{"id":143,"date":"2009-11-09T10:10:30","date_gmt":"2009-11-09T18:10:30","guid":{"rendered":"http:\/\/wordpress.reed.edu\/chem201202\/2009\/11\/sn2-kinetics-and-geometry.html"},"modified":"2014-03-18T10:13:03","modified_gmt":"2014-03-18T17:13:03","slug":"sn2-kinetics-and-geometry","status":"publish","type":"post","link":"https:\/\/blogs.reed.edu\/chem201202\/2009\/11\/sn2-kinetics-and-geometry\/","title":{"rendered":"SN2 kinetics and geometry"},"content":{"rendered":"<p>First, if you would like to see the MO pictures that I used in class today, check out last year&#8217;s post on <a href=\"https:\/\/blogs.reed.edu\/chem201202\/2008\/10\/back-side-attack-homo-lumo-overlap\"><b>back side attack &amp; HOMO-LUMO overlap<\/b><\/a> (Oct 13, 2008).<\/p>\n<p>There are also a couple of points that I want to add concerning reaction rates (kinetics):<br \/><!--more--><br \/>Carefully read section 9.3, <b>Reaction Rates<\/b>. I couldn&#8217;t find a convenient way to fit this material into lecture, but it is vitally important. Now for some specific points:<\/p>\n<p>#1 &#8211; We usually <b>compare competing reactions by comparing their rate constants<\/b>.<br \/>\nLoudon says this is legitimate when the competing reactions are both<br \/>\nperformed under &#8220;standard conditions&#8221; and he&#8217;s right. It is also a fair comparison when<br \/>\nthe rate laws for the competing reactions are the same, i.e., they<br \/>\ndiffer only in the numerical values of their respective rate constants. When this condition is satisfied,<br \/>\nwe don&#8217;t have to be limited to standard conditions.<\/p>\n<p>#2 &#8211; The lifetime of a species is inversely related to the rate<br \/>\nconstant for its disappearance. A large rate constant (for<br \/>\ndisappearing) means a short life. This also means that the &#8220;time to<br \/>\ncompletion&#8221; for a chemical reaction is inversely related to the rate<br \/>\nconstant for the reaction. A large rate constant means a short time to<br \/>\ncompletion.<\/p>\n<p>#3 &#8211; The ratio between two rate constants is <i>exponentially<\/i><br \/>\nrelated to the difference in their activation energies. See equations<br \/>\n9.22. Also compare data in different columns of Table 9.2. Reactions<br \/>\nthat have &#8220;time to completion&#8221; of 12 minutes, 20 hours, and 83 days<br \/>\nhave nearly the same free energies of activation. The exponential<br \/>\nrelationship makes this possible.<\/p>\n<p>#4 &#8211; (see<br \/>\nsection 9.4B) Acid-base reactions happen much, much faster than SN2<br \/>\nreactions (and much, much faster than just about any other kind of<br \/>\norganic reaction). A mechanistic hypothesis can&#8217;t be valid if it<br \/>\nrequires favorable acid-base reactions to &#8220;wait&#8221; for other reactions.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>First, if you would like to see the MO pictures that I used in class today, check out last year&#8217;s post on back side attack &amp; HOMO-LUMO overlap (Oct 13, 2008).There are also a couple of points that I want&#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":[46,47,30,43],"class_list":["post-143","post","type-post","status-publish","format-standard","hentry","category-post-lecture","tag-backside-attack","tag-homo-lumo-overlap","tag-mo","tag-sn2"],"_links":{"self":[{"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/posts\/143","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=143"}],"version-history":[{"count":3,"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/posts\/143\/revisions"}],"predecessor-version":[{"id":5152,"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/posts\/143\/revisions\/5152"}],"wp:attachment":[{"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/media?parent=143"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/categories?post=143"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/tags?post=143"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}