{"id":1045,"date":"2017-10-19T14:24:54","date_gmt":"2017-10-19T14:24:54","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/?post_type=chapter&#038;p=1045"},"modified":"2018-10-03T20:07:29","modified_gmt":"2018-10-03T20:07:29","slug":"the-hammond-postulate","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/chapter\/the-hammond-postulate\/","title":{"raw":"The Hammond Postulate","rendered":"The Hammond Postulate"},"content":{"raw":"<div class=\"elm-header\">\r\n<div class=\"textbox learning-objectives\">\r\n<h3>Objective<\/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 use the Hammond postulate to explain the formation of the most stable carbocation during the addition of a protic acid, HX, to an alkene.\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<div class=\"textbox key-takeaways\">\r\n<div class=\"elm-header\">\r\n<h3 class=\"elm-header-custom\">Key Term<\/h3>\r\n<\/div>\r\n<div id=\"elm-main-content\" class=\"elm-content-container\">\r\n<div>\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>Hammond postulate<\/li>\r\n<\/ul>\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\r\nNow, back to transition states. Chemists are often very interested in trying to learn about what the transition state for a given reaction looks like, but addressing this question requires an indirect approach because the transition state itself cannot be observed. In order to gain some insight into what a particular transition state looks like, chemists often invoke the <strong>Hammond postulate<\/strong>, which states that <em>a transition state resembles the structure of the nearest stable species<\/em>. For an exergonic reaction, therefore, the transition state resembles the reactants more than it does the products.\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\/05141215\/image037.png\" alt=\"image038.png\" width=\"359\" height=\"212\" \/>\r\n\r\nIf we consider a hypothetical exergonic reaction between compounds A and B to form AB, the distance between A and B would be relatively large at the transition state, resembling the starting state where A and B are two isolated species. In the hypothetical endergonic reaction between C and D to form CD, however, the bond formation process would be much further along at the TS point, resembling the product.\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\/05141218\/image039.png\" alt=\"image040.png\" width=\"376\" height=\"232\" \/>\r\n\r\nThe Hammond Postulate is a very simplistic idea, which relies on an assumption that potential energy surfaces are parabolic.\u00a0 Although such an assumption is not rigorously true, it is fairly reliable and allows chemists to make energetic arguments about transition states by employing arguments about the stability of a related species.\u00a0 Since the formation of a reactive intermediate is very reliably <strong>endergonic<\/strong>, arguments about the stability of reactive intermediates can serve as proxy arguments about transition state stability.\r\n<div id=\"section_1\">\r\n<h3 class=\"editable\">The Hammond Postulate and the S<sub>N<\/sub>1 Reaction<\/h3>\r\nthe <a class=\"external\" href=\"http:\/\/www2.chemistry.msu.edu\/faculty\/reusch\/VirtTxtJml\/addene1.htm#add1bc\" target=\"_blank\" rel=\"external nofollow noopener\">Hammond postulate<\/a> suggests that the activation energy of the rate-determining first step will be inversely proportional to the stability of the carbocation intermediate. The stability of carbocations was discussed earlier, and a qualitative relationship is given below:\r\n\r\n<img class=\"internal aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05141221\/sn1diag.gif\" alt=\"\" width=\"264px\" height=\"180px\" \/>\r\n<table cellpadding=\"3\">\r\n<tbody>\r\n<tr>\r\n<td>Carbocation\r\nStability<\/td>\r\n<td>CH<sub>3<\/sub><sup>(+)<\/sup><\/td>\r\n<td>&lt;<\/td>\r\n<td>CH<sub>3<\/sub>CH<sub>2<\/sub><sup>(+)<\/sup><\/td>\r\n<td>&lt;<\/td>\r\n<td>(CH<sub>3<\/sub>)<sub>2<\/sub>CH<sup>(+)<\/sup><\/td>\r\n<td>\u2248<\/td>\r\n<td>CH<sub>2<\/sub>=CH-CH<sub>2<\/sub><sup>(+)<\/sup><\/td>\r\n<td>&lt;<\/td>\r\n<td>C<sub>6<\/sub>H<sub>5<\/sub>CH<sub>2<\/sub><sup>(+)<\/sup><\/td>\r\n<td>\u2248<\/td>\r\n<td>(CH<sub>3<\/sub>)<sub>3<\/sub>C<sup>(+)<\/sup><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\nConsequently, we expect that 3\u00ba-alkyl halides will be more reactive than their 2\u00ba and 1\u00ba-counterparts in reactions that follow an S<sub>N<\/sub>1 mechanism. This is opposite to the reactivity order observed for the S<sub>N<\/sub>2 mechanism. Allylic and benzylic halides are exceptionally reactive by either mechanism.\r\n\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=\"s61712\">\r\n<div id=\"section_33\">\r\n<h3 id=\"Questions-61712\">Question<\/h3>\r\nConsider the second step in the electrophilic addition of HBr to an alkene. Is this step exergonic or endergonic does the intermediate represent the product or the reactant? Draw out an energy diagram of this step reaction.\r\n\r\n<\/div>\r\n<div id=\"section_34\">\r\n<h3 id=\"Solutions-61712\">Solutions<\/h3>\r\n[reveal-answer q=\"593126\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"593126\"]<img class=\"internal default aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05141222\/7-10sol.gif\" alt=\"\" width=\"276\" height=\"231\" \/>\r\nExergonic, and represents the products. A \u2013 Reactant; B \u2013 Intermediate, Cation, resembling the product; C \u2013 Product As shown to go from B to C the step is exergonic.[\/hidden-answer]\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=\"textbox learning-objectives\">\n<h3>Objective<\/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 use the Hammond postulate to explain the formation of the most stable carbocation during the addition of a protic acid, HX, to an alkene.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox key-takeaways\">\n<div class=\"elm-header\">\n<h3 class=\"elm-header-custom\">Key Term<\/h3>\n<\/div>\n<div id=\"elm-main-content\" class=\"elm-content-container\">\n<div>\n<div>\n<p>Make certain that you can define, and use in context, the key term below.<\/p>\n<ul>\n<li>Hammond postulate<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div id=\"elm-main-content\" class=\"elm-content-container\">\n<div>\n<p>Now, back to transition states. Chemists are often very interested in trying to learn about what the transition state for a given reaction looks like, but addressing this question requires an indirect approach because the transition state itself cannot be observed. In order to gain some insight into what a particular transition state looks like, chemists often invoke the <strong>Hammond postulate<\/strong>, which states that <em>a transition state resembles the structure of the nearest stable species<\/em>. For an exergonic reaction, therefore, the transition state resembles the reactants more than it does the products.<\/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\/05141215\/image037.png\" alt=\"image038.png\" width=\"359\" height=\"212\" \/><\/p>\n<p>If we consider a hypothetical exergonic reaction between compounds A and B to form AB, the distance between A and B would be relatively large at the transition state, resembling the starting state where A and B are two isolated species. In the hypothetical endergonic reaction between C and D to form CD, however, the bond formation process would be much further along at the TS point, resembling the product.<\/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\/05141218\/image039.png\" alt=\"image040.png\" width=\"376\" height=\"232\" \/><\/p>\n<p>The Hammond Postulate is a very simplistic idea, which relies on an assumption that potential energy surfaces are parabolic.\u00a0 Although such an assumption is not rigorously true, it is fairly reliable and allows chemists to make energetic arguments about transition states by employing arguments about the stability of a related species.\u00a0 Since the formation of a reactive intermediate is very reliably <strong>endergonic<\/strong>, arguments about the stability of reactive intermediates can serve as proxy arguments about transition state stability.<\/p>\n<div id=\"section_1\">\n<h3 class=\"editable\">The Hammond Postulate and the S<sub>N<\/sub>1 Reaction<\/h3>\n<p>the <a class=\"external\" href=\"http:\/\/www2.chemistry.msu.edu\/faculty\/reusch\/VirtTxtJml\/addene1.htm#add1bc\" target=\"_blank\" rel=\"external nofollow noopener\">Hammond postulate<\/a> suggests that the activation energy of the rate-determining first step will be inversely proportional to the stability of the carbocation intermediate. The stability of carbocations was discussed earlier, and a qualitative relationship is given below:<\/p>\n<p><img decoding=\"async\" class=\"internal aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1518\/2017\/10\/05141221\/sn1diag.gif\" alt=\"\" width=\"264px\" height=\"180px\" \/><\/p>\n<table cellpadding=\"3\">\n<tbody>\n<tr>\n<td>Carbocation<br \/>\nStability<\/td>\n<td>CH<sub>3<\/sub><sup>(+)<\/sup><\/td>\n<td>&lt;<\/td>\n<td>CH<sub>3<\/sub>CH<sub>2<\/sub><sup>(+)<\/sup><\/td>\n<td>&lt;<\/td>\n<td>(CH<sub>3<\/sub>)<sub>2<\/sub>CH<sup>(+)<\/sup><\/td>\n<td>\u2248<\/td>\n<td>CH<sub>2<\/sub>=CH-CH<sub>2<\/sub><sup>(+)<\/sup><\/td>\n<td>&lt;<\/td>\n<td>C<sub>6<\/sub>H<sub>5<\/sub>CH<sub>2<\/sub><sup>(+)<\/sup><\/td>\n<td>\u2248<\/td>\n<td>(CH<sub>3<\/sub>)<sub>3<\/sub>C<sup>(+)<\/sup><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>Consequently, we expect that 3\u00ba-alkyl halides will be more reactive than their 2\u00ba and 1\u00ba-counterparts in reactions that follow an S<sub>N<\/sub>1 mechanism. This is opposite to the reactivity order observed for the S<sub>N<\/sub>2 mechanism. Allylic and benzylic halides are exceptionally reactive by either mechanism.<\/p>\n<\/div>\n<div id=\"section_2\">\n<div class=\"textbox exercises\">\n<h3 class=\"editable\">Exercises<\/h3>\n<div id=\"s61712\">\n<div id=\"section_33\">\n<h3 id=\"Questions-61712\">Question<\/h3>\n<p>Consider the second step in the electrophilic addition of HBr to an alkene. Is this step exergonic or endergonic does the intermediate represent the product or the reactant? Draw out an energy diagram of this step reaction.<\/p>\n<\/div>\n<div id=\"section_34\">\n<h3 id=\"Solutions-61712\">Solutions<\/h3>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q593126\">Show Answer<\/span><\/p>\n<div id=\"q593126\" class=\"hidden-answer\" style=\"display: none\"><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\/05141222\/7-10sol.gif\" alt=\"\" width=\"276\" height=\"231\" \/><br \/>\nExergonic, and represents the products. A \u2013 Reactant; B \u2013 Intermediate, Cation, resembling the product; C \u2013 Product As shown to go from B to C the step is exergonic.<\/div>\n<\/div>\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":11,"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-1045","chapter","type-chapter","status-publish","hentry"],"part":23,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/1045","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":5,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/1045\/revisions"}],"predecessor-version":[{"id":2293,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/1045\/revisions\/2293"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/parts\/23"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/1045\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/wp\/v2\/media?parent=1045"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/pressbooks\/v2\/chapter-type?post=1045"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/wp\/v2\/contributor?post=1045"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/wp-json\/wp\/v2\/license?post=1045"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}