out.<\/p>\n<\/div>\n\n<div class=\"sn2-stage\" id=\"sn2Stage\">\n  <canvas class=\"sn2-canvas\" id=\"sn2Canvas\"><\/canvas>\n  <div class=\"sn2-overlay\" id=\"sn2Overlay\"><\/div>\n  <div class=\"sn2-caption\" id=\"sn2Caption\"><\/div>\n  <div class=\"sn2-hint\" id=\"sn2Hint\">Drag to rotate<\/div>\n<\/div>\n\n<div class=\"sn2-eqbar\" id=\"sn2Badges\"><\/div>\n\n<div class=\"sn2-controls\">\n  <div class=\"sn2-row\">\n    <span class=\"sn2-rowlab\">Step<\/span>\n    <div class=\"sn2-seg\" role=\"group\" aria-label=\"Step\">\n      <button type=\"button\" data-step=\"1\" aria-pressed=\"true\">Reactants<\/button>\n      <button type=\"button\" data-step=\"2\" aria-pressed=\"false\">Attack<\/button>\n      <button type=\"button\" data-step=\"3\" aria-pressed=\"false\">Transition state<\/button>\n      <button type=\"button\" data-step=\"4\" aria-pressed=\"false\">Products<\/button>\n    <\/div>\n  <\/div>\n\n  <div class=\"sn2-row\">\n    <span class=\"sn2-rowlab\">View<\/span>\n    <div class=\"sn2-seg\" role=\"group\" aria-label=\"Camera view\">\n      <button type=\"button\" data-v=\"front\" aria-pressed=\"false\">Front<\/button>\n      <button type=\"button\" data-v=\"side\" aria-pressed=\"false\">Side<\/button>\n      <button type=\"button\" data-v=\"top\" aria-pressed=\"false\">Top<\/button>\n    <\/div>\n  <\/div>\n\n  <div class=\"sn2-row\">\n    <span class=\"sn2-rowlab\">Show<\/span>\n    <button type=\"button\" class=\"sn2-pill\" id=\"sn2TogArr\" aria-pressed=\"true\"><i class=\"dot\"><\/i>Curly arrows<\/button>\n    <button type=\"button\" class=\"sn2-pill\" id=\"sn2TogDelta\" aria-pressed=\"true\"><i class=\"dot\"><\/i>&delta; labels<\/button>\n    <button type=\"button\" class=\"sn2-pill\" id=\"sn2TogLp\" aria-pressed=\"true\"><i class=\"dot\"><\/i>Lone pair<\/button>\n    <button type=\"button\" class=\"sn2-pill p-gold\" id=\"sn2TogProf\" aria-pressed=\"true\"><i class=\"dot\"><\/i>Reaction profile<\/button>\n  <\/div>\n\n  <div class=\"sn2-row\">\n    <span class=\"sn2-rowlab\">Motion<\/span>\n    <button type=\"button\" class=\"sn2-pill\" id=\"sn2Spin\" aria-pressed=\"false\"><i class=\"dot\"><\/i>Rotation<\/button>\n    <button type=\"button\" class=\"sn2-pill\" id=\"sn2ResetV\">Reset view<\/button>\n  <\/div>\n<\/div>\n\n<div class=\"sn2-info\">\n  <h3 id=\"sn2InfoTitle\"><\/h3>\n  <p id=\"sn2InfoText\"><\/p>\n  <div class=\"sn2-facts\" id=\"sn2Facts\"><\/div>\n  <div class=\"sn2-key\">\n    <span><i style=\"background:#484e5c\"><\/i>Carbon<\/span>\n    <span><i style=\"background:#eef0f5\"><\/i>Hydrogen<\/span>\n    <span><i style=\"background:#8c1e1e\"><\/i>Bromine<\/span>\n    <span><i style=\"background:#cd342c\"><\/i>Oxygen<\/span>\n    <span><i style=\"background:#8c92a0\"><\/i>Bond<\/span>\n    <span><i style=\"background:#ffffff; border:1.5px dashed #5a6070\"><\/i>Partial bond<\/span>\n    <span><i style=\"background:rgba(28,36,75,0.45)\"><\/i>Curly arrow<\/span>\n    <span><i style=\"background:#3b78dc; border-radius:50%\"><\/i>Lone pair<\/span>\n    <span><i style=\"background:#c9961c\"><\/i>Reaction profile<\/span>\n  <\/div>\n<\/div>\n<\/section>\n\n<p class=\"ols-cc-sn2-001\">&copy; Dr. Mohammed Al-Fatah &#8211; <a href=\"https:\/\/www.onlinelearningsystem.net\" target=\"_blank\" rel=\"noopener\">onlinelearningsystem.net<\/a><\/p>\n\n<script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/sn2-bromoethane.js?v=20260926c\"><\/script>\n\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> Two arrows, two partial charges, one lone pair, one leaving group with its negative charge. Draw the arrow from the lone pair to the carbon, never from the carbon to the nucleophile.<\/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>Check: The Substitution Mechanism<\/h2>\n<p>Draw and judge mechanisms for halogenoalkanes other than 1-bromopropane.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-953\" class=\"h5p-iframe\" data-content-id=\"953\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Alcohols, Halogenoalkanes and Spectra Summary: The Substitution Mechanism\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">2<\/div>\n<h2>Water as the Nucleophile<\/h2>\n<\/div>\n<p>Water is a much weaker nucleophile than the hydroxide ion, because it has no negative charge, but it hydrolyses halogenoalkanes slowly: CH\u2083CH\u2082Br + H\u2082O \u2192 CH\u2083CH\u2082OH + H\u207a + Br\u207b. The reaction is used as the basis of the <strong>silver nitrate test<\/strong>: the halogenoalkane is warmed with aqueous silver nitrate dissolved in ethanol, and as the halide ion is released it reacts with silver ions to give a precipitate of the silver halide, Ag\u207a(aq) + Br\u207b(aq) \u2192 AgBr(s). Ethanol is the solvent because the halogenoalkane does not dissolve in water. The colour of the precipitate identifies the halogen (white AgCl, cream AgBr, yellow AgI) and the time taken for it to appear compares the rates of hydrolysis.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Aqueous silver nitrate in ethanol: water is the nucleophile, the halide ion that leaves gives the precipitate.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">3<\/div>\n<h2>Ammonia: Making Amines<\/h2>\n<\/div>\n<p>Heating a halogenoalkane with <strong>ammonia dissolved in ethanol<\/strong> under pressure in a sealed tube gives a <strong>primary amine<\/strong>: CH\u2083CH\u2082CH\u2082Br + 2NH\u2083 \u2192 CH\u2083CH\u2082CH\u2082NH\u2082 + NH\u2084Br. The nitrogen lone pair of ammonia attacks the \u03b4+ carbon exactly as the hydroxide ion did; the product at that stage is a positively charged ion, CH\u2083CH\u2082CH\u2082NH\u2083\u207a, and a second ammonia molecule removes a proton from it to give the amine and an ammonium ion. That is why two molecules of ammonia appear in the equation.<\/p>\n<p>The mechanism needs three arrows in two steps: lone pair on N to the carbon and C\u2013Br bond to Br in the first step; then a lone pair on a second NH\u2083 to one of the N\u2013H hydrogens and the N\u2013H bond back onto the nitrogen in the second. The sealed tube keeps the volatile ammonia in the mixture, and an excess of ammonia is used because the amine formed is itself a nucleophile and would otherwise attack more halogenoalkane to give secondary and tertiary amines.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> &#8220;Ethanolic ammonia, heated under pressure in a sealed tube, excess ammonia.&#8221; Products: the amine and ammonium bromide.<\/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>Check: Ammonia and Amines<\/h2>\n<p>Write equations and conditions for amine preparations not used above.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"954\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">4<\/div>\n<h2>Cyanide Ions: Making Nitriles and Lengthening the Chain<\/h2>\n<\/div>\n<p>Heating a halogenoalkane under reflux with <strong>potassium cyanide dissolved in ethanol<\/strong> gives a <strong>nitrile<\/strong>: CH\u2083CH\u2082CH\u2082Br + CN\u207b \u2192 CH\u2083CH\u2082CH\u2082CN + Br\u207b. The cyanide ion is a nucleophile through the lone pair on its carbon atom, and the mechanism is the same two-arrow substitution as with hydroxide. The important feature is that the carbon of the cyanide group is added to the chain, so the product has <strong>one more carbon atom<\/strong> than the starting compound: 1-bromopropane (three carbons) gives butanenitrile (four carbons, counting the nitrile carbon in the name). This is one of the few ways of lengthening a carbon chain in synthesis, and the nitrile can then be hydrolysed to a carboxylic acid or reduced to an amine.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Reagent and conditions<\/th><th>Nucleophile<\/th><th>Product from 1-bromopropane<\/th><th>Type of compound<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>KOH(aq), heat under reflux<\/strong><\/td><td>OH\u207b<\/td><td>CH\u2083CH\u2082CH\u2082OH, propan-1-ol<\/td><td>alcohol<\/td><\/tr>\n<tr><td><strong>H\u2082O with AgNO\u2083 in ethanol, warm<\/strong><\/td><td>H\u2082O<\/td><td>propan-1-ol slowly, AgBr precipitate<\/td><td>alcohol<\/td><\/tr>\n<tr><td><strong>NH\u2083 in ethanol, heat under pressure, excess NH\u2083<\/strong><\/td><td>NH\u2083<\/td><td>CH\u2083CH\u2082CH\u2082NH\u2082, propylamine (propan-1-amine)<\/td><td>primary amine<\/td><\/tr>\n<tr><td><strong>KCN in ethanol, heat under reflux<\/strong><\/td><td>CN\u207b<\/td><td>CH\u2083CH\u2082CH\u2082CN, butanenitrile<\/td><td>nitrile<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> Name nitriles from the whole chain including the CN carbon: butanenitrile, not propanenitrile, from a three-carbon bromoalkane.<\/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>Check: Choosing the Nucleophile<\/h2>\n<p>Pick reagents and predict products for target molecules not made on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-955\" class=\"h5p-iframe\" data-content-id=\"955\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Alcohols, Halogenoalkanes and Spectra Drag: Choosing the Nucleophile\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">5<\/div>\n<h2>S\u20991 and S\u20992: Which Mechanism?<\/h2>\n<\/div>\n<p>Primary halogenoalkanes react by the one-step <strong>S\u20992 mechanism<\/strong> described above: the nucleophile attacks as the leaving group departs, both species are in the rate-determining step (bimolecular), and the carbon is inverted as the three groups on it flip over. Tertiary halogenoalkanes react by the two-step <strong>S\u20991 mechanism<\/strong>: the C\u2013X bond breaks first, heterolytically, to give a <strong>carbocation<\/strong> intermediate and a halide ion, and only then does the nucleophile attack the positive carbon. The slow step involves one molecule (unimolecular). Secondary compounds react by a mixture of the two, depending on their structure.<\/p>\n<p>The reason is the stability of the carbocation. Alkyl groups have a positive <strong>inductive effect<\/strong>: they push electron density towards the carbon they are bonded to. A tertiary carbocation has three alkyl groups feeding electron density onto the positive carbon, so it is stabilised and forms readily; a primary carbocation has one and is too unstable to form, so the primary compound must wait for the nucleophile. The three bulky alkyl groups on a tertiary carbon also block the approach of a nucleophile from behind, which stops the S\u20992 route.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> S\u20992: one step, primary, inversion. S\u20991: two steps through a carbocation, tertiary, stabilised by the inductive effect of the alkyl groups.<\/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>Check: S\u20991 or S\u20992<\/h2>\n<p>Assign mechanisms and explain them for compounds not used above.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"957\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">6<\/div>\n<h2>Common Exam Points<\/h2>\n<\/div>\n<h3>Say<\/h3><p>&#8220;The lone pair on the nucleophile forms a bond to the \u03b4+ carbon while the C\u2013X bond breaks heterolytically.&#8221; &#8220;Aqueous KOH, heat under reflux, substitution.&#8221; &#8220;KCN in ethanol adds one carbon to the chain.&#8221;<\/p>\n<h3>Do not say<\/h3><p>&#8220;KOH in ethanol&#8221; for substitution (that gives elimination). &#8220;The bromine leaves as a bromine atom&#8221; (it leaves as a bromide ion).<\/p>\n<h3>Watch for<\/h3><p>State the mechanism (S\u20991 or S\u20992) before you draw it, and show the carbocation in an S\u20991 mechanism.<\/p>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: Substitution Round-up<\/h2>\n<p>Match reagents, conditions and products across nucleophilic substitutions of halogenoalkanes not seen above.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-956\" class=\"h5p-iframe\" data-content-id=\"956\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Alcohols, Halogenoalkanes and Spectra Quick Choice: Substitution Round-up\"><\/iframe><\/div><\/div>\n<\/section>\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check nucleophilic substitution.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Why is the reaction with KOH done in water rather than ethanol?<\/h3>\n<p>In water the hydroxide ion acts as a nucleophile and substitutes the halogen. In ethanol it acts as a base and an elimination reaction takes over.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why are two molecules of ammonia needed?<\/h3>\n<p>The first attacks the carbon and forms an alkylammonium ion; the second removes a proton from it to leave the free amine and an ammonium ion.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why does the cyanide reaction add a carbon to the chain?<\/h3>\n<p>The nucleophile is the carbon of the cyanide ion, so the CN carbon becomes part of the chain: 1-bromopropane (three carbons) gives butanenitrile (four carbons).<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why is silver nitrate used with water as the nucleophile?<\/h3>\n<p>Water hydrolyses the compound slowly and releases a halide ion; silver ions turn that halide ion into a precipitate, which makes the slow reaction visible and identifies the halogen.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>How do I decide between S\u20991 and S\u20992?<\/h3>\n<p>By the class of the compound: primary reacts by S\u20992 in one step, tertiary by S\u20991 through a carbocation that the inductive effect of three alkyl groups stabilises, secondary by a mixture.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<section class=\"ols-related-card\">\n<h2>Related Topic 15 Halogen Compounds Pages<\/h2>\n<p>Use these pages to connect the ideas across Topic 15 Halogen Compounds and the rest of the course.<\/p>\n<div class=\"ols-related-grid\">\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-15-halogen-compounds\/reaction-types-mechanisms-and-nucleophiles\/\">Reaction Types, Mechanisms and Nucleophiles<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-15-halogen-compounds\/halogenoalkanes-naming-classification-and-bonding\/\">Halogenoalkanes: Naming, Classification and the C\u2013X Bond<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-15-halogen-compounds\/elimination-reactions-of-halogenoalkanes\/\">Elimination Reactions of Halogenoalkanes<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-15-halogen-compounds\/rates-of-hydrolysis-and-bond-enthalpy\/\">Rates of Hydrolysis and Bond Enthalpy<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-15-halogen-compounds\/\">Topic 15 Halogen Compounds Overview<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-16-hydroxy-compounds\/\">Topic 16 Hydroxy Compounds<\/a>\n<\/div>\n<\/section>\n<section class=\"ols-attribution-card\">\n        <p><strong>Copyright and author footprint:<\/strong> This OLS revision page was written for Online Learning System by <strong>Dr. Mohammed Al-Fatah<\/strong>. It is designed for A Level Chemistry revision and should not be copied or redistributed without permission.<\/p>\n      <\/section>\n\n      <div class=\"ols-image-lightbox\" id=\"olsImageLightboxNatureCovalentBonding9ch0\" aria-hidden=\"true\" role=\"dialog\" aria-modal=\"true\" aria-label=\"Expanded revision image\">\n        <div class=\"ols-image-lightbox-inner\">\n          <button class=\"ols-image-lightbox-close\" type=\"button\" aria-label=\"Close enlarged image\">\u00d7<\/button>\n          <img decoding=\"async\" class=\"ols-image-lightbox-img\" src=\"\" alt=\"\">\n        <\/div>\n      <\/div>\n\n      <script>\n        (function(){\n          var page = document.querySelector(\".ols-nature-of-covalent-bonding-9ch0-page\");\n          if (!page) { return; }\n          var lightbox = page.querySelector(\"#olsImageLightboxNatureCovalentBonding9ch0\");\n          if (!lightbox) { return; }\n          var lightboxImage = lightbox.querySelector(\".ols-image-lightbox-img\");\n    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