{"id":10914,"date":"2026-09-26T21:56:57","date_gmt":"2026-09-26T20:56:57","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-10-alcohols-halogenoalkanes-and-spectra\/10c-spectra\/identifying-unknown-compounds\/"},"modified":"2026-10-04T23:03:55","modified_gmt":"2026-10-04T22:03:55","slug":"identifying-unknown-compounds","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-10-alcohols-halogenoalkanes-and-spectra\/10c-spectra\/identifying-unknown-compounds\/","title":{"rendered":"Identifying Unknown Compounds"},"content":{"rendered":"\n<section class=\"ols-revision-page ols-alcohols-halogenoalkanes-spectra-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  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border-radius: 18px;\n      padding: 18px 20px;\n      box-shadow: var(--inner-shadow);\n    }\n.ols-faq-item h3 {\n      margin: 0 0 8px;\n      font-size: 20px;\n      line-height: 1.3;\n      color: var(--navy);\n    }\n.ols-faq-item p {\n      margin: 0;\n      font-size: 16px;\n      line-height: 1.65;\n      color: var(--body-text);\n    }\n.ols-faq-card,\n      .ols-quicksnap-card,\n      .ols-attribution-card {\n        padding: 24px 18px;\n        border-radius: 22px;\n      }\n  <\/style>\n\n    <aside class=\"ols-sidebar\">\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 10 Alcohols, Halogenoalkanes &amp; Spectra<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-10-alcohols-halogenoalkanes-and-spectra\/\">Topic 10 Overview<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-10-alcohols-halogenoalkanes-and-spectra\/10a-alcohols\/\">Topic 10A Alcohols<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-10-alcohols-halogenoalkanes-and-spectra\/10a-alcohols\/alcohols-naming-classification-and-properties\/\">Alcohols: Naming, Classification and Properties<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-10-alcohols-halogenoalkanes-and-spectra\/10a-alcohols\/oxidation-of-alcohols\/\">Oxidation of Alcohols<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-10-alcohols-halogenoalkanes-and-spectra\/10a-alcohols\/reactions-of-alcohols\/\">Substitution and Elimination Reactions of Alcohols<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-10-alcohols-halogenoalkanes-and-spectra\/10a-alcohols\/preparing-and-purifying-organic-liquids\/\">Preparing and Purifying Organic Liquids<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/core-practicals\/cp-6-chlorination-of-2-methylpropan-2-ol\/\">Core Practical 6<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/core-practicals\/cp-7-oxidation-of-propan-1-ol\/\">Core Practical 7<\/a>\n          <\/li>\n        <\/ul>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-10-alcohols-halogenoalkanes-and-spectra\/10b-halogenoalkanes\/\">Topic 10B Halogenoalkanes<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-10-alcohols-halogenoalkanes-and-spectra\/10b-halogenoalkanes\/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\/edexcel-international\/topic-10-alcohols-halogenoalkanes-and-spectra\/10b-halogenoalkanes\/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\/edexcel-international\/topic-10-alcohols-halogenoalkanes-and-spectra\/10b-halogenoalkanes\/nucleophilic-substitution-reactions\/\">Nucleophilic Substitution Reactions<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-10-alcohols-halogenoalkanes-and-spectra\/10b-halogenoalkanes\/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\/edexcel-international\/topic-10-alcohols-halogenoalkanes-and-spectra\/10b-halogenoalkanes\/rates-of-hydrolysis-and-bond-enthalpy\/\">Rates of Hydrolysis and Bond Enthalpy<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/core-practicals\/cp-5-hydrolysis-of-halogenoalkanes\/\">Core Practical 5<\/a>\n          <\/li>\n        <\/ul>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-10-alcohols-halogenoalkanes-and-spectra\/10c-spectra\/\">Topic 10C Spectra<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-10-alcohols-halogenoalkanes-and-spectra\/10c-spectra\/mass-spectra-of-organic-compounds\/\">Mass Spectra of Organic Compounds<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-10-alcohols-halogenoalkanes-and-spectra\/10c-spectra\/infrared-spectroscopy\/\">Infrared Spectroscopy<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-10-alcohols-halogenoalkanes-and-spectra\/10c-spectra\/identifying-unknown-compounds\/\">Identifying Unknown Compounds<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/core-practicals\/cp-8-analysis-of-inorganic-and-organic-unknowns\/\">Core Practical 8<\/a>\n          <\/li>\n        <\/ul>\n      <\/li>\n    <\/ul>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Other Topics<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-4-organic-chemistry-and-alkanes\/\">Topic 4 Organic Chemistry &amp; Alkanes<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/\">Topic 5 Alkenes<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-9-kinetics-and-equilibria\/\">Topic 9 Kinetics &amp; Equilibria<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/core-practicals\/\">Core Practicals<\/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    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href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-10-alcohols-halogenoalkanes-and-spectra\/10c-spectra\/\">Topic 10C Spectra<\/a> \/\n<span>Identifying Unknown Compounds<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Identifying Unknown Compounds<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to combining mass spectra, infrared spectra and test-tube reactions to deduce the structure of an unknown organic compound, with the table of functional group tests and a fully worked example.<\/p>\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">Exam board: Edexcel International<\/div>\n<div class=\"ols-badge\">Unit 2: WCH12\/01<\/div>\n<div class=\"ols-badge\">Topic 10C Spectra<\/div>\n<\/div>\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 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>Putting the Evidence Together<\/h2>\n<\/div>\n<p>An exam question rarely gives one piece of evidence. It gives a <strong>mass spectrum<\/strong>, an <strong>infrared spectrum<\/strong>, perhaps an elemental analysis or a molecular formula, and one or two <strong>test-tube observations<\/strong>, and asks for the structure. Each technique answers a different question, so use them in order. The mass spectrum gives the relative molecular mass from the molecular ion and pieces of the structure from the fragments. The infrared spectrum gives the functional group. The test-tube reactions confirm the functional group and distinguish between the remaining possibilities. A logical answer states each piece of evidence, what it shows, and how the pieces combine.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Question<\/th><th>Technique<\/th><th>What to read<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>What is the relative molecular mass?<\/strong><\/td><td>mass spectrum<\/td><td>m\/z of the molecular ion (highest, ignoring M+1)<\/td><\/tr>\n<tr><td><strong>Which halogen, if any?<\/strong><\/td><td>mass spectrum<\/td><td>M and M+2 peaks: 3 : 1 chlorine, 1 : 1 bromine<\/td><\/tr>\n<tr><td><strong>What pieces is it made of?<\/strong><\/td><td>mass spectrum<\/td><td>fragment ions and the losses from M<\/td><\/tr>\n<tr><td><strong>Which functional group?<\/strong><\/td><td>infrared spectrum<\/td><td>absorptions above 1500 cm\u207b\u00b9 and the data sheet<\/td><\/tr>\n<tr><td><strong>Is it pure and which isomer?<\/strong><\/td><td>infrared spectrum<\/td><td>fingerprint region against a database<\/td><\/tr>\n<tr><td><strong>Confirm the group<\/strong><\/td><td>test-tube reactions<\/td><td>reagent, condition, observation<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Mass spectrum for Mr and pieces, infrared for the functional group, chemical tests to confirm. Say which evidence gives which conclusion.<\/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: Which Evidence Answers Which Question<\/h2>\n<p>Match evidence to conclusions for an unknown that is not the worked example below.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-974\" class=\"h5p-iframe\" data-content-id=\"974\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Alcohols, Halogenoalkanes and Spectra Drag: Which Evidence Answers Which Question\"><\/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>Test-Tube Reactions for Functional Groups<\/h2>\n<\/div>\n<p>The chemical tests met across the course are collected here because the analysis question uses them together. Each is written as reagent, condition and observation, and a negative result is as useful as a positive one: a compound that does not fizz with sodium carbonate is not a carboxylic acid.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Functional group<\/th><th>Test<\/th><th>Positive result<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>C=C (alkene)<\/strong><\/td><td>shake with bromine water<\/td><td>orange to colourless<\/td><\/tr>\n<tr><td><strong>Aldehyde<\/strong><\/td><td>warm with Benedict&#8217;s or Fehling&#8217;s solution<\/td><td>blue solution to brick-red precipitate<\/td><\/tr>\n<tr><td><strong>Aldehyde<\/strong><\/td><td>warm with Tollens&#8217; reagent<\/td><td>silver mirror<\/td><\/tr>\n<tr><td><strong>Ketone<\/strong><\/td><td>either aldehyde reagent<\/td><td>no change (carbonyl present if 2,4-DNPH gives an orange precipitate)<\/td><\/tr>\n<tr><td><strong>Carboxylic acid<\/strong><\/td><td>add sodium carbonate or sodium hydrogencarbonate<\/td><td>fizzing, CO\u2082 turns limewater milky<\/td><\/tr>\n<tr><td><strong>O\u2013H (alcohol, acid, water)<\/strong><\/td><td>add PCl\u2085<\/td><td>steamy white fumes of HCl<\/td><\/tr>\n<tr><td><strong>Primary or secondary alcohol, aldehyde<\/strong><\/td><td>warm with acidified potassium dichromate(VI)<\/td><td>orange to green<\/td><\/tr>\n<tr><td><strong>Tertiary alcohol<\/strong><\/td><td>warm with acidified potassium dichromate(VI)<\/td><td>stays orange<\/td><\/tr>\n<tr><td><strong>Halogenoalkane<\/strong><\/td><td>warm with aqueous silver nitrate in ethanol<\/td><td>white (Cl), cream (Br) or yellow (I) precipitate<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/core-practicals\/cp-8-analysis-of-inorganic-and-organic-unknowns\/\">Core Practical 8 (analysis of inorganic and organic unknowns)<\/a> uses these tests to identify unknown organic liquids alongside the tests for inorganic ions.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> Three parts to every test: reagent, condition, observation. &#8220;Add sodium carbonate solution: effervescence&#8221; scores; &#8220;test with carbonate&#8221; does not.<\/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\/09\/alcohols-halogenoalkanes-spectra-testtubes.jpg\" target=\"_blank\" rel=\"noopener\" aria-label=\"Open image fullscreen\">\n<img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/alcohols-halogenoalkanes-spectra-testtubes.jpg\" alt=\"Grid of before-and-after test tubes for alkene, aldehyde, carboxylic acid, O\u2013H group, alcohol and halogenoalkane tests.\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/alcohols-halogenoalkanes-spectra-testtubes.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>Each functional group test is a reagent, a condition and an observation, and the colour change or precipitate is the evidence the examiner wants.<\/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>Check: Choosing a Test<\/h2>\n<p>Pick a single test that separates pairs of compounds not listed above.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"975\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">3<\/div>\n<h2>Worked Example: an Unknown Liquid<\/h2>\n<\/div>\n<p>An unknown liquid has a molecular ion peak at m\/z 60, fragment peaks at 45, 43 and 15, a very broad infrared absorption from 2500 to 3300 cm\u207b\u00b9 and a strong absorption at 1715 cm\u207b\u00b9, and it fizzes with sodium hydrogencarbonate solution.<\/p>\n<p><strong>Step 1, Mr:<\/strong> the molecular ion at 60 gives Mr = 60. Formulae that fit include C\u2083H\u2088O (propan-1-ol, propan-2-ol), C\u2082H\u2084O\u2082 (ethanoic acid, methyl methanoate) and C\u2083H\u2088O with an ether structure.<\/p>\n<p><strong>Step 2, functional group:<\/strong> the very broad absorption at 2500\u20133300 cm\u207b\u00b9 is the O\u2013H of a carboxylic acid, and 1715 cm\u207b\u00b9 is a C=O. Together they point to a carboxylic acid, so C\u2082H\u2084O\u2082 as ethanoic acid, CH\u2083COOH.<\/p>\n<p><strong>Step 3, fragments:<\/strong> m\/z 45 is COOH\u207a (M \u2212 15, loss of CH\u2083\u2022), m\/z 43 is CH\u2083CO\u207a (M \u2212 17, loss of \u2022OH) and m\/z 15 is CH\u2083\u207a. All three fit CH\u2083COOH and none fits methyl methanoate, which would show m\/z 31 (OCH\u2083\u207a).<\/p>\n<p><strong>Step 4, confirm:<\/strong> effervescence with sodium hydrogencarbonate shows an acid. The unknown is ethanoic acid.<\/p>\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\/09\/alcohols-halogenoalkanes-spectra-identify.jpg\" target=\"_blank\" rel=\"noopener\" aria-label=\"Open image fullscreen\">\n<img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/alcohols-halogenoalkanes-spectra-identify.jpg\" alt=\"Flow chart for identifying an unknown organic compound from mass spectra, infrared spectra and chemical tests\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/alcohols-halogenoalkanes-spectra-identify.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>The four-step route from spectra and tests to a structure, worked for the unknown with Mr 60.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam technique:<\/strong> Number the steps, quote every value you use (m\/z 60, 2500\u20133300 cm\u207b\u00b9), and finish with the name AND the structural formula.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">4<\/div>\n<h2>Common Exam Points<\/h2>\n<\/div>\n<h3>Say<\/h3><p>&#8220;Molecular ion at m\/z 60, so Mr = 60.&#8221; &#8220;Absorption at 1680\u20131750 cm\u207b\u00b9 shows C=O; the very broad absorption at 2500\u20133300 cm\u207b\u00b9 shows the O\u2013H of a carboxylic acid.&#8221; &#8220;Effervescence with sodium carbonate confirms the acid.&#8221;<\/p>\n<h3>Do not say<\/h3><p>&#8220;The IR shows it is ethanoic acid&#8221; (infrared shows the functional group; the mass spectrum and tests give the rest). &#8220;The peak at 45 is 45.&#8221;<\/p>\n<h3>Watch for<\/h3><p>Isomers with the same Mr and functional group: use a fragment that only one isomer can give, or a test such as tri-iodomethane or the rate of oxidation.<\/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: Solving an Unknown<\/h2>\n<p>Work through evidence for an unknown compound that is not ethanoic acid.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-976\" class=\"h5p-iframe\" data-content-id=\"976\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Alcohols, Halogenoalkanes and Spectra Explain: Solving an Unknown\"><\/iframe><\/div><\/div>\n<\/section>\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check the identification method.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>In what order should I use the evidence?<\/h3>\n<p>Molecular ion for Mr, infrared for the functional group, fragments to fit the pieces together, then a chemical test to confirm. State what each piece shows as you go.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>What if two isomers fit all the spectra?<\/h3>\n<p>Look for a fragment that only one of them can give, or choose a test that distinguishes them: rate of oxidation with dichromate(VI), or the tri-iodomethane test for a CH\u2083CH(OH)\u2013 or CH\u2083CO\u2013 group.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>How do I show a compound is an acid rather than an alcohol?<\/h3>\n<p>The infrared O\u2013H band of an acid is very broad (2500\u20133300 cm\u207b\u00b9) and comes with a strong C=O at 1680\u20131750 cm\u207b\u00b9; an alcohol has a broad O\u2013H at 3200\u20133600 and no C=O. Sodium carbonate then fizzes only with the acid.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why quote wavenumber ranges rather than single values?<\/h3>\n<p>Because the data sheet gives ranges and the mark scheme expects them. An absorption &#8220;at 1715 cm\u207b\u00b9, in the C=O range 1680\u20131750 cm\u207b\u00b9&#8221; covers both.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Do I need to draw the structure at the end?<\/h3>\n<p>Yes. Give the name and the structural or displayed formula; a name alone can lose the final mark if the question asked for a structure.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<section class=\"ols-related-card\">\n<h2>Related Topic 10C Spectra Pages<\/h2>\n<p>Use these pages to connect the ideas across spectra and the rest of Topic 10.<\/p>\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-10-alcohols-halogenoalkanes-and-spectra\/10c-spectra\/mass-spectra-of-organic-compounds\/\">Mass Spectra of Organic Compounds<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-10-alcohols-halogenoalkanes-and-spectra\/10c-spectra\/infrared-spectroscopy\/\">Infrared Spectroscopy<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-10-alcohols-halogenoalkanes-and-spectra\/10c-spectra\/\">Topic 10C Spectra Overview<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/core-practicals\/cp-8-analysis-of-inorganic-and-organic-unknowns\/\">Core Practical 8<\/a>\n<\/div>\n<\/section>\n<section class=\"ols-attribution-card\">\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\n<script type=\"application\/ld+json\">\n{\n  \"@context\": \"https:\/\/schema.org\",\n  \"@graph\": [\n    {\n      \"@type\": \"WebPage\",\n      \"@id\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-10-alcohols-halogenoalkanes-and-spectra\/10c-spectra\/identifying-unknown-compounds\/#webpage\",\n      \"url\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-10-alcohols-halogenoalkanes-and-spectra\/10c-spectra\/identifying-unknown-compounds\/\",\n      \"name\": \"Identifying Unknown Compounds | Topic 10C Spectra | Online Learning System\",\n      \"description\": \"Edexcel International A Level Chemistry revision notes on identifying unknown organic compounds: combining the molecular ion, fragmentation, infrared absorptions and 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