{"id":152,"date":"2009-11-30T23:35:21","date_gmt":"2009-12-01T07:35:21","guid":{"rendered":"http:\/\/wordpress.reed.edu\/chem201202\/2009\/11\/notes-on-epoxides-and-other-chapter-11-stuff.html"},"modified":"2014-03-18T10:13:03","modified_gmt":"2014-03-18T17:13:03","slug":"notes-on-epoxides-and-other-chapter-11-stuff","status":"publish","type":"post","link":"https:\/\/blogs.reed.edu\/chem201202\/2009\/11\/notes-on-epoxides-and-other-chapter-11-stuff\/","title":{"rendered":"Notes on epoxides and other Chapter 11 stuff"},"content":{"rendered":"<p>A few reflections on the material from Monday morning&#8217;s lecture.<\/p>\n<p><b>Epoxide ring-opening. <\/b>First, epoxides can react with <b>strong<\/b> nucleophiles without adding acid. The strained ring makes the epoxide reactive. These reactions look like SN2 reactions.<\/p>\n<p>Second, epoxides can react with much <b>weaker<\/b> nucleophiles by adding acid. Examples of weak nucleophiles include water, alcohols, carboxylic acids. The mechanisms of these reactions are a little strange. If one epoxide carbon is tertiary, the nucleophile adds there, but regardless of where the nucleophile adds, a backside attack occurs. So a little SN1 and a little SN2-like behavior all at the same time.<br \/><!--more-->The strange change in epoxide behavior parallels that of alkyl bromides and cyclic bromonium ions. The former contains a good leaving group (Br-), but the latter contains an even better leaving group (Br) and gets another boost from relief of angle strain. Take another look at section 5.2, p. 181-185, and you will be surprised by the similarity between cyclic bromonium ions and protonated epoxides.<\/p>\n<p><b>Organometallic reagents.<\/b> When you draw these reagents as synthetic reagents, please draw:<\/p>\n<ul>\n<li>RMgX &#8211; Grignard<\/li>\n<li>RLi &#8211; organolithium<\/li>\n<li>R2CuLi &#8211; lithium dialkylcuprate (FYI &#8211; you don&#8217;t need to learn the more complicated <b>higher-order cuprates<\/b> that Loudon mentions. Life is complicated enough already. Just assume that all cuprates can do the same chemistry.)<\/li>\n<\/ul>\n<p>However, when you draw these reagents as part of a reaction mechanism, i.e., with curved arrows to show electron flow, its OK to draw them as ionic compounds, R- M+ (include whatever metal is appropriate).<\/p>\n<p><b>Glycols.<\/b> I don&#8217;t think many students reached the model that described the preparation of cis-glycols using OsO4. Your book&#8217;s discussion of this reagent gets pretty complicated. Although I included one amine oxide in the ChemActivity, you don&#8217;t need to learn these reagents &#8211; simply draw &#8220;OsO4&#8221; whenever you want to turn an alkene into a cis-glycol. The reagent may be toxic and expensive, but we&#8217;re just drawing plans on paper and that&#8217;s very safe and (nearly) free.<\/p>\n<p>Your book also mentions the use of KMnO4 to make glycols. This is not a very satisfactory reaction. Skip it.<\/p>\n<p>Finally, your book mentions the &#8220;oxidative cleavage&#8221; of glycols with periodic acid. This is an important reaction, especially in the chemistry of carbohydrates (section 24.8). However, we have to draw the line somewhere and I&#8217;m going to draw it here. Formation of glycols &#8211; learn that. Oxidative cleavage of glycols &#8211; skip it (until Pat says go back and learn it).<\/p>\n","protected":false},"excerpt":{"rendered":"<p>A few reflections on the material from Monday morning&apos;s lecture.Epoxide ring-opening. First, epoxides can react with strong nucleophiles without adding acid. The strained ring makes the epoxide reactive. These reactions look like SN2 reactions.Second, epoxides can react with much weaker&#8230;<\/p>\n","protected":false},"author":55,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[3],"tags":[],"class_list":["post-152","post","type-post","status-publish","format-standard","hentry","category-post-lecture"],"_links":{"self":[{"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/posts\/152","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/users\/55"}],"replies":[{"embeddable":true,"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/comments?post=152"}],"version-history":[{"count":2,"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/posts\/152\/revisions"}],"predecessor-version":[{"id":5143,"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/posts\/152\/revisions\/5143"}],"wp:attachment":[{"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/media?parent=152"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/categories?post=152"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/tags?post=152"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}