{"id":528,"date":"2017-10-04T20:56:53","date_gmt":"2017-10-04T20:56:53","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/?post_type=chapter&#038;p=528"},"modified":"2017-10-24T15:19:31","modified_gmt":"2017-10-24T15:19:31","slug":"conformers-of-alkanes","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/chapter\/conformers-of-alkanes\/","title":{"raw":"Conformers of Alkanes","rendered":"Conformers of Alkanes"},"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>explain the concept of free rotation about a carbon-carbon single bond.<\/li>\r\n \t<li>explain the difference between conformational isomerism and the other types of isomerism which you have encountered.<\/li>\r\n \t<li>represent the conformers of ethane by both sawhorse representation and Newman projection.<\/li>\r\n \t<li>sketch a graph of energy versus bond rotation for ethane, and discuss the graph in terms of torsional strain.<\/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<h3>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>conformation (conformer, conformational isomer)<\/li>\r\n \t<li>eclipsed conformation<\/li>\r\n \t<li>Newman projection<\/li>\r\n \t<li>sawhorse representation<\/li>\r\n \t<li>staggered conformation<\/li>\r\n \t<li>strain energy<\/li>\r\n \t<li>torsional strain (eclipsing strain)<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n<div id=\"note\">\r\n<div class=\"textbox\">\r\n<h3 class=\"boxtitle\">Study Notes<\/h3>\r\nYou should be prepared to sketch various conformers using both sawhorse representations and Newman projections. Each method has its own advantages, depending upon the circumstances. Notice that when drawing the Newman projection of the eclipsed conformation of ethane, you cannot draw the rear hydrogens exactly behind the front ones. This is an inherent limitation associated with representing a 3-D structure in two dimensions.\r\n\r\n<\/div>\r\n\r\n\r\n<\/div>\r\n<strong>Conformational isomerism <\/strong>involves rotation about sigma bonds, and does not involve any differences in the connectivity or geometry of bonding.\u00a0 Two or more structures that are categorized as conformational isomers, or <strong>conformers<\/strong>, are really just two of the exact same molecule that differ only in terms of the angle about one or more sigma bonds.\r\n<div id=\"section_1\">\r\n\r\n\r\n<h3 class=\"editable\">Ethane Conformations<\/h3>\r\nAlthough there are seven sigma bonds in the ethane molecule, rotation about the six carbon-hydrogen bonds does not result in any change in the shape of the molecule because the hydrogen atoms are essentially spherical.\u00a0 Rotation about the carbon-carbon bond, however, results in many different possible molecular conformations.\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\/04205627\/image008.png\" alt=\"image008.png\" width=\"575\" height=\"110\" \/>\r\n\r\nIn order to better visualize these different conformations, it is convenient to use a drawing convention called the <strong>Newman projection<\/strong>.\u00a0 In a Newman projection, we look lengthwise down a specific bond of interest \u2013 in this case, the carbon-carbon bond in ethane.\u00a0 We depict the \u2018front\u2019 atom as a dot, and the \u2018back\u2019 atom as a larger circle.\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\/04205630\/image010.png\" alt=\"image010.png\" width=\"497\" height=\"151\" \/>\r\n\r\nThe six carbon-hydrogen bonds are shown as solid lines protruding from the two carbons at 120\u00b0angles, which is what the actual tetrahedral geometry looks like when viewed from this perspective and flattened into two dimensions.\r\n\r\nThe lowest energy conformation of ethane, shown in the figure above,\u00a0 is called the \u2018staggered\u2019 conformation, in which all of the C-H bonds on the front carbon are positioned at dihedral angles of 60\u00b0relative to the C-H bonds on the back carbon.\u00a0 In this conformation, the distance between the bonds (and the electrons in them) is maximized.\r\n\r\nIf we now rotate the front CH<sub>3<\/sub> group 60\u00b0 clockwise, the molecule is in the highest energy \u2018eclipsed' conformation, where the hydrogens on the front carbon are as close as possible to the hydrogens on the back carbon.\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\/04205633\/image012.png\" alt=\"image012.png\" width=\"349\" height=\"135\" \/>\r\n\r\nThis is the highest energy conformation because of unfavorable interactions between the electrons in the front and back C-H bonds.\u00a0 The energy of the eclipsed conformation is approximately 3 kcal\/mol higher than that of the staggered conformation. Another 60\u00b0rotation returns the molecule to a second eclipsed conformation. This process can be continued all around the 360\u00b0circle, with three possible eclipsed conformations and three staggered conformations, in addition to an infinite number of variations in between.\r\n\r\n<\/div>\r\n<div id=\"section_2\">\r\n\r\n\r\n<h3 class=\"editable\">Free Rotations Do Not Exist in Ethane<\/h3>\r\nThe carbon-carbon bond is not <em>completely<\/em> free to rotate \u2013 there is indeed a small, 3 kcal\/mol barrier to rotation that must be overcome for the bond to rotate from one staggered conformation to another.\u00a0 This rotational barrier is not high enough to prevent constant rotation except at extremely cold temperatures.\u00a0 However, at any given moment the molecule is more likely to be in a staggered conformation - one of the rotational \u2018energy valleys\u2019 - than in any other state. The potential energy associated with the various conformations of ethane varies with the dihedral angle of the bonds, as shown below.\r\n\r\n[caption id=\"attachment_2089\" align=\"aligncenter\" width=\"300\"]<img class=\"wp-image-2089 size-medium\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/24151658\/Graph-X-300x275.gif\" alt=\"\" width=\"300\" height=\"275\" \/> Figure X: The potential energy associated with the various conformations of ethane varies with the dihedral angle of the bonds.[\/caption]\r\n\r\nAlthough the conformers of ethane are in rapid equilibrium with each other, the 3 kcal\/mol energy difference leads to a substantial preponderance of staggered conformers (&gt; 99.9%) at any given time. The animation below illustrates the relationship between ethane's potential energy and its dihedral angle\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"403\"]<img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/04205636\/ethancfm.gif\" alt=\"image\" width=\"403\" height=\"397\" \/> Figure 2.X: Animation of potential energy vs. dihedral angle in ethane[\/caption]\r\n\r\n\r\n\r\n<\/div>\r\n<div id=\"section_3\">\r\n<div class=\"textbox exercises\">\r\n<h3>Exercises<\/h3>\r\n<div id=\"section_3\">\r\n<div id=\"s61690\">\r\n<div id=\"section_20\">\r\n\r\n\r\n<h3 id=\"Questions-61690\">Question<\/h3>\r\n<span><span>What is the most stable rotational conformation of ethane and explain why it is preferred over the other conformation?<\/span><\/span>\r\n\r\n<\/div>\r\n<div id=\"section_21\">\r\n<h3 id=\"Solutions-61690\">Solution<\/h3>\r\n[reveal-answer q=\"668953\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"668953\"]Staggered, as there is less repulsion between the hydrogen atoms.[\/hidden-answer]\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n\r\n\r\n<\/div>\r\n<div id=\"section_4\">\r\n<h3 class=\"editable\">Contributors<\/h3>\r\n<ul>\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 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<\/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>explain the concept of free rotation about a carbon-carbon single bond.<\/li>\n<li>explain the difference between conformational isomerism and the other types of isomerism which you have encountered.<\/li>\n<li>represent the conformers of ethane by both sawhorse representation and Newman projection.<\/li>\n<li>sketch a graph of energy versus bond rotation for ethane, and discuss the graph in terms of torsional strain.<\/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<h3>Key Terms<\/h3>\n<p>Make certain that you can define, and use in context, the key terms below.<\/p>\n<ul>\n<li>conformation (conformer, conformational isomer)<\/li>\n<li>eclipsed conformation<\/li>\n<li>Newman projection<\/li>\n<li>sawhorse representation<\/li>\n<li>staggered conformation<\/li>\n<li>strain energy<\/li>\n<li>torsional strain (eclipsing strain)<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div id=\"note\">\n<div class=\"textbox\">\n<h3 class=\"boxtitle\">Study Notes<\/h3>\n<p>You should be prepared to sketch various conformers using both sawhorse representations and Newman projections. Each method has its own advantages, depending upon the circumstances. Notice that when drawing the Newman projection of the eclipsed conformation of ethane, you cannot draw the rear hydrogens exactly behind the front ones. This is an inherent limitation associated with representing a 3-D structure in two dimensions.<\/p>\n<\/div>\n<\/div>\n<p><strong>Conformational isomerism <\/strong>involves rotation about sigma bonds, and does not involve any differences in the connectivity or geometry of bonding.\u00a0 Two or more structures that are categorized as conformational isomers, or <strong>conformers<\/strong>, are really just two of the exact same molecule that differ only in terms of the angle about one or more sigma bonds.<\/p>\n<div id=\"section_1\">\n<h3 class=\"editable\">Ethane Conformations<\/h3>\n<p>Although there are seven sigma bonds in the ethane molecule, rotation about the six carbon-hydrogen bonds does not result in any change in the shape of the molecule because the hydrogen atoms are essentially spherical.\u00a0 Rotation about the carbon-carbon bond, however, results in many different possible molecular conformations.<\/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\/04205627\/image008.png\" alt=\"image008.png\" width=\"575\" height=\"110\" \/><\/p>\n<p>In order to better visualize these different conformations, it is convenient to use a drawing convention called the <strong>Newman projection<\/strong>.\u00a0 In a Newman projection, we look lengthwise down a specific bond of interest \u2013 in this case, the carbon-carbon bond in ethane.\u00a0 We depict the \u2018front\u2019 atom as a dot, and the \u2018back\u2019 atom as a larger circle.<\/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\/04205630\/image010.png\" alt=\"image010.png\" width=\"497\" height=\"151\" \/><\/p>\n<p>The six carbon-hydrogen bonds are shown as solid lines protruding from the two carbons at 120\u00b0angles, which is what the actual tetrahedral geometry looks like when viewed from this perspective and flattened into two dimensions.<\/p>\n<p>The lowest energy conformation of ethane, shown in the figure above,\u00a0 is called the \u2018staggered\u2019 conformation, in which all of the C-H bonds on the front carbon are positioned at dihedral angles of 60\u00b0relative to the C-H bonds on the back carbon.\u00a0 In this conformation, the distance between the bonds (and the electrons in them) is maximized.<\/p>\n<p>If we now rotate the front CH<sub>3<\/sub> group 60\u00b0 clockwise, the molecule is in the highest energy \u2018eclipsed&#8217; conformation, where the hydrogens on the front carbon are as close as possible to the hydrogens on the back carbon.<\/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\/04205633\/image012.png\" alt=\"image012.png\" width=\"349\" height=\"135\" \/><\/p>\n<p>This is the highest energy conformation because of unfavorable interactions between the electrons in the front and back C-H bonds.\u00a0 The energy of the eclipsed conformation is approximately 3 kcal\/mol higher than that of the staggered conformation. Another 60\u00b0rotation returns the molecule to a second eclipsed conformation. This process can be continued all around the 360\u00b0circle, with three possible eclipsed conformations and three staggered conformations, in addition to an infinite number of variations in between.<\/p>\n<\/div>\n<div id=\"section_2\">\n<h3 class=\"editable\">Free Rotations Do Not Exist in Ethane<\/h3>\n<p>The carbon-carbon bond is not <em>completely<\/em> free to rotate \u2013 there is indeed a small, 3 kcal\/mol barrier to rotation that must be overcome for the bond to rotate from one staggered conformation to another.\u00a0 This rotational barrier is not high enough to prevent constant rotation except at extremely cold temperatures.\u00a0 However, at any given moment the molecule is more likely to be in a staggered conformation &#8211; one of the rotational \u2018energy valleys\u2019 &#8211; than in any other state. The potential energy associated with the various conformations of ethane varies with the dihedral angle of the bonds, as shown below.<\/p>\n<div id=\"attachment_2089\" style=\"width: 310px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-2089\" class=\"wp-image-2089 size-medium\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/24151658\/Graph-X-300x275.gif\" alt=\"\" width=\"300\" height=\"275\" \/><\/p>\n<p id=\"caption-attachment-2089\" class=\"wp-caption-text\">Figure X: The potential energy associated with the various conformations of ethane varies with the dihedral angle of the bonds.<\/p>\n<\/div>\n<p>Although the conformers of ethane are in rapid equilibrium with each other, the 3 kcal\/mol energy difference leads to a substantial preponderance of staggered conformers (&gt; 99.9%) at any given time. The animation below illustrates the relationship between ethane&#8217;s potential energy and its dihedral angle<\/p>\n<div style=\"width: 413px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/04205636\/ethancfm.gif\" alt=\"image\" width=\"403\" height=\"397\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 2.X: Animation of potential energy vs. dihedral angle in ethane<\/p>\n<\/div>\n<\/div>\n<div id=\"section_3\">\n<div class=\"textbox exercises\">\n<h3>Exercises<\/h3>\n<div id=\"section_3\">\n<div id=\"s61690\">\n<div id=\"section_20\">\n<h3 id=\"Questions-61690\">Question<\/h3>\n<p><span><span>What is the most stable rotational conformation of ethane and explain why it is preferred over the other conformation?<\/span><\/span><\/p>\n<\/div>\n<div id=\"section_21\">\n<h3 id=\"Solutions-61690\">Solution<\/h3>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q668953\">Show Answer<\/span><\/p>\n<div id=\"q668953\" class=\"hidden-answer\" style=\"display: none\">Staggered, as there is less repulsion between the hydrogen atoms.<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div id=\"section_4\">\n<h3 class=\"editable\">Contributors<\/h3>\n<ul>\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 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<\/ul>\n<\/div>\n<\/div>\n<\/div>\n","protected":false},"author":311,"menu_order":8,"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-528","chapter","type-chapter","status-publish","hentry"],"part":21,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/528","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\/311"}],"version-history":[{"count":5,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/528\/revisions"}],"predecessor-version":[{"id":2091,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/528\/revisions\/2091"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/parts\/21"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/528\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/wp\/v2\/media?parent=528"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/chapter-type?post=528"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/wp\/v2\/contributor?post=528"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/wp\/v2\/license?post=528"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}