{"id":10994,"date":"2026-09-26T22:01:32","date_gmt":"2026-09-26T21:01:32","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-22-analytical-techniques\/infrared-spectroscopy\/"},"modified":"2026-09-27T10:07:37","modified_gmt":"2026-09-27T09:07:37","slug":"infrared-spectroscopy","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-22-analytical-techniques\/infrared-spectroscopy\/","title":{"rendered":"Infrared Spectroscopy"},"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      --soft-green: #f0f7f1;\n      --soft-orange: 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26px !important; border-top-right-radius: 26px !important; position: relative !important; z-index: 2 !important; }\n    .ols-zoom-card img.ols-zoomable-img { display: block !important; max-width: 100% !important; height: auto !important; border-radius: 18px !important; cursor: zoom-in !important; pointer-events: auto !important; user-select: none !important; -webkit-user-drag: none !important; transform: translateZ(0) scale(1) !important; transform-origin: center center !important; transition: transform 0.32s ease, box-shadow 0.32s ease, filter 0.32s ease !important; position: relative !important; z-index: 2 !important; }\n    @media (hover: hover) and (pointer: fine) { .ols-zoom-card img.ols-zoomable-img:hover { transform: translateZ(0) scale(1.35) !important; box-shadow: 0 28px 70px rgba(28, 36, 75, 0.34) !important; filter: saturate(1.02) contrast(1.01) !important; z-index: 100 !important; } }\n    .ols-zoom-card-caption { padding: 18px 22px 20px !important; background: linear-gradient(135deg, #ffffff, #f8fbff) !important; border-bottom-left-radius: 26px !important; border-bottom-right-radius: 26px !important; position: relative !important; z-index: 1 !important; }\n    .ols-zoom-card-caption p { margin: 0 !important; color: #5f6b85 !important; font-size: 15px !important; line-height: 1.65 !important; font-weight: 300 !important; font-style: italic !important; font-family: Poppins, Arial, sans-serif !important; }\n    .ols-image-lightbox { position: fixed; inset: 0; z-index: 999999; display: none; align-items: center; justify-content: center; padding: 34px; background: rgba(10, 15, 35, 0.86); backdrop-filter: blur(8px); -webkit-backdrop-filter: blur(8px); }\n    .ols-image-lightbox.is-open { display: flex; }\n    .ols-image-lightbox-inner { position: relative; width: min(96vw, 1500px); max-height: 92vh; display: flex; align-items: center; justify-content: center; }\n    .ols-image-lightbox-img { display: block; max-width: 100%; max-height: 92vh; height: auto; width: auto; border-radius: 22px; background: #ffffff; box-shadow: 0 32px 90px rgba(0, 0, 0, 0.45); object-fit: contain; }\n    .ols-image-lightbox-close { position: absolute; top: -18px; right: -18px; width: 46px; height: 46px; border: 0; border-radius: 50%; background: #ffffff; color: var(--navy); font-family: Poppins, Arial, sans-serif; font-size: 28px; line-height: 1; font-weight: 700; cursor: pointer; box-shadow: 0 16px 34px rgba(0, 0, 0, 0.28); display: flex; align-items: center; justify-content: center; transition: transform 0.2s ease, background 0.2s ease, color 0.2s ease; }\n    .ols-image-lightbox-close:hover { transform: scale(1.08); background: var(--blue); color: #ffffff; }\n    @media (max-width: 760px) { .ols-zoom-card { border-radius: 22px !important; overflow: hidden !important; } .ols-zoom-card-image { min-height: auto !important; padding: 12px !important; overflow: hidden !important; border-top-left-radius: 22px !important; border-top-right-radius: 22px 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.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 var(--border); padding: 14px 16px; overflow-wrap: anywhere; }\n      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   .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 22 Analytical Techniques<\/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-22-analytical-techniques\/\">Topic 22 Overview<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-22-analytical-techniques\/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\/cie\/topic-22-analytical-techniques\/infrared-spectroscopy\/\">Infrared Spectroscopy<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-22-analytical-techniques\/identifying-unknown-compounds\/\">Identifying Unknown Compounds<\/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-15-halogen-compounds\/\">Topic 15 Halogen Compounds<\/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 = 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href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/\">Cambridge International (CIE)<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-22-analytical-techniques\/\">Topic 22 Analytical Techniques<\/a> \/\n<span>Infrared Spectroscopy<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Infrared Spectroscopy<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to infrared spectroscopy: why bonds absorb infrared radiation, wavenumbers and transmittance, the characteristic absorptions of C\u2013H, C=C, C=O, O\u2013H, N\u2013H and C\u2013X bonds, the fingerprint region.<\/p>\n\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">AS Level<\/div>\n<div class=\"ols-badge\">Topic 22: Analytical Techniques<\/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      <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: Bonds and Energy<\/h2>\n<p>Three quick questions on covalent bonds, the electromagnetic spectrum and why greenhouse gases warm the atmosphere.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"970\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">1<\/div>\n<h2>Why Bonds Absorb Infrared<\/h2>\n<\/div>\n<p>The atoms joined by a covalent bond are always vibrating: the bond stretches and compresses, and the angles between bonds bend. Each vibration has its own natural frequency, which depends on the masses of the atoms and the strength of the bond. When <strong>infrared radiation<\/strong> of that same frequency passes through the sample, the bond absorbs it and vibrates more energetically. An infrared spectrometer records which frequencies are absorbed, so the spectrum shows which bonds are present.<\/p>\n<p>The frequency is given as a <strong>wavenumber<\/strong> in cm\u207b\u00b9 (the number of waves per centimetre), plotted from about 4000 on the left to 500 on the right. The y-axis is <strong>transmittance<\/strong>, the percentage of the radiation that passes through, so an absorption appears as a <strong>trough<\/strong> pointing downwards. Stronger bonds and lighter atoms vibrate faster and absorb at higher wavenumbers: O\u2013H and N\u2013H absorb near 3300 cm\u207b\u00b9, C=O near 1700 cm\u207b\u00b9, and C\u2013Cl near 700 cm\u207b\u00b9.<\/p>\n<!-- 3D card: infrared-bonds (26 Sep 2026) -->\n<section class=\"ols-irs-001\" id=\"olsIrs001\">\n<style>\n@import url('https:\/\/fonts.googleapis.com\/css2?family=Poppins:wght@300;400;500;600&display=swap');\n\n.ols-irs-001{\n  font-family:'Poppins',system-ui,-apple-system,'Segoe UI',Roboto,Helvetica,Arial,sans-serif;\n  color:#1C244B; background:#ffffff;\n  border:1px solid rgba(28,36,75,0.14);\n  border-radius:28px;\n  box-shadow:0 18px 45px rgba(28,36,75,0.10);\n  padding:44px; margin:26px auto; max-width:980px;\n  box-sizing:border-box; -webkit-font-smoothing:antialiased;\n}\n.ols-irs-001 *{box-sizing:border-box;}\n\n.ols-irs-001 .irs-head{margin:0 0 22px;}\n.ols-irs-001 h2.irs-title{\n  font-size:26px; line-height:1.25; font-weight:600; letter-spacing:-0.012em;\n  margin:0 0 9px; 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padding:1px 5px;}\n  .ols-irs-001 .irs-badge.b-eq{font-size:11px;}\n  .ols-irs-001 .irs-read{padding:5px 9px;}\n  .ols-irs-001 .irs-panel{left:auto; right:14px; transform:none; font-size:11px; padding:6px 10px;}\n}\n@media (prefers-reduced-motion:reduce){\n  .ols-irs-001 .irs-seg button,.ols-irs-001 .irs-pill{transition:none;}\n}\n\n.ols-cc-irs-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-irs-001 a{color:#aab0c0; text-decoration:none;}\n.ols-cc-irs-001 a:hover{text-decoration:underline;}\n<\/style>\n\n<div class=\"irs-head\">\n  <h2 class=\"irs-title\">Infrared Spectroscopy: Bonds That Vibrate<\/h2>\n  <p class=\"irs-sub\">Pick a bond, watch it stretch and bend, and see where it absorbs in the infrared spectrum of ethanol, ethanal, ethanoic acid or chloroethane.<\/p>\n<\/div>\n\n<div class=\"irs-stage\" id=\"irsStage\">\n  <canvas class=\"irs-canvas\" id=\"irsCanvas\"><\/canvas>\n  <div class=\"irs-overlay\" id=\"irsOverlay\"><\/div>\n  <div class=\"irs-caption\" id=\"irsCaption\"><\/div>\n  <div class=\"irs-hint\" id=\"irsHint\">Drag to rotate<\/div>\n  <div class=\"irs-badges\" id=\"irsBadges\"><\/div>\n<\/div>\n\n<div class=\"irs-readbar hide\" id=\"irsRead\" aria-live=\"polite\"><\/div>\n\n<div class=\"irs-controls\">\n  <div class=\"irs-row\">\n    <span class=\"irs-rowlab\">Molecule<\/span>\n    <div class=\"irs-seg\" role=\"group\" aria-label=\"Molecule\" id=\"irsMols\">\n      <button type=\"button\" data-m=\"ethanol\" aria-pressed=\"true\">Ethanol CH<sub>3<\/sub>CH<sub>2<\/sub>OH<\/button>\n      <button type=\"button\" data-m=\"ethanal\" aria-pressed=\"false\">Ethanal CH<sub>3<\/sub>CHO<\/button>\n      <button type=\"button\" data-m=\"acid\" aria-pressed=\"false\">Ethanoic acid CH<sub>3<\/sub>COOH<\/button>\n      <button type=\"button\" data-m=\"chloro\" aria-pressed=\"false\">Chloroethane CH<sub>3<\/sub>CH<sub>2<\/sub>Cl<\/button>\n    <\/div>\n  <\/div>\n\n  <div class=\"irs-row\">\n    <span class=\"irs-rowlab\">Bond<\/span>\n    <div class=\"irs-seg b-teal\" role=\"group\" aria-label=\"Bond\" id=\"irsBonds\"><\/div>\n  <\/div>\n\n  <div class=\"irs-row\">\n    <span class=\"irs-rowlab\">Bonds<\/span>\n    <div class=\"irs-seg\" role=\"group\" aria-label=\"Bonds\">\n      <button type=\"button\" data-step=\"2\" aria-pressed=\"true\">Stretch<\/button>\n      <button type=\"button\" data-step=\"3\" aria-pressed=\"false\">Bend<\/button>\n    <\/div>\n  <\/div>\n\n  <div class=\"irs-row\">\n    <span class=\"irs-rowlab\">Atmosphere<\/span>\n    <button type=\"button\" class=\"irs-pill\" id=\"irsGreen\" aria-pressed=\"false\"><i class=\"dot\"><\/i>Greenhouse<\/button>\n  <\/div>\n\n  <div class=\"irs-row\">\n    <span class=\"irs-rowlab\">View<\/span>\n    <div class=\"irs-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=\"irs-row\">\n    <span class=\"irs-rowlab\">Show<\/span>\n    <button type=\"button\" class=\"irs-pill\" id=\"irsTogLab\" aria-pressed=\"true\"><i class=\"dot\"><\/i>Bond labels<\/button>\n    <button type=\"button\" class=\"irs-pill p-gold\" id=\"irsTogRead\" aria-pressed=\"true\"><i class=\"dot\"><\/i>Wavenumber readout<\/button>\n  <\/div>\n\n  <div class=\"irs-row\">\n    <span class=\"irs-rowlab\">Motion<\/span>\n    <button type=\"button\" class=\"irs-pill\" id=\"irsSpin\" aria-pressed=\"false\"><i class=\"dot\"><\/i>Rotation<\/button>\n    <button type=\"button\" class=\"irs-pill\" id=\"irsResetV\">Reset view<\/button>\n  <\/div>\n<\/div>\n\n<div class=\"irs-info\">\n  <h3 id=\"irsInfoTitle\"><\/h3>\n  <p id=\"irsInfoText\"><\/p>\n  <div class=\"irs-facts\" id=\"irsFacts\"><\/div>\n  <div class=\"irs-key\">\n    <span><i style=\"background:#484e5c\"><\/i>Carbon<\/span>\n    <span><i style=\"background:#eef0f5\"><\/i>Hydrogen<\/span>\n    <span><i style=\"background:#cd342c\"><\/i>Oxygen<\/span>\n    <span><i style=\"background:#3aa048\"><\/i>Chlorine<\/span>\n    <span><i style=\"background:#8c92a0\"><\/i>Bond<\/span>\n    <span><i style=\"background:#14a0a0\"><\/i>Selected bond and its absorption<\/span>\n    <span><i style=\"background:#c9961c\"><\/i>Bending angle<\/span>\n    <span><i style=\"background:#d9433a\"><\/i>Infrared from the Earth<\/span>\n  <\/div>\n<\/div>\n<\/section>\n\n<p class=\"ols-cc-irs-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\/infrared-bonds.js?v=20260926c\"><\/script>\n\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> A bond absorbs infrared radiation at the frequency of its own vibration. The wavenumber of the trough identifies the bond.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">2<\/div>\n<h2>Characteristic Absorptions<\/h2>\n<\/div>\n<p>The region above 1500 cm\u207b\u00b9 is used for <strong>functional group identification<\/strong>, because the absorptions there belong to particular bonds and are found in a data booklet. The exam provides the table; the skill is to match each trough to a bond and to give the range in the answer.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Bond<\/th><th>Where it is found<\/th><th>Wavenumber \/ cm\u207b\u00b9<\/th><th>Appearance<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>C\u2013H<\/strong><\/td><td>alkanes, alkenes, aldehydes, almost every organic compound<\/td><td>2850\u20133100<\/td><td>sharp, medium<\/td><\/tr>\n<tr><td><strong>C=C<\/strong><\/td><td>alkenes<\/td><td>1620\u20131680<\/td><td>medium<\/td><\/tr>\n<tr><td><strong>C=O<\/strong><\/td><td>aldehydes, ketones, carboxylic acids, esters<\/td><td>1680\u20131750<\/td><td>strong, sharp<\/td><\/tr>\n<tr><td><strong>O\u2013H<\/strong><\/td><td>alcohols<\/td><td>3200\u20133600<\/td><td>broad<\/td><\/tr>\n<tr><td><strong>O\u2013H<\/strong><\/td><td>carboxylic acids<\/td><td>2500\u20133300<\/td><td>very broad, overlaps C\u2013H<\/td><\/tr>\n<tr><td><strong>N\u2013H<\/strong><\/td><td>amines<\/td><td>3300\u20133500<\/td><td>medium, often two peaks<\/td><\/tr>\n<tr><td><strong>C\u2013O<\/strong><\/td><td>alcohols, acids, esters<\/td><td>1000\u20131300<\/td><td>strong<\/td><\/tr>\n<tr><td><strong>C\u2013X<\/strong><\/td><td>halogenoalkanes: C\u2013Cl, C\u2013Br<\/td><td>600\u2013800 (C\u2013Cl), 500\u2013600 (C\u2013Br)<\/td><td>in the fingerprint region<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>The O\u2013H absorptions are <strong>broad<\/strong> because hydrogen bonding between molecules spreads the vibration over a range of frequencies; a carboxylic acid O\u2013H is the broadest of all and runs into the C\u2013H region. An alcohol shows a broad O\u2013H near 3350 and no C=O; a carboxylic acid shows both a very broad O\u2013H and a strong C=O near 1715; an aldehyde or ketone shows the C=O but no O\u2013H. An alkene&#8217;s C=C is weaker than a C=O and is easy to miss.<\/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-irspectra-v2.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-irspectra-v2.jpg\" alt=\"Infrared spectra of ethanol and ethanoic acid with the key absorptions labelled, and a table of characteristic absorptions\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>The infrared spectra of ethanol and ethanoic acid with the O\u2013H, C=O, C\u2013H and C\u2013O absorptions labelled and the fingerprint region shaded, beside the wavenumber table.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> Write &#8220;absorption at 1680\u20131750 cm\u207b\u00b9 due to C=O&#8221;. The bond, the range from the data sheet and the word absorption (or trough) are the three parts of the mark.<\/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 Bond, Which Trough<\/h2>\n<p>Assign absorptions in spectra of compounds not discussed on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-971\" class=\"h5p-iframe\" data-content-id=\"971\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Alcohols, Halogenoalkanes and Spectra Drag: Which Bond, Which Trough\"><\/iframe><\/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>The Fingerprint Region<\/h2>\n<\/div>\n<p>Below about 1500 cm\u207b\u00b9 the spectrum contains many overlapping absorptions from bending vibrations and from C\u2013C and C\u2013O stretches. This <strong>fingerprint region<\/strong> is too complicated to assign bond by bond, but its exact pattern is unique to each compound. A computer compares the fingerprint region of an unknown with a database of spectra of pure compounds; an exact match identifies the compound and confirms its purity.<\/p>\n\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Above 1500 cm\u207b\u00b9: identify the functional groups. Below 1500 cm\u207b\u00b9: match the fingerprint to a database.<\/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: Telling Compounds Apart<\/h2>\n<p>Use spectra to distinguish between compounds that are not the examples above.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"972\"><\/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>Common Exam Points<\/h2>\n<\/div>\n<h3>Say<\/h3><p>&#8220;A broad absorption at 3200\u20133600 cm\u207b\u00b9 shows an O\u2013H group in an alcohol.&#8221; &#8220;No absorption at 1680\u20131750 cm\u207b\u00b9, so no C=O bond.&#8221; &#8220;The fingerprint region matches the database spectrum of the pure compound.&#8221;<\/p>\n<h3>Do not say<\/h3><p>&#8220;A peak at 1700&#8221; (say absorption or trough, and quote the range). &#8220;The O\u2013H peak&#8221; without saying alcohol or acid.<\/p>\n<h3>Watch for<\/h3><p>Distinguishing questions: name the absorption that one compound has and the other lacks, with the wavenumber range.<\/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: Predicting a Spectrum<\/h2>\n<p>Predict the absorptions of compounds not shown on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-973\" class=\"h5p-iframe\" data-content-id=\"973\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Alcohols, Halogenoalkanes and Spectra Quick Choice: Predicting a Spectrum\"><\/iframe><\/div><\/div>\n<\/section>\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check infrared spectroscopy.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Why does the spectrum have troughs rather than peaks?<\/h3>\n<p>The y-axis is transmittance, the fraction of radiation that gets through. Where a bond absorbs, less radiation is transmitted, so the line dips.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why is the O\u2013H absorption broad?<\/h3>\n<p>Hydrogen bonding between molecules varies the strength of each O\u2013H bond slightly, so the vibrations are spread over a range of wavenumbers. Carboxylic acids hydrogen bond most strongly and give the broadest band.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Can I tell an aldehyde from a ketone by infrared?<\/h3>\n<p>Not reliably. Both show the C=O absorption near 1700 cm\u207b\u00b9. Use the mass spectrum or a chemical test such as Tollens&#8217; reagent or Fehling&#8217;s solution.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>What is the fingerprint region for?<\/h3>\n<p>Matching. Below 1500 cm\u207b\u00b9 the pattern is unique to each compound, so comparing it with a database spectrum identifies the compound and shows whether it is pure.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Do I have to remember the wavenumbers?<\/h3>\n<p>No. The exam gives a data sheet; the skill is to look up the right bond and quote the range in your answer.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<section class=\"ols-related-card\">\n<h2>Related Topic 22 Analytical Techniques Pages<\/h2>\n<p>Use these pages to connect the ideas across Topic 22 Analytical Techniques 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-22-analytical-techniques\/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\/cie\/topic-22-analytical-techniques\/identifying-unknown-compounds\/\">Identifying Unknown Compounds<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-22-analytical-techniques\/\">Topic 22 Analytical Techniques Overview<\/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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