{"id":229051,"date":"2025-06-07T20:10:01","date_gmt":"2025-06-07T20:10:01","guid":{"rendered":"https:\/\/learnexams.com\/blog\/?p=229051"},"modified":"2025-06-07T20:10:04","modified_gmt":"2025-06-07T20:10:04","slug":"draw-the-fischer-projection-of-a-d-aldopentose-b-provide-the-arrow-pushing-mechanism-showing-how-this-sugar-can-isomerize-under-basic-conditions-to-its-epimer","status":"publish","type":"post","link":"https:\/\/www.learnexams.com\/blog\/2025\/06\/07\/draw-the-fischer-projection-of-a-d-aldopentose-b-provide-the-arrow-pushing-mechanism-showing-how-this-sugar-can-isomerize-under-basic-conditions-to-its-epimer\/","title":{"rendered":"Draw the Fischer projection of a D-aldopentose B) Provide the arrow-pushing mechanism showing how this sugar can isomerize under basic conditions to its epimer"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\">(8 pts )A) Draw the Fischer projection of a D-aldopentose B) Provide the arrow-pushing mechanism showing how this sugar can isomerize under basic conditions to its epimer<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong><mark style=\"background-color:rgba(0, 0, 0, 0)\" class=\"has-inline-color has-ast-global-color-1-color\">The Correct Answer and Explanation is:<\/mark><\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>A) Fischer Projection of a D-Aldopentose:<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">There are several D-aldopentoses, but one of the most common is <strong>D-ribose<\/strong>. Here&#8217;s the Fischer projection of <strong>D-ribose<\/strong> (an example of a D-aldopentose):<\/p>\n\n\n\n<pre class=\"wp-block-preformatted\">mathematicaCopyEdit<code>     CHO\n      |\nH \u2013 C \u2013 OH\n      |\nH \u2013 C \u2013 OH\n      |\nH \u2013 C \u2013 OH\n      |\n     CH2OH\n<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">The key feature of a D-aldopentose is:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>A five-carbon sugar (pentose),<\/li>\n\n\n\n<li>An aldehyde group at C1 (making it an aldose),<\/li>\n\n\n\n<li>The OH on the highest-numbered chiral carbon (C4 in this case) is on the <strong>right<\/strong>, making it <strong>D<\/strong>.<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>B) Mechanism: Base-Catalyzed Epimerization of a D-Aldopentose<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Under <strong>basic conditions<\/strong>, an aldopentose such as D-ribose can epimerize at <strong>C2<\/strong>, converting into its epimer (e.g., D-arabinose). This happens via an <strong>enediol intermediate<\/strong>. Here&#8217;s the <strong>arrow-pushing mechanism<\/strong>:<\/p>\n\n\n\n<ol class=\"wp-block-list\">\n<li><strong>Deprotonation<\/strong>:\n<ul class=\"wp-block-list\">\n<li>A base (like OH\u207b) abstracts the proton from the \u03b1-hydrogen next to the aldehyde (at C2), forming an enolate ion.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Enediol Formation<\/strong>:\n<ul class=\"wp-block-list\">\n<li>The electrons from the enolate form a double bond between C1 and C2, and the \u03c0 electrons of the carbonyl move to oxygen, forming an <strong>enediol<\/strong> intermediate (with OH groups on both C1 and C2 and a C=C bond between them).<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Reprotonation<\/strong>:\n<ul class=\"wp-block-list\">\n<li>The enediol tautomerizes back to the aldose form, but now the proton can be added to <strong>either face<\/strong> of C2, giving either the original sugar (D-ribose) or its <strong>C2-epimer<\/strong> (D-arabinose).<\/li>\n<\/ul>\n<\/li>\n<\/ol>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Explanation<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">A <strong>D-aldopentose<\/strong> is a five-carbon sugar with an aldehyde group and D-configuration, meaning the OH group on the highest numbered chiral carbon is on the right in a Fischer projection. An example is <strong>D-ribose<\/strong>, with the configuration (from top to bottom): CHO, H\u2013C\u2013OH, H\u2013C\u2013OH, H\u2013C\u2013OH, CH\u2082OH. D-aldopentoses can undergo isomerization reactions under <strong>basic conditions<\/strong>, one common type being <strong>epimerization<\/strong>\u2014a change in configuration at a single chiral center.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The mechanism proceeds via an <strong>enediol intermediate<\/strong>. In base, the \u03b1-hydrogen next to the aldehyde (on C2) is slightly acidic. Hydroxide abstracts this proton, forming a resonance-stabilized <strong>enolate ion<\/strong>. The enolate tautomerizes to form an <strong>enediol<\/strong>: a molecule with a double bond between C1 and C2 and hydroxyl groups on both.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">This enediol is a symmetrical intermediate. When it tautomerizes back to an aldose, protonation can occur on either face of the planar C2. As a result, the hydroxyl group at C2 may end up on either the same side (reforming the original sugar) or the opposite side (forming the <strong>epimer<\/strong> at C2).<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Thus, <strong>D-ribose<\/strong> can convert into <strong>D-arabinose<\/strong> through this reversible process. This mechanism is important in carbohydrate chemistry and underlies the Lobry de Bruyn\u2013Alberda van Ekenstein transformation, a base-catalyzed isomerization involving aldoses, ketoses, and their epimers.<\/p>\n\n\n\n<figure class=\"wp-block-image size-full\"><img decoding=\"async\" src=\"https:\/\/learnexams.com\/blog\/wp-content\/uploads\/2025\/06\/learnexams-banner4-685.jpeg\" alt=\"\" class=\"wp-image-229052\"\/><\/figure>\n\n\n","protected":false},"excerpt":{"rendered":"<p>(8 pts )A) Draw the Fischer projection of a D-aldopentose B) Provide the arrow-pushing mechanism showing how this sugar can isomerize under basic conditions to its epimer The Correct Answer and Explanation is: A) Fischer Projection of a D-Aldopentose: There are several D-aldopentoses, but one of the most common is D-ribose. Here&#8217;s the Fischer projection [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"site-sidebar-layout":"default","site-content-layout":"","ast-site-content-layout":"default","site-content-style":"default","site-sidebar-style":"default","ast-global-header-display":"","ast-banner-title-visibility":"","ast-main-header-display":"","ast-hfb-above-header-display":"","ast-hfb-below-header-display":"","ast-hfb-mobile-header-display":"","site-post-title":"","ast-breadcrumbs-content":"","ast-featured-img":"","footer-sml-layout":"","ast-disable-related-posts":"","theme-transparent-header-meta":"","adv-header-id-meta":"","stick-header-meta":"","header-above-stick-meta":"","header-main-stick-meta":"","header-below-stick-meta":"","astra-migrate-meta-layouts":"default","ast-page-background-enabled":"default","ast-page-background-meta":{"desktop":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"ast-content-background-meta":{"desktop":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"footnotes":""},"categories":[25],"tags":[],"class_list":["post-229051","post","type-post","status-publish","format-standard","hentry","category-exams-certification"],"_links":{"self":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/posts\/229051","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/comments?post=229051"}],"version-history":[{"count":0,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/posts\/229051\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/media?parent=229051"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/categories?post=229051"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/tags?post=229051"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}