{"id":3736,"date":"2018-07-16T18:51:11","date_gmt":"2018-07-16T18:51:11","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/?post_type=chapter&#038;p=3736"},"modified":"2018-08-06T11:55:13","modified_gmt":"2018-08-06T11:55:13","slug":"4-5-stereochemistry-of-reactions","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/chapter\/4-5-stereochemistry-of-reactions\/","title":{"raw":"4.5. Stereochemistry of reactions","rendered":"4.5. Stereochemistry of reactions"},"content":{"raw":"Soon we will begin to study organic reactions, so it is useful for us to consider how stereochemistry will affect the outcome of a reaction.\u00a0 There are several questions that often arise when predicting what products will form:\r\n<div class=\"textbox shaded\">\r\n<div class=\"textbox shaded\"><strong><big>Some key questions<\/big><\/strong><\/div>\r\n<div>Q: When a chiral molecule reacts, will the chiral center be preserved?<\/div>\r\n<div>A: Yes, if the product is still chiral, and if no bonds are broken to that center<\/div>\r\n<div><\/div>\r\n<div>Q: Will optical activity in the reactant carry over into the product?<\/div>\r\n<div>A: If the molecule remains chiral at every stage - including intermediates - then yes.<\/div>\r\n<div><\/div>\r\n<div>Q: Can we get an optically active product from an inactive reactant?<\/div>\r\n<div>A: No, unless another optically active agent (a reagent, catalyst, etc.) is used in the process.<\/div>\r\n<\/div>\r\nTo understand such things in detail, we will need to examine some specific scenarios.\r\n<h2>Reactions at a single chiral carbon, leading to a chiral product (chiral -&gt; chiral)<\/h2>\r\nIf a bond to a chiral carbon breaks during a reaction, there are several possibilities:\r\n<ul>\r\n \t<li>(a) Bond is broken and new bond is made in a single step, or in an ordered way.\u00a0 In this situation, the stereochemistry will be either retained or inverted.<\/li>\r\n \t<li>(b) Bond is broken and a new bond is made in two or more steps (i.e., via intermediates), or in a less ordered way.\u00a0 In this case, the stereochemistry may become less distinct, and it may lead to a racemic mixture (50:50 mixture of enantiomers).<\/li>\r\n<\/ul>\r\nTo illustrate each of these cases, we will look at the stereochemistry of some reactions we will be learning later in the semester.\r\n\r\n<strong>Example: S<sub>N<\/sub>2 reaction<\/strong>\r\n\r\n<img class=\"alignnone size-full wp-image-4781\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3369\/2018\/07\/06115456\/Haloalkanes_111.png\" alt=\"\" width=\"611\" height=\"267\" \/>\r\n\r\nHere, iodide (I-) attacks the alkyl halide from the back and forms a product where the chiral center has been inverted.\u00a0 Because the process happens all in one step, there is no scrambling of the stereochemistry, and the product would remain fully optically active.\r\n\r\n<strong>Example: S<sub>N<\/sub>1 reaction<\/strong>\r\n\r\n<sub><img class=\"size-medium aligncenter\" src=\"https:\/\/upload.wikimedia.org\/wikipedia\/commons\/5\/5b\/SN1Stereochemistry.png\" width=\"761\" height=\"329\" \/><\/sub>\r\n\r\nAgain, iodide ion is attacking the alkyl halide, but In this case, the reaction takes place in two steps.\u00a0 After the first step an achiral intermediate is formed, and this will lead to a racemic (50:50) mixture of enantiomers for the chiral product.\r\n<h2>Reactions that form a new chiral center<\/h2>\r\nIf the starting material is achiral, then we will always get a racemic mixture of our product enantiomers, unless some other chiral agent is involved.\u00a0 This is for the same reason that the S<sub>N<\/sub>1 reaction above gives a racemic product - there is an equal chance of attacking the achiral starting material from either side.\u00a0 Thus <strong>Achiral -&gt; Chiral<\/strong> will give a racemic product.\r\n\r\nIf the starting material is already chiral, and we introduce a new chiral center, the situation is more complicated because we may produce a mixture of diastereomers, which have differing stabilities.\u00a0 We will not examine this scenario further in this class.\r\n<h2>Optically active compounds and their \"pixie dust\"!<\/h2>\r\nIt can be helpful - if rather silly - to consider that an optically active reactant somehow has some special \"magical powers\", which I imagine come from having some magical pixie dust.\u00a0 If we have a case like in the S<sub>N<\/sub>2 reaction above, the molecule remains chiral throughout the reaction, so it keeps its pixie dust - so it stays optically active.\u00a0 In a case like the S<sub>N<\/sub>1 reaction above, the molecule loses its chirality in the middle, and now it has dropped its pixie dust!\u00a0 Disaster!\u00a0 Now it has lost its optical activity, so any chiral product after that must be racemic - no pixie dust.\u00a0 (The only way to get it back is if it meets another magical molecule which shares some of its own pixie dust.)\r\n\r\nLater in this course we will design multi-step syntheses, where we will design a sequence of reactions to make a target product.\u00a0 The \"pixie dust\" principle still applies here - once we lose the pixie dust in our reaction sequence, any molecule that follows will remain optically inactive.","rendered":"<p>Soon we will begin to study organic reactions, so it is useful for us to consider how stereochemistry will affect the outcome of a reaction.\u00a0 There are several questions that often arise when predicting what products will form:<\/p>\n<div class=\"textbox shaded\">\n<div class=\"textbox shaded\"><strong><span style=\"font-size: larger;\">Some key questions<\/span><\/strong><\/div>\n<div>Q: When a chiral molecule reacts, will the chiral center be preserved?<\/div>\n<div>A: Yes, if the product is still chiral, and if no bonds are broken to that center<\/div>\n<div><\/div>\n<div>Q: Will optical activity in the reactant carry over into the product?<\/div>\n<div>A: If the molecule remains chiral at every stage &#8211; including intermediates &#8211; then yes.<\/div>\n<div><\/div>\n<div>Q: Can we get an optically active product from an inactive reactant?<\/div>\n<div>A: No, unless another optically active agent (a reagent, catalyst, etc.) is used in the process.<\/div>\n<\/div>\n<p>To understand such things in detail, we will need to examine some specific scenarios.<\/p>\n<h2>Reactions at a single chiral carbon, leading to a chiral product (chiral -&gt; chiral)<\/h2>\n<p>If a bond to a chiral carbon breaks during a reaction, there are several possibilities:<\/p>\n<ul>\n<li>(a) Bond is broken and new bond is made in a single step, or in an ordered way.\u00a0 In this situation, the stereochemistry will be either retained or inverted.<\/li>\n<li>(b) Bond is broken and a new bond is made in two or more steps (i.e., via intermediates), or in a less ordered way.\u00a0 In this case, the stereochemistry may become less distinct, and it may lead to a racemic mixture (50:50 mixture of enantiomers).<\/li>\n<\/ul>\n<p>To illustrate each of these cases, we will look at the stereochemistry of some reactions we will be learning later in the semester.<\/p>\n<p><strong>Example: S<sub>N<\/sub>2 reaction<\/strong><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-4781\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3369\/2018\/07\/06115456\/Haloalkanes_111.png\" alt=\"\" width=\"611\" height=\"267\" \/><\/p>\n<p>Here, iodide (I-) attacks the alkyl halide from the back and forms a product where the chiral center has been inverted.\u00a0 Because the process happens all in one step, there is no scrambling of the stereochemistry, and the product would remain fully optically active.<\/p>\n<p><strong>Example: S<sub>N<\/sub>1 reaction<\/strong><\/p>\n<p><sub><img loading=\"lazy\" decoding=\"async\" class=\"size-medium aligncenter\" src=\"https:\/\/upload.wikimedia.org\/wikipedia\/commons\/5\/5b\/SN1Stereochemistry.png\" width=\"761\" height=\"329\" alt=\"image\" \/><\/sub><\/p>\n<p>Again, iodide ion is attacking the alkyl halide, but In this case, the reaction takes place in two steps.\u00a0 After the first step an achiral intermediate is formed, and this will lead to a racemic (50:50) mixture of enantiomers for the chiral product.<\/p>\n<h2>Reactions that form a new chiral center<\/h2>\n<p>If the starting material is achiral, then we will always get a racemic mixture of our product enantiomers, unless some other chiral agent is involved.\u00a0 This is for the same reason that the S<sub>N<\/sub>1 reaction above gives a racemic product &#8211; there is an equal chance of attacking the achiral starting material from either side.\u00a0 Thus <strong>Achiral -&gt; Chiral<\/strong> will give a racemic product.<\/p>\n<p>If the starting material is already chiral, and we introduce a new chiral center, the situation is more complicated because we may produce a mixture of diastereomers, which have differing stabilities.\u00a0 We will not examine this scenario further in this class.<\/p>\n<h2>Optically active compounds and their &#8220;pixie dust&#8221;!<\/h2>\n<p>It can be helpful &#8211; if rather silly &#8211; to consider that an optically active reactant somehow has some special &#8220;magical powers&#8221;, which I imagine come from having some magical pixie dust.\u00a0 If we have a case like in the S<sub>N<\/sub>2 reaction above, the molecule remains chiral throughout the reaction, so it keeps its pixie dust &#8211; so it stays optically active.\u00a0 In a case like the S<sub>N<\/sub>1 reaction above, the molecule loses its chirality in the middle, and now it has dropped its pixie dust!\u00a0 Disaster!\u00a0 Now it has lost its optical activity, so any chiral product after that must be racemic &#8211; no pixie dust.\u00a0 (The only way to get it back is if it meets another magical molecule which shares some of its own pixie dust.)<\/p>\n<p>Later in this course we will design multi-step syntheses, where we will design a sequence of reactions to make a target product.\u00a0 The &#8220;pixie dust&#8221; principle still applies here &#8211; once we lose the pixie dust in our reaction sequence, any molecule that follows will remain optically inactive.<\/p>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-3736\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Original<\/div><ul class=\"citation-list\"><li><strong>Authored by<\/strong>: Martin Walker. <strong>Provided by<\/strong>: SUNY Potsdam. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/directory.potsdam.edu\/?function=user=walkerma\">http:\/\/directory.potsdam.edu\/?function=user=walkerma<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/\">CC BY-SA: Attribution-ShareAlike<\/a><\/em><\/li><\/ul><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Diagram of SN2 stereochemistry. <strong>Authored by<\/strong>: Racheal Curtis. <strong>Provided by<\/strong>: UC Davis. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/chem.libretexts.org\/Textbook_Maps\/Organic_Chemistry\/Supplemental_Modules_(Organic_Chemistry)\/Reactions\/Substitution_Reactions\/SN2\/Sterochemistry\">https:\/\/chem.libretexts.org\/Textbook_Maps\/Organic_Chemistry\/Supplemental_Modules_(Organic_Chemistry)\/Reactions\/Substitution_Reactions\/SN2\/Sterochemistry<\/a>. <strong>Project<\/strong>: Libretexts. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/about\/pdm\">Public Domain: No Known Copyright<\/a><\/em><\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":96103,"menu_order":5,"template":"","meta":{"_candela_citation":"[{\"type\":\"original\",\"description\":\"\",\"author\":\"Martin Walker\",\"organization\":\"SUNY Potsdam\",\"url\":\"http:\/\/directory.potsdam.edu\/?function=user=walkerma\",\"project\":\"\",\"license\":\"cc-by-sa\",\"license_terms\":\"\"},{\"type\":\"cc\",\"description\":\"Diagram of SN2 stereochemistry\",\"author\":\"Racheal Curtis\",\"organization\":\"UC Davis\",\"url\":\"https:\/\/chem.libretexts.org\/Textbook_Maps\/Organic_Chemistry\/Supplemental_Modules_(Organic_Chemistry)\/Reactions\/Substitution_Reactions\/SN2\/Sterochemistry\",\"project\":\"Libretexts\",\"license\":\"pd\",\"license_terms\":\"\"}]","CANDELA_OUTCOMES_GUID":"","pb_show_title":"on","pb_short_title":"Stereochemistry of reactions","pb_subtitle":"","pb_authors":["martin-walker"],"pb_section_license":"cc-by-sa"},"chapter-type":[],"contributor":[54],"license":[57],"class_list":["post-3736","chapter","type-chapter","status-publish","hentry","contributor-martin-walker","license-cc-by-sa"],"part":76,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/3736","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/wp\/v2\/users\/96103"}],"version-history":[{"count":15,"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/3736\/revisions"}],"predecessor-version":[{"id":4782,"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/3736\/revisions\/4782"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/pressbooks\/v2\/parts\/76"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/3736\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/wp\/v2\/media?parent=3736"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/pressbooks\/v2\/chapter-type?post=3736"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/wp\/v2\/contributor?post=3736"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/wp\/v2\/license?post=3736"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}