{"id":1458,"date":"2017-10-12T14:18:11","date_gmt":"2017-10-12T14:18:11","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/?post_type=chapter&#038;p=1458"},"modified":"2018-10-05T19:03:11","modified_gmt":"2018-10-05T19:03:11","slug":"aromatic-heterocyclic-compounds","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/chapter\/aromatic-heterocyclic-compounds\/","title":{"raw":"Aromatic Heterocyclic Compounds","rendered":"Aromatic Heterocyclic Compounds"},"content":{"raw":"<div class=\"elm-header\">\r\n<div class=\"elm-header-custom\">\r\n<div class=\"textbox learning-objectives\">\r\n<h3>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>draw the structure of the common aromatic heterocycles pyridine and pyrrole.<\/li>\r\n \t<li>use the H\u00fcckel 4<em>n<\/em> + 2 rule to explain the aromaticity of each of pyridine and pyrrole.<\/li>\r\n \t<li>draw a diagram to show the orbitals involved in forming the conjugated six\u2011pi\u2011electron systems present in aromatic heterocycles such as pyridine, pyrrole, etc.<\/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 id=\"elm-main-content\" class=\"elm-content-container\">\r\n<div>\r\n<div>\r\n<div class=\"textbox key-takeaways\">\r\n<div>\r\n<h3 class=\"boxtitle\">Key Terms<\/h3>\r\nMake certain that you can define, and use in context, the key terms below.\r\n<ul>\r\n \t<li><a href=\"https:\/\/chem.libretexts.org\/Reference\/Organic_Chemistry_Glossary\/Carbocycle\" target=\"_blank\" rel=\"internal noopener\">carbocycles<\/a><\/li>\r\n \t<li><a href=\"https:\/\/chem.libretexts.org\/Reference\/Organic_Chemistry_Glossary\/Heterocycle\" target=\"_blank\" rel=\"internal noopener\">heterocycles<\/a><\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n<h3 class=\"boxtitle\">Aromatic Heterocycles<\/h3>\r\n<\/div>\r\n<div id=\"section_1\">\r\n\r\nMany unsaturated cyclic compounds have exceptional properties that we now consider characteristic of \"aromatic\" systems. The following cases are illustrative:\r\n<table style=\"width: 697px\" border=\"0\" cellpadding=\"0\">\r\n<tbody>\r\n<tr>\r\n<td><strong>Compound<\/strong><\/td>\r\n<td><strong>Structural\r\nFormula<\/strong><\/td>\r\n<td><strong>Reaction\r\nwith Br<sub>2<\/sub><\/strong><\/td>\r\n<td><strong>Thermodynamic\r\nStabilization<\/strong><\/td>\r\n<\/tr>\r\n<tr>\r\n<td colspan=\"7\"><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>1,3-Cyclopentadiene<\/td>\r\n<td rowspan=\"7\">\u00a0 \u00a0 \u00a0 \u00a0\u00a0 \u00a0 <img src=\"https:\/\/chem.libretexts.org\/Textbook_Maps\/Organic_Chemistry_Textbook_Maps\/Map%3A_Organic_Chemistry_(McMurry)\/Chapter_15%3A_Benzene_and_Aromaticity\/denied:data:image\/png;base64,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#fixme\" alt=\"\" \/><\/td>\r\n<td>Addition ( 0 \u00baC )<\/td>\r\n<td>\u00a0Slight<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>1,3,5-Cycloheptatriene<\/td>\r\n<td>Addition ( 0 \u00baC )<\/td>\r\n<td>Slight<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>1,3,5,7-Cyclooctatetraene<\/td>\r\n<td>Addition ( 0 \u00baC )<\/td>\r\n<td>Slight<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Benzene<\/td>\r\n<td>Substitution<\/td>\r\n<td>Large<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Pyridine<\/td>\r\n<td>Substitution<\/td>\r\n<td>Large<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Furan<\/td>\r\n<td>Substitution ( 0 \u00baC )<\/td>\r\n<td>Moderate<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Pyrrole<\/td>\r\n<td>Substitution<\/td>\r\n<td>Moderate<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<div>\r\n\r\nBenzene is the archetypical aromatic compound. It is planar, bond angles=120\u00ba, all carbon atoms in the ring are sp<sup>2<\/sup> hybridized, and the pi-orbitals are occupied by 6 electrons. The aromatic heterocycle pyridine is similar to benzene, and is often used as a weak base for scavenging protons. Furan and pyrrole have heterocyclic five-membered rings, in which the heteroatom has at least one pair of non-bonding valence shell electrons. By hybridizing this heteroatom to a sp2 state, a p-orbital occupied by a pair of electrons and oriented parallel to the carbon p-orbitals is created. The resulting planar ring meets the first requirement for aromaticity, and the \u03c0-system is occupied by 6 electrons, 4 from the two double bonds and 2 from the heteroatom, thus satisfying the H\u00fcckel Rule.\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\/05151254\/1.jpg\" alt=\"http:\/\/www2.chemistry.msu.edu\/faculty\/reusch\/VirtTxtJml\/Images\/annulen1.gif\" \/>\r\n\r\nFour illustrative examples of aromatic compounds are shown above. The sp2 hybridized ring atoms are connected by brown bonds, the \u03c0-electron pairs and bonds that constitute the aromatic ring are colored blue. Electron pairs that are not part of the aromatic \u03c0-electron system are black. The first example is azulene, a blue-colored 10 \u03c0-electron aromatic hydrocarbon isomeric with naphthalene. The second and third compounds are heterocycles having aromatic properties. Pyridine has a benzene-like six-membered ring incorporating one nitrogen atom. The non-bonding electron pair on the nitrogen is not part of the aromatic \u03c0-electron sextet, and may bond to a proton or other electrophile without disrupting the aromatic system. In the case of thiophene, a sulfur analog of furan, one of the sulfur electron pairs (colored blue) participates in the aromatic ring \u03c0-electron conjugation. The last compound is imidazole, a heterocycle having two nitrogen atoms. Note that only one of the nitrogen non-bonding electron pairs is used for the aromatic \u03c0-electron sextet. The other electron pair (colored black) behaves similarly to the electron pair in pyridine.\r\n\r\nHeterocycles - cyclic structures in which the ring atoms may include oxygen or nitrogen - can also be aromatic.\u00a0 Pyridine, for example, is an aromatic heterocycle.\u00a0 In the bonding picture for pyridine, the nitrogen is <em>sp<sup>2<\/sup><\/em>-hybridized, with two of the three <em>sp<sup>2<\/sup><\/em> orbitals forming sigma overlaps with the <em>sp<sup>2<\/sup><\/em> orbitals of neighboring carbon atoms, and the third nitrogen <em>sp<sup>2<\/sup><\/em> orbital containing the lone pair.\u00a0 The unhybridized <em>p<\/em> orbital contains a single electron, which is part of the 6 pi-electron system delocalized around the ring.\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\/05151259\/fig2-2-16.png\" alt=\"fig2-2-16.png\" width=\"681\" height=\"385\" \/>\r\n\r\n<a class=\"external\" title=\"http:\/\/wps.prenhall.com\/wps\/media\/objects\/724\/741576\/Instructor_Resources\/Chapter_15\/Text_Images\/FG15_01-13-2UN.JPG\" href=\"http:\/\/wps.prenhall.com\/wps\/media\/objects\/724\/741576\/Instructor_Resources\/Chapter_15\/Text_Images\/FG15_01-13-2UN.JPG\" target=\"_blank\" rel=\"external nofollow noopener\">another image of orbitals in pyridine<\/a>\r\n\r\nWhy do we not assume that the nitrogen in pyrrole is s<em>p<sup>3<\/sup><\/em>-hybridized, like a normal secondary amine? The answer is simple: if it were, then pyrrole could not be aromatic, and thus it would not have the stability associated with aromaticity.\u00a0 In general,<em> if a molecule or group can be aromatic, it will be,<\/em> just as water will always flow downhill if there is a downhill pathway available.\r\n\r\nImidazole is another important example of an aromatic heterocycle found in biomolecules - the side chain of the amino acid histidine contains an imidazole ring.\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\/05151302\/fig2-2-21.png\" alt=\"\" width=\"319\" height=\"153\" \/>\r\n\r\nIn imidazole, one nitrogen is 'pyrrole-like' (the lone pair contributes to the aromatic sextet) and one is 'pyridine-like' (the lone pair is located in an <em>sp<sup>2<\/sup><\/em> orbital, and is <em>not<\/em> part of the aromatic sextet).\r\n\r\n<\/div>\r\n<\/div>\r\n<div id=\"section_2\">\r\n<div class=\"textbox exercises\">\r\n<h3 class=\"editable\">Exercises<\/h3>\r\n<div id=\"s61720\">\r\n<div id=\"section_20\">\r\n\r\n<b>1.<\/b>\r\n\r\nDraw the orbitals of thiophene to show that is aromatic.\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\/05151304\/15-5-1qu.png\" alt=\"\" width=\"97\" height=\"106\" \/>\r\n\r\n<strong>2.<\/strong>\r\n\r\nThe following ring is called a thiazolium ring. Describe how it is aromatic.\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\/05151305\/15-5-2qu.png\" alt=\"\" width=\"112\" height=\"101\" \/>\r\n\r\n<\/div>\r\n<div id=\"section_21\">\r\n<h3 id=\"Solutions-61720\">Solutions<\/h3>\r\n<strong>\r\n[reveal-answer q=\"740202\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"740202\"]1.<\/strong>\r\n\r\nThis drawing shows it has 6 electrons in the pi-orbital.\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\/05151307\/15-5-1sol.gif\" alt=\"\" width=\"229\" height=\"142\" \/>\r\n\r\n<strong>2.<\/strong>\r\n\r\nSimilar to the last question, the drawing shows that there is only 6 electrons in the pi-system.\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\/05151310\/15-5-2sol.gif\" alt=\"\" width=\"264\" height=\"139\" \/><strong>[\/hidden-answer]<\/strong>\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<div id=\"section_3\">\r\n<h3 class=\"editable\">Contributors<\/h3>\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=\"Organic_Chemistry_With_a_Biological_Emphasis\" href=\"https:\/\/chem.libretexts.org\/Textbook_Maps\/Organic_Chemistry_Textbook_Maps\/Map%3A_Organic_Chemistry_with_a_Biological_Emphasis_(Soderberg)\" rel=\"internal\">Organic Chemistry With a Biological Emphasis <\/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>\r\n<\/div>\r\n<\/div>","rendered":"<div class=\"elm-header\">\n<div class=\"elm-header-custom\">\n<div class=\"textbox learning-objectives\">\n<h3>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>draw the structure of the common aromatic heterocycles pyridine and pyrrole.<\/li>\n<li>use the H\u00fcckel 4<em>n<\/em> + 2 rule to explain the aromaticity of each of pyridine and pyrrole.<\/li>\n<li>draw a diagram to show the orbitals involved in forming the conjugated six\u2011pi\u2011electron systems present in aromatic heterocycles such as pyridine, pyrrole, etc.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div id=\"elm-main-content\" class=\"elm-content-container\">\n<div>\n<div>\n<div class=\"textbox key-takeaways\">\n<div>\n<h3 class=\"boxtitle\">Key Terms<\/h3>\n<p>Make certain that you can define, and use in context, the key terms below.<\/p>\n<ul>\n<li><a href=\"https:\/\/chem.libretexts.org\/Reference\/Organic_Chemistry_Glossary\/Carbocycle\" target=\"_blank\" rel=\"internal noopener\">carbocycles<\/a><\/li>\n<li><a href=\"https:\/\/chem.libretexts.org\/Reference\/Organic_Chemistry_Glossary\/Heterocycle\" target=\"_blank\" rel=\"internal noopener\">heterocycles<\/a><\/li>\n<\/ul>\n<\/div>\n<\/div>\n<h3 class=\"boxtitle\">Aromatic Heterocycles<\/h3>\n<\/div>\n<div id=\"section_1\">\n<p>Many unsaturated cyclic compounds have exceptional properties that we now consider characteristic of &#8220;aromatic&#8221; systems. The following cases are illustrative:<\/p>\n<table style=\"width: 697px\" cellpadding=\"0\">\n<tbody>\n<tr>\n<td><strong>Compound<\/strong><\/td>\n<td><strong>Structural<br \/>\nFormula<\/strong><\/td>\n<td><strong>Reaction<br \/>\nwith Br<sub>2<\/sub><\/strong><\/td>\n<td><strong>Thermodynamic<br \/>\nStabilization<\/strong><\/td>\n<\/tr>\n<tr>\n<td colspan=\"7\"><\/td>\n<\/tr>\n<tr>\n<td>1,3-Cyclopentadiene<\/td>\n<td rowspan=\"7\">\u00a0 \u00a0 \u00a0 \u00a0\u00a0 \u00a0 <img decoding=\"async\" src=\"https:\/\/chem.libretexts.org\/Textbook_Maps\/Organic_Chemistry_Textbook_Maps\/Map%3A_Organic_Chemistry_(McMurry)\/Chapter_15%3A_Benzene_and_Aromaticity\/denied:data:image\/png;base64,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#fixme\" alt=\"\" \/><\/td>\n<td>Addition ( 0 \u00baC )<\/td>\n<td>\u00a0Slight<\/td>\n<\/tr>\n<tr>\n<td>1,3,5-Cycloheptatriene<\/td>\n<td>Addition ( 0 \u00baC )<\/td>\n<td>Slight<\/td>\n<\/tr>\n<tr>\n<td>1,3,5,7-Cyclooctatetraene<\/td>\n<td>Addition ( 0 \u00baC )<\/td>\n<td>Slight<\/td>\n<\/tr>\n<tr>\n<td>Benzene<\/td>\n<td>Substitution<\/td>\n<td>Large<\/td>\n<\/tr>\n<tr>\n<td>Pyridine<\/td>\n<td>Substitution<\/td>\n<td>Large<\/td>\n<\/tr>\n<tr>\n<td>Furan<\/td>\n<td>Substitution ( 0 \u00baC )<\/td>\n<td>Moderate<\/td>\n<\/tr>\n<tr>\n<td>Pyrrole<\/td>\n<td>Substitution<\/td>\n<td>Moderate<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<div>\n<p>Benzene is the archetypical aromatic compound. It is planar, bond angles=120\u00ba, all carbon atoms in the ring are sp<sup>2<\/sup> hybridized, and the pi-orbitals are occupied by 6 electrons. The aromatic heterocycle pyridine is similar to benzene, and is often used as a weak base for scavenging protons. Furan and pyrrole have heterocyclic five-membered rings, in which the heteroatom has at least one pair of non-bonding valence shell electrons. By hybridizing this heteroatom to a sp2 state, a p-orbital occupied by a pair of electrons and oriented parallel to the carbon p-orbitals is created. The resulting planar ring meets the first requirement for aromaticity, and the \u03c0-system is occupied by 6 electrons, 4 from the two double bonds and 2 from the heteroatom, thus satisfying the H\u00fcckel Rule.<\/p>\n<p><img decoding=\"async\" class=\"internal default\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05151254\/1.jpg\" alt=\"http:\/\/www2.chemistry.msu.edu\/faculty\/reusch\/VirtTxtJml\/Images\/annulen1.gif\" \/><\/p>\n<p>Four illustrative examples of aromatic compounds are shown above. The sp2 hybridized ring atoms are connected by brown bonds, the \u03c0-electron pairs and bonds that constitute the aromatic ring are colored blue. Electron pairs that are not part of the aromatic \u03c0-electron system are black. The first example is azulene, a blue-colored 10 \u03c0-electron aromatic hydrocarbon isomeric with naphthalene. The second and third compounds are heterocycles having aromatic properties. Pyridine has a benzene-like six-membered ring incorporating one nitrogen atom. The non-bonding electron pair on the nitrogen is not part of the aromatic \u03c0-electron sextet, and may bond to a proton or other electrophile without disrupting the aromatic system. In the case of thiophene, a sulfur analog of furan, one of the sulfur electron pairs (colored blue) participates in the aromatic ring \u03c0-electron conjugation. The last compound is imidazole, a heterocycle having two nitrogen atoms. Note that only one of the nitrogen non-bonding electron pairs is used for the aromatic \u03c0-electron sextet. The other electron pair (colored black) behaves similarly to the electron pair in pyridine.<\/p>\n<p>Heterocycles &#8211; cyclic structures in which the ring atoms may include oxygen or nitrogen &#8211; can also be aromatic.\u00a0 Pyridine, for example, is an aromatic heterocycle.\u00a0 In the bonding picture for pyridine, the nitrogen is <em>sp<sup>2<\/sup><\/em>-hybridized, with two of the three <em>sp<sup>2<\/sup><\/em> orbitals forming sigma overlaps with the <em>sp<sup>2<\/sup><\/em> orbitals of neighboring carbon atoms, and the third nitrogen <em>sp<sup>2<\/sup><\/em> orbital containing the lone pair.\u00a0 The unhybridized <em>p<\/em> orbital contains a single electron, which is part of the 6 pi-electron system delocalized around the ring.<\/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\/05151259\/fig2-2-16.png\" alt=\"fig2-2-16.png\" width=\"681\" height=\"385\" \/><\/p>\n<p><a class=\"external\" title=\"http:\/\/wps.prenhall.com\/wps\/media\/objects\/724\/741576\/Instructor_Resources\/Chapter_15\/Text_Images\/FG15_01-13-2UN.JPG\" href=\"http:\/\/wps.prenhall.com\/wps\/media\/objects\/724\/741576\/Instructor_Resources\/Chapter_15\/Text_Images\/FG15_01-13-2UN.JPG\" target=\"_blank\" rel=\"external nofollow noopener\">another image of orbitals in pyridine<\/a><\/p>\n<p>Why do we not assume that the nitrogen in pyrrole is s<em>p<sup>3<\/sup><\/em>-hybridized, like a normal secondary amine? The answer is simple: if it were, then pyrrole could not be aromatic, and thus it would not have the stability associated with aromaticity.\u00a0 In general,<em> if a molecule or group can be aromatic, it will be,<\/em> just as water will always flow downhill if there is a downhill pathway available.<\/p>\n<p>Imidazole is another important example of an aromatic heterocycle found in biomolecules &#8211; the side chain of the amino acid histidine contains an imidazole ring.<\/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\/05151302\/fig2-2-21.png\" alt=\"\" width=\"319\" height=\"153\" \/><\/p>\n<p>In imidazole, one nitrogen is &#8216;pyrrole-like&#8217; (the lone pair contributes to the aromatic sextet) and one is &#8216;pyridine-like&#8217; (the lone pair is located in an <em>sp<sup>2<\/sup><\/em> orbital, and is <em>not<\/em> part of the aromatic sextet).<\/p>\n<\/div>\n<\/div>\n<div id=\"section_2\">\n<div class=\"textbox exercises\">\n<h3 class=\"editable\">Exercises<\/h3>\n<div id=\"s61720\">\n<div id=\"section_20\">\n<p><b>1.<\/b><\/p>\n<p>Draw the orbitals of thiophene to show that is aromatic.<\/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\/05151304\/15-5-1qu.png\" alt=\"\" width=\"97\" height=\"106\" \/><\/p>\n<p><strong>2.<\/strong><\/p>\n<p>The following ring is called a thiazolium ring. Describe how it is aromatic.<\/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\/05151305\/15-5-2qu.png\" alt=\"\" width=\"112\" height=\"101\" \/><\/p>\n<\/div>\n<div id=\"section_21\">\n<h3 id=\"Solutions-61720\">Solutions<\/h3>\n<p><strong><\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q740202\">Show Answer<\/span><\/p>\n<div id=\"q740202\" class=\"hidden-answer\" style=\"display: none\">1.<\/strong><\/p>\n<p>This drawing shows it has 6 electrons in the pi-orbital.<\/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\/05151307\/15-5-1sol.gif\" alt=\"\" width=\"229\" height=\"142\" \/><\/p>\n<p><strong>2.<\/strong><\/p>\n<p>Similar to the last question, the drawing shows that there is only 6 electrons in the pi-system.<\/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\/05151310\/15-5-2sol.gif\" alt=\"\" width=\"264\" height=\"139\" \/><strong><\/div>\n<\/div>\n<p><\/strong><\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div id=\"section_3\">\n<h3 class=\"editable\">Contributors<\/h3>\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=\"Organic_Chemistry_With_a_Biological_Emphasis\" href=\"https:\/\/chem.libretexts.org\/Textbook_Maps\/Organic_Chemistry_Textbook_Maps\/Map%3A_Organic_Chemistry_with_a_Biological_Emphasis_(Soderberg)\" rel=\"internal\">Organic Chemistry With a Biological Emphasis <\/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<\/div>\n<\/div>\n","protected":false},"author":44985,"menu_order":4,"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-1458","chapter","type-chapter","status-publish","hentry"],"part":27,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/1458","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":6,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/1458\/revisions"}],"predecessor-version":[{"id":2335,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/1458\/revisions\/2335"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/parts\/27"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/1458\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/wp\/v2\/media?parent=1458"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/chapter-type?post=1458"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/wp\/v2\/contributor?post=1458"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/wp\/v2\/license?post=1458"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}