{"id":1627,"date":"2017-10-10T16:46:11","date_gmt":"2017-10-10T16:46:11","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/?post_type=chapter&#038;p=1627"},"modified":"2017-11-20T19:59:28","modified_gmt":"2017-11-20T19:59:28","slug":"interpreting-infrared-spectra","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/chapter\/interpreting-infrared-spectra\/","title":{"raw":"Interpreting Infrared Spectra","rendered":"Interpreting Infrared Spectra"},"content":{"raw":"<div id=\"elm-main-content\" class=\"elm-content-container\">\r\n<div class=\"textbox learning-objectives\">\r\n<h3>Learning Objectives<\/h3>\r\n<div id=\"elm-main-content\" class=\"elm-content-container\">\r\n<div>\r\n<div id=\"skills\">\r\n\r\nAfter completing this section, you should be able to:\r\n<ol>\r\n \t<li>describe how the so-called \u201cfingerprint region\u201d of an infrared spectrum can assist in the identification of an unknown compound.<\/li>\r\n \t<li>identify the functional group or groups present in a compound, given a list of the most prominent absorptions in the infrared spectrum and a table of characteristic absorption frequencies.<\/li>\r\n \t<li>identify the broad regions of the infrared spectrum in which occur absorptions caused by\r\n<ol>\r\n \t<li><span><span>N\u2212H<\/span><\/span>,\u00a0<span><span>C\u2212H<\/span><\/span>, and\u00a0<span><span>O\u2212H<\/span><\/span><\/li>\r\n \t<li><span><span>C\u2261C<\/span><\/span>\u00a0and\u00a0<span><span>C\u2261N<\/span><\/span><\/li>\r\n \t<li><span><span>C=O<\/span><\/span>,\u00a0<span><span>C=N<\/span><\/span>, and C<span><span><span><span><span><span><span><span>=C<\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<div>\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Key Terms<\/h3>\r\n<div>\r\n\r\nMake certain that you can define, and use in context, the key term below.\r\n<ul>\r\n \t<li>fingerprint region<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n<div class=\"textbox\">\r\n<div>\r\n<h3 class=\"boxtitle\">Study Notes<\/h3>\r\n<\/div>\r\n<div id=\"note\">\r\n\r\nWhen answering assignment questions, you may use this <a href=\"https:\/\/chem.libretexts.org\/Reference\/Reference_Tables\/Spectroscopic_Parameters\/Infrared_Spectroscopy_Absorption_Table\" target=\"_blank\" rel=\"internal noopener\">IR table<\/a> to find the characteristic infrared absorptions of the various functional groups. However, you should be able to indicate in broad terms where certain characteristic absorptions occur. You can achieve this objective by memorizing the following table.\r\n<table>\r\n<tbody>\r\n<tr>\r\n<th scope=\"col\">Region of Spectrum (cm<sup>\u22121<\/sup>)<\/th>\r\n<th scope=\"col\">Absorption<\/th>\r\n<\/tr>\r\n<tr>\r\n<td>2500-4000<\/td>\r\n<td><span><span>N\u2212H<\/span><\/span>,\u00a0<span><span>O\u2212H<\/span><\/span>,\u00a0<span><span>C\u2212H<\/span><\/span><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>2000-2500<\/td>\r\n<td><span><span>C\u2261C<\/span><\/span>,\u00a0<span><span>C\u2261N<\/span><\/span><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>1500-2000<\/td>\r\n<td><span><span>C=O<\/span><\/span>,\u00a0<span><span>C=N<\/span><\/span>,\u00a0<span><span>C=C<\/span><\/span><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>below 1500<\/td>\r\n<td>Fingerprint region<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n<\/div>\r\n<p class=\"boxtitle\">The Origin of Group Frequencies<\/p>\r\n\r\n<\/div>\r\n<div id=\"section_1\">\r\n\r\nAn important observation made by early researchers is that many functional group absorb infrared radiation at about the same wavenumber, regardless of the structure of the rest of the molecule. For example, C-H stretching vibrations usually appear between 3200 and 2800cm<sup>-1<\/sup> and\u00a0carbonyl(C=O) stretching vibrations usually appear between 1800 and 1600cm<sup>-1<\/sup>. This makes these bands diagnostic markers for the presence of a functional group in a sample. These types of infrared bands are called group\u00a0frequencies because they tell us about the presence or absence of specific functional groups in a sample.\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"857\"]<img class=\"internal default\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153444\/group_frequency1_1.gif\" alt=\"group frequency1 (1).gif\" width=\"857\" height=\"400\" \/> Figure 2. Group frequency and fingerprint regions of the mid-infrared spectrum[\/caption]\r\n\r\nThe region of the infrared spectrum from 1200 to 700 cm<sup>-1 <\/sup>is called the fingerprint region. This region is\u00a0notable for the large number of infrared bands that are found there. Many different vibrations, including C-O, C-C and C-N single bond stretches, C-H bending vibrations, and some bands due to benzene rings are found in this region. The fingerprint region is often the most complex and confusing region to interpret, and is usually the last section of a spectrum to be interpreted. However, the utility of the fingerprint region is that the many bands there provide a fingerprint for a molecule.\r\n\r\n<img class=\"internal default aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153447\/image015.png\" alt=\"image016.png\" width=\"750\" height=\"445\" \/>\r\n\r\nThe key absorption peak in this spectrum is that from the carbonyl double bond, at 1716 cm<sup>-1<\/sup> (corresponding to a wavelength of 5.86 mm, a frequency of 5.15 x 10<sup>13<\/sup> Hz, and a <span><strong>\u0394<\/strong><\/span>E value of 4.91 kcal\/mol).\u00a0 Notice how strong this peak is, relative to the others on the spectrum:\u00a0 <em>a strong peak in the 1650-1750 cm<sup>-1<\/sup> region is a dead giveaway for the presence of a carbonyl group<\/em>.\u00a0 Within that range, carboxylic acids, esters, ketones, and aldehydes tend to absorb in the shorter wavelength end (1700-1750 cm-1), while conjugated unsaturated ketones and amides tend to absorb on the longer wavelength end (1650-1700 cm<sup>-1<\/sup>).\r\n\r\nThe jagged peak at approximately 2900-3000 cm<sup>-1<\/sup> is characteristic of tetrahedral carbon-hydrogen bonds.\u00a0 This peak is not terribly useful, as just about every organic molecule that you will have occasion to analyze has these bonds.\u00a0 Nevertheless, it can serve as a familiar reference point to orient yourself in a spectrum.\r\n\r\nYou will notice that there are many additional peaks in this spectrum in the longer-wavelength 400 -1400 cm<sup>-1<\/sup> region.\u00a0 This part of the spectrum is called the <strong>fingerprint region<\/strong>.\u00a0 While it is usually very difficult to pick out any specific functional group identifications from this region, it does, nevertheless, contain valuable information.\u00a0 The reason for this is suggested by the name: just like a human fingerprint, the pattern of absorbance peaks in the fingerprint region is unique to every molecule, meaning that the data from an unknown sample can be compared to the IR spectra of known standards in order to make a positive identification.\u00a0 In the mid-1990's, for example, several paintings were identified as forgeries because scientists were able to identify the IR footprint region of red and yellow pigment compounds that would not have been available to the artist who supposedly created the painting (for more details see <a class=\"external\" title=\"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/cen-v085n037.p028\" href=\"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/cen-v085n037.p028\" target=\"_blank\" rel=\"external nofollow noopener\">Chemical and Engineering News, Sept 10, 2007, p. 28<\/a>).\r\n\r\nNow, let\u2019s take a look at the IR spectrum for 1-hexanol.\r\n\r\n<img class=\"internal default aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153450\/image017.png\" alt=\"image018.png\" width=\"750\" height=\"462\" \/>\r\n\r\nAs you can see, the carbonyl peak is gone, and in its place is a very broad \u2018mountain\u2019 centered at about 3400 cm<sup>-1<\/sup>.\u00a0 This signal is characteristic of the O-H stretching mode of alcohols, and is a dead giveaway for the presence of an alcohol group.\u00a0 The breadth of this signal is a consequence of hydrogen bonding between molecules.\r\n\r\nIn the spectrum of octanoic acid we see, as expected, the characteristic carbonyl peak, this time at 1709 cm<sup>-1<\/sup>.\r\n\r\n<img class=\"internal default aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153453\/image019.png\" alt=\"image020.png\" width=\"750\" height=\"487\" \/>\r\n\r\nWe also see a low, broad absorbance band that looks like an alcohol, except that it is displaced slightly to the right (long-wavelength) side of the spectrum, causing it to overlap to some degree with the C-H region.\u00a0 This is the characteristic carboxylic acid O-H single bond stretching absorbance.\r\n\r\nThe spectrum for 1-octene shows two peaks that are characteristic of alkenes: the one at 1642 cm<sup>-1<\/sup> is due to stretching of the carbon-carbon double bond, and the one at 3079 cm-1 is due to stretching of the s bond between the alkene carbons and their attached hydrogens.\r\n\r\n<img class=\"internal default aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153456\/image021.png\" alt=\"image022.png\" width=\"750\" height=\"444\" \/>\r\n\r\nAlkynes have characteristic IR absorbance peaks in the range of 2100-2250 cm<sup>-1<\/sup> due to stretching of the carbon-carbon triple bond, and terminal alkenes can be identified by their absorbance at about 3300 cm-1, due to stretching of the bond between the sp-hybridized carbon and the terminal hydrogen.\r\n\r\nIt is possible to identify other functional groups such as amines and ethers, but the characteristic peaks for these groups are considerably more subtle and\/or variable, and often are overlapped with peaks from the fingerprint region.\u00a0 For this reason, we will limit our discussion here to the most easily recognized functional groups, which are summarized in this <a title=\"Some characteristic absorption frequencies in IR spectroscopy\" href=\"https:\/\/chem.libretexts.org\/Textbook_Maps\/Organic_Chemistry_Textbook_Maps\/Map%3A_Organic_Chemistry_with_a_Biological_Emphasis_(Soderberg)\/Reference_Tables\/Some_characteristic_absorption_frequencies_in_IR_spectroscopy\" rel=\"internal\">table<\/a>.\r\n\r\nAs you can imagine, obtaining an IR spectrum for a compound will not allow us to figure out the complete structure of even a simple molecule, unless we happen to have a reference spectrum for comparison.\u00a0 In conjunction with other analytical methods, however, IR spectroscopy can prove to be a very valuable tool, given the information it provides about the presence or absence of key functional groups. IR can also be a quick and convenient way for a chemist to check to see if a reaction has proceeded as planned.\u00a0 If we were to run a reaction in which we wished to convert cyclohexanone to cyclohexanol, for example, a quick comparison of the IR spectra of starting compound and product would tell us if we had successfully converted the ketone group to an alcohol.\r\n\r\n<\/div>\r\n<div id=\"section_2\">\r\n<h4 class=\"editable\">More examples of IR spectra<\/h4>\r\nTo illustrate the usefulness of infrared absorption spectra, examples for five C<sub>4<\/sub>H<sub>8<\/sub>O isomers are presented below their corresponding structural formulas.\u00a0 Try to associate each spectrum with one of the isomers in the row above it.\r\n\r\n<map id=\"spectrum\"><\/map><img class=\"internal default aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153459\/c4h8omer.gif\" alt=\"c4h8omer.gif\" \/>\r\n<img class=\"internal default aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153502\/spectr1.gif\" alt=\"spectr1.gif\" \/><img class=\"internal default aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153504\/spectr2.gif\" alt=\"spectr2.gif\" \/><img class=\"internal default aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153506\/spectr3.gif\" alt=\"spectr3.gif\" \/><img class=\"internal default aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153508\/spectr4.gif\" alt=\"spectr4.gif\" \/><img class=\"internal default aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153511\/spectr5.gif\" alt=\"spectr5.gif\" \/>\r\n\r\n<\/div>\r\n<div id=\"section_3\">\r\n<div class=\"textbox exercises\">\r\n<div id=\"section_3\">\r\n<h3 class=\"editable\">Exercises<\/h3>\r\n<div id=\"s61717\">\r\n<div id=\"section_20\">\r\n<h4 id=\"Questions-61717\">Questions<\/h4>\r\n<b>1.\u00a0<\/b>What functional groups give the following signals in an IR spectrum?\r\n\r\nA) 1700 cm<sup>-1<\/sup>\r\n\r\nB) 1550 cm<sup>-1<\/sup>\r\n\r\n<span><span>C) 1700 cm<sup>-1 <\/sup>and 2510-3000 cm<sup>-1 <\/sup><\/span><\/span>\r\n\r\n<strong>2.\u00a0<\/strong>How can you distinguish the following pairs of compounds through IR analysis?\r\n\r\nA)\u00a0 CH<sub>3<\/sub>OH (Methanol) and CH<sub>3<\/sub>CH<sub>2<\/sub>OCH<sub>2<\/sub>CH<sub>3 <\/sub>(Diethylether)\r\n\r\nB)\u00a0 Cyclopentane and 1-pentene.\r\n\r\nC)\r\n\r\n<img class=\"internal default alignnone\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153513\/12.72.png\" alt=\"\" width=\"425\" height=\"114\" \/>\r\n\r\n<\/div>\r\n<div id=\"section_21\">\r\n\r\n[reveal-answer q=\"265841\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"265841\"]\r\n<p id=\"Solutions-61717\">Solutions<\/p>\r\n<strong>1.<\/strong>\r\n\r\n<img class=\"internal default\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153515\/12.7.png\" alt=\"\" width=\"434\" height=\"400\" \/>\r\n\r\n<strong>2.<\/strong>\r\n\r\nA)\u00a0 A OH peak will be present around 3300 cm-1 for methanol and will be absent in the ether.\r\n\r\n\r\nB) \u00a01-pentene will have a alkene peak around 1650 cm-1 for the C=C and there will be another peak around 3100 cm-1 for the sp2 C-H group on the alkene\r\n\r\nC)\u00a0 Cannot distinguish these two isomers. They both have the same functional groups and therefore would have the same peaks on an IR spectra\r\n<\/div>\r\n<\/div>\r\n<\/div>[\/hidden-answer]\r\n\r\n<\/div>\r\n<h3 class=\"editable\">Contributors<\/h3>\r\n<\/div>\r\n<div id=\"section_4\">\r\n<ul>\r\n \t<li><a class=\"external\" title=\"http:\/\/science.athabascau.ca\/staff-pages\/dietmark\" href=\"http:\/\/science.athabascau.ca\/staff-pages\/dietmark\" target=\"_blank\" rel=\"external nofollow noopener\">Dr. Dietmar Kennepohl<\/a> FCIC (Professor of Chemistry, <a class=\"external\" title=\"http:\/\/www.athabascau.ca\/\" href=\"http:\/\/www.athabascau.ca\/\" target=\"_blank\" rel=\"external nofollow noopener\">Athabasca University<\/a>)<\/li>\r\n \t<li>Prof. Steven Farmer (<a class=\"external\" title=\"http:\/\/www.sonoma.edu\" href=\"http:\/\/www.sonoma.edu\" target=\"_blank\" rel=\"external nofollow noopener\">Sonoma State University<\/a>)<\/li>\r\n \t<li>William Reusch, Professor Emeritus (<a class=\"external\" title=\"http:\/\/www.msu.edu\/\" href=\"http:\/\/www.msu.edu\/\" target=\"_blank\" rel=\"external nofollow noopener\">Michigan State U.<\/a>), <a class=\"external\" title=\"http:\/\/www.cem.msu.edu\/~reusch\/VirtualText\/intro1.htm\" href=\"http:\/\/www.cem.msu.edu\/%7Ereusch\/VirtualText\/intro1.htm\" target=\"_blank\" rel=\"external nofollow noopener\">Virtual Textbook of\u00a0Organic\u00a0Chemistry<\/a><\/li>\r\n \t<li><a title=\"http:\/\/chemwiki.ucdavis.edu\/Organic_Chemistry\/Organic_Chemistry_With_a_Biological_Emphasis\" href=\"\/Organic_Chemistry\/Organic_Chemistry_With_a_Biological_Emphasis\" rel=\"internal\"><strong>Organic Chemistry With a Biological Emphasis <\/strong><\/a>by\u00a0<a class=\"external\" title=\"http:\/\/facultypages.morris.umn.edu\/~soderbt\/\" href=\"http:\/\/facultypages.morris.umn.edu\/%7Esoderbt\/\" target=\"_blank\" rel=\"external nofollow noopener\">Tim Soderberg<\/a>\u00a0(University of Minnesota, Morris)<\/li>\r\n<\/ul>\r\n<\/div>","rendered":"<div id=\"elm-main-content\" class=\"elm-content-container\">\n<div class=\"textbox learning-objectives\">\n<h3>Learning Objectives<\/h3>\n<div id=\"elm-main-content\" class=\"elm-content-container\">\n<div>\n<div id=\"skills\">\n<p>After completing this section, you should be able to:<\/p>\n<ol>\n<li>describe how the so-called \u201cfingerprint region\u201d of an infrared spectrum can assist in the identification of an unknown compound.<\/li>\n<li>identify the functional group or groups present in a compound, given a list of the most prominent absorptions in the infrared spectrum and a table of characteristic absorption frequencies.<\/li>\n<li>identify the broad regions of the infrared spectrum in which occur absorptions caused by\n<ol>\n<li><span><span>N\u2212H<\/span><\/span>,\u00a0<span><span>C\u2212H<\/span><\/span>, and\u00a0<span><span>O\u2212H<\/span><\/span><\/li>\n<li><span><span>C\u2261C<\/span><\/span>\u00a0and\u00a0<span><span>C\u2261N<\/span><\/span><\/li>\n<li><span><span>C=O<\/span><\/span>,\u00a0<span><span>C=N<\/span><\/span>, and C<span><span><span><span><span><span><span><span>=C<\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/span><\/li>\n<\/ol>\n<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div>\n<div class=\"textbox key-takeaways\">\n<h3>Key Terms<\/h3>\n<div>\n<p>Make certain that you can define, and use in context, the key term below.<\/p>\n<ul>\n<li>fingerprint region<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div class=\"textbox\">\n<div>\n<h3 class=\"boxtitle\">Study Notes<\/h3>\n<\/div>\n<div id=\"note\">\n<p>When answering assignment questions, you may use this <a href=\"https:\/\/chem.libretexts.org\/Reference\/Reference_Tables\/Spectroscopic_Parameters\/Infrared_Spectroscopy_Absorption_Table\" target=\"_blank\" rel=\"internal noopener\">IR table<\/a> to find the characteristic infrared absorptions of the various functional groups. However, you should be able to indicate in broad terms where certain characteristic absorptions occur. You can achieve this objective by memorizing the following table.<\/p>\n<table>\n<tbody>\n<tr>\n<th scope=\"col\">Region of Spectrum (cm<sup>\u22121<\/sup>)<\/th>\n<th scope=\"col\">Absorption<\/th>\n<\/tr>\n<tr>\n<td>2500-4000<\/td>\n<td><span><span>N\u2212H<\/span><\/span>,\u00a0<span><span>O\u2212H<\/span><\/span>,\u00a0<span><span>C\u2212H<\/span><\/span><\/td>\n<\/tr>\n<tr>\n<td>2000-2500<\/td>\n<td><span><span>C\u2261C<\/span><\/span>,\u00a0<span><span>C\u2261N<\/span><\/span><\/td>\n<\/tr>\n<tr>\n<td>1500-2000<\/td>\n<td><span><span>C=O<\/span><\/span>,\u00a0<span><span>C=N<\/span><\/span>,\u00a0<span><span>C=C<\/span><\/span><\/td>\n<\/tr>\n<tr>\n<td>below 1500<\/td>\n<td>Fingerprint region<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n<p class=\"boxtitle\">The Origin of Group Frequencies<\/p>\n<\/div>\n<div id=\"section_1\">\n<p>An important observation made by early researchers is that many functional group absorb infrared radiation at about the same wavenumber, regardless of the structure of the rest of the molecule. For example, C-H stretching vibrations usually appear between 3200 and 2800cm<sup>-1<\/sup> and\u00a0carbonyl(C=O) stretching vibrations usually appear between 1800 and 1600cm<sup>-1<\/sup>. This makes these bands diagnostic markers for the presence of a functional group in a sample. These types of infrared bands are called group\u00a0frequencies because they tell us about the presence or absence of specific functional groups in a sample.<\/p>\n<div style=\"width: 867px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"internal default\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153444\/group_frequency1_1.gif\" alt=\"group frequency1 (1).gif\" width=\"857\" height=\"400\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 2. Group frequency and fingerprint regions of the mid-infrared spectrum<\/p>\n<\/div>\n<p>The region of the infrared spectrum from 1200 to 700 cm<sup>-1 <\/sup>is called the fingerprint region. This region is\u00a0notable for the large number of infrared bands that are found there. Many different vibrations, including C-O, C-C and C-N single bond stretches, C-H bending vibrations, and some bands due to benzene rings are found in this region. The fingerprint region is often the most complex and confusing region to interpret, and is usually the last section of a spectrum to be interpreted. However, the utility of the fingerprint region is that the many bands there provide a fingerprint for a molecule.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"internal default aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153447\/image015.png\" alt=\"image016.png\" width=\"750\" height=\"445\" \/><\/p>\n<p>The key absorption peak in this spectrum is that from the carbonyl double bond, at 1716 cm<sup>-1<\/sup> (corresponding to a wavelength of 5.86 mm, a frequency of 5.15 x 10<sup>13<\/sup> Hz, and a <span><strong>\u0394<\/strong><\/span>E value of 4.91 kcal\/mol).\u00a0 Notice how strong this peak is, relative to the others on the spectrum:\u00a0 <em>a strong peak in the 1650-1750 cm<sup>-1<\/sup> region is a dead giveaway for the presence of a carbonyl group<\/em>.\u00a0 Within that range, carboxylic acids, esters, ketones, and aldehydes tend to absorb in the shorter wavelength end (1700-1750 cm-1), while conjugated unsaturated ketones and amides tend to absorb on the longer wavelength end (1650-1700 cm<sup>-1<\/sup>).<\/p>\n<p>The jagged peak at approximately 2900-3000 cm<sup>-1<\/sup> is characteristic of tetrahedral carbon-hydrogen bonds.\u00a0 This peak is not terribly useful, as just about every organic molecule that you will have occasion to analyze has these bonds.\u00a0 Nevertheless, it can serve as a familiar reference point to orient yourself in a spectrum.<\/p>\n<p>You will notice that there are many additional peaks in this spectrum in the longer-wavelength 400 -1400 cm<sup>-1<\/sup> region.\u00a0 This part of the spectrum is called the <strong>fingerprint region<\/strong>.\u00a0 While it is usually very difficult to pick out any specific functional group identifications from this region, it does, nevertheless, contain valuable information.\u00a0 The reason for this is suggested by the name: just like a human fingerprint, the pattern of absorbance peaks in the fingerprint region is unique to every molecule, meaning that the data from an unknown sample can be compared to the IR spectra of known standards in order to make a positive identification.\u00a0 In the mid-1990&#8217;s, for example, several paintings were identified as forgeries because scientists were able to identify the IR footprint region of red and yellow pigment compounds that would not have been available to the artist who supposedly created the painting (for more details see <a class=\"external\" title=\"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/cen-v085n037.p028\" href=\"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/cen-v085n037.p028\" target=\"_blank\" rel=\"external nofollow noopener\">Chemical and Engineering News, Sept 10, 2007, p. 28<\/a>).<\/p>\n<p>Now, let\u2019s take a look at the IR spectrum for 1-hexanol.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"internal default aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153450\/image017.png\" alt=\"image018.png\" width=\"750\" height=\"462\" \/><\/p>\n<p>As you can see, the carbonyl peak is gone, and in its place is a very broad \u2018mountain\u2019 centered at about 3400 cm<sup>-1<\/sup>.\u00a0 This signal is characteristic of the O-H stretching mode of alcohols, and is a dead giveaway for the presence of an alcohol group.\u00a0 The breadth of this signal is a consequence of hydrogen bonding between molecules.<\/p>\n<p>In the spectrum of octanoic acid we see, as expected, the characteristic carbonyl peak, this time at 1709 cm<sup>-1<\/sup>.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"internal default aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153453\/image019.png\" alt=\"image020.png\" width=\"750\" height=\"487\" \/><\/p>\n<p>We also see a low, broad absorbance band that looks like an alcohol, except that it is displaced slightly to the right (long-wavelength) side of the spectrum, causing it to overlap to some degree with the C-H region.\u00a0 This is the characteristic carboxylic acid O-H single bond stretching absorbance.<\/p>\n<p>The spectrum for 1-octene shows two peaks that are characteristic of alkenes: the one at 1642 cm<sup>-1<\/sup> is due to stretching of the carbon-carbon double bond, and the one at 3079 cm-1 is due to stretching of the s bond between the alkene carbons and their attached hydrogens.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"internal default aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153456\/image021.png\" alt=\"image022.png\" width=\"750\" height=\"444\" \/><\/p>\n<p>Alkynes have characteristic IR absorbance peaks in the range of 2100-2250 cm<sup>-1<\/sup> due to stretching of the carbon-carbon triple bond, and terminal alkenes can be identified by their absorbance at about 3300 cm-1, due to stretching of the bond between the sp-hybridized carbon and the terminal hydrogen.<\/p>\n<p>It is possible to identify other functional groups such as amines and ethers, but the characteristic peaks for these groups are considerably more subtle and\/or variable, and often are overlapped with peaks from the fingerprint region.\u00a0 For this reason, we will limit our discussion here to the most easily recognized functional groups, which are summarized in this <a title=\"Some characteristic absorption frequencies in IR spectroscopy\" href=\"https:\/\/chem.libretexts.org\/Textbook_Maps\/Organic_Chemistry_Textbook_Maps\/Map%3A_Organic_Chemistry_with_a_Biological_Emphasis_(Soderberg)\/Reference_Tables\/Some_characteristic_absorption_frequencies_in_IR_spectroscopy\" rel=\"internal\">table<\/a>.<\/p>\n<p>As you can imagine, obtaining an IR spectrum for a compound will not allow us to figure out the complete structure of even a simple molecule, unless we happen to have a reference spectrum for comparison.\u00a0 In conjunction with other analytical methods, however, IR spectroscopy can prove to be a very valuable tool, given the information it provides about the presence or absence of key functional groups. IR can also be a quick and convenient way for a chemist to check to see if a reaction has proceeded as planned.\u00a0 If we were to run a reaction in which we wished to convert cyclohexanone to cyclohexanol, for example, a quick comparison of the IR spectra of starting compound and product would tell us if we had successfully converted the ketone group to an alcohol.<\/p>\n<\/div>\n<div id=\"section_2\">\n<h4 class=\"editable\">More examples of IR spectra<\/h4>\n<p>To illustrate the usefulness of infrared absorption spectra, examples for five C<sub>4<\/sub>H<sub>8<\/sub>O isomers are presented below their corresponding structural formulas.\u00a0 Try to associate each spectrum with one of the isomers in the row above it.<\/p>\n<map id=\"spectrum\" name=\"name\"><\/map>\n<p><img decoding=\"async\" class=\"internal default aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153459\/c4h8omer.gif\" alt=\"c4h8omer.gif\" \/><br \/>\n<img decoding=\"async\" class=\"internal default aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153502\/spectr1.gif\" alt=\"spectr1.gif\" \/><img decoding=\"async\" class=\"internal default aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153504\/spectr2.gif\" alt=\"spectr2.gif\" \/><img decoding=\"async\" class=\"internal default aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153506\/spectr3.gif\" alt=\"spectr3.gif\" \/><img decoding=\"async\" class=\"internal default aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153508\/spectr4.gif\" alt=\"spectr4.gif\" \/><img decoding=\"async\" class=\"internal default aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153511\/spectr5.gif\" alt=\"spectr5.gif\" \/><\/p>\n<\/div>\n<div id=\"section_3\">\n<div class=\"textbox exercises\">\n<div id=\"section_3\">\n<h3 class=\"editable\">Exercises<\/h3>\n<div id=\"s61717\">\n<div id=\"section_20\">\n<h4 id=\"Questions-61717\">Questions<\/h4>\n<p><b>1.\u00a0<\/b>What functional groups give the following signals in an IR spectrum?<\/p>\n<p>A) 1700 cm<sup>-1<\/sup><\/p>\n<p>B) 1550 cm<sup>-1<\/sup><\/p>\n<p><span><span>C) 1700 cm<sup>-1 <\/sup>and 2510-3000 cm<sup>-1 <\/sup><\/span><\/span><\/p>\n<p><strong>2.\u00a0<\/strong>How can you distinguish the following pairs of compounds through IR analysis?<\/p>\n<p>A)\u00a0 CH<sub>3<\/sub>OH (Methanol) and CH<sub>3<\/sub>CH<sub>2<\/sub>OCH<sub>2<\/sub>CH<sub>3 <\/sub>(Diethylether)<\/p>\n<p>B)\u00a0 Cyclopentane and 1-pentene.<\/p>\n<p>C)<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"internal default alignnone\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153513\/12.72.png\" alt=\"\" width=\"425\" height=\"114\" \/><\/p>\n<\/div>\n<div id=\"section_21\">\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q265841\">Show Answer<\/span><\/p>\n<div id=\"q265841\" class=\"hidden-answer\" style=\"display: none\">\n<p id=\"Solutions-61717\">Solutions<\/p>\n<p><strong>1.<\/strong><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"internal default\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05153515\/12.7.png\" alt=\"\" width=\"434\" height=\"400\" \/><\/p>\n<p><strong>2.<\/strong><\/p>\n<p>A)\u00a0 A OH peak will be present around 3300 cm-1 for methanol and will be absent in the ether.<\/p>\n<p>B) \u00a01-pentene will have a alkene peak around 1650 cm-1 for the C=C and there will be another peak around 3100 cm-1 for the sp2 C-H group on the alkene<\/p>\n<p>C)\u00a0 Cannot distinguish these two isomers. They both have the same functional groups and therefore would have the same peaks on an IR spectra\n<\/p><\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<h3 class=\"editable\">Contributors<\/h3>\n<\/div>\n<div id=\"section_4\">\n<ul>\n<li><a class=\"external\" title=\"http:\/\/science.athabascau.ca\/staff-pages\/dietmark\" href=\"http:\/\/science.athabascau.ca\/staff-pages\/dietmark\" target=\"_blank\" rel=\"external nofollow noopener\">Dr. Dietmar Kennepohl<\/a> FCIC (Professor of Chemistry, <a class=\"external\" title=\"http:\/\/www.athabascau.ca\/\" href=\"http:\/\/www.athabascau.ca\/\" target=\"_blank\" rel=\"external nofollow noopener\">Athabasca University<\/a>)<\/li>\n<li>Prof. Steven Farmer (<a class=\"external\" title=\"http:\/\/www.sonoma.edu\" href=\"http:\/\/www.sonoma.edu\" target=\"_blank\" rel=\"external nofollow noopener\">Sonoma State University<\/a>)<\/li>\n<li>William Reusch, Professor Emeritus (<a class=\"external\" title=\"http:\/\/www.msu.edu\/\" href=\"http:\/\/www.msu.edu\/\" target=\"_blank\" rel=\"external nofollow noopener\">Michigan State U.<\/a>), <a class=\"external\" title=\"http:\/\/www.cem.msu.edu\/~reusch\/VirtualText\/intro1.htm\" href=\"http:\/\/www.cem.msu.edu\/%7Ereusch\/VirtualText\/intro1.htm\" target=\"_blank\" rel=\"external nofollow noopener\">Virtual Textbook of\u00a0Organic\u00a0Chemistry<\/a><\/li>\n<li><a title=\"http:\/\/chemwiki.ucdavis.edu\/Organic_Chemistry\/Organic_Chemistry_With_a_Biological_Emphasis\" href=\"\/Organic_Chemistry\/Organic_Chemistry_With_a_Biological_Emphasis\" rel=\"internal\"><strong>Organic Chemistry With a Biological Emphasis <\/strong><\/a>by\u00a0<a class=\"external\" title=\"http:\/\/facultypages.morris.umn.edu\/~soderbt\/\" href=\"http:\/\/facultypages.morris.umn.edu\/%7Esoderbt\/\" target=\"_blank\" rel=\"external nofollow noopener\">Tim Soderberg<\/a>\u00a0(University of Minnesota, Morris)<\/li>\n<\/ul>\n<\/div>\n","protected":false},"author":44985,"menu_order":3,"template":"","meta":{"_candela_citation":"[]","CANDELA_OUTCOMES_GUID":"","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-1627","chapter","type-chapter","status-publish","hentry"],"part":29,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/1627","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/wp\/v2\/users\/44985"}],"version-history":[{"count":12,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/1627\/revisions"}],"predecessor-version":[{"id":2141,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/1627\/revisions\/2141"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/parts\/29"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/1627\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/wp\/v2\/media?parent=1627"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/chapter-type?post=1627"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/wp\/v2\/contributor?post=1627"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/wp\/v2\/license?post=1627"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}