{"id":76,"date":"2008-11-12T23:06:39","date_gmt":"2008-11-13T07:06:39","guid":{"rendered":"http:\/\/wordpress.reed.edu\/chem201202\/2008\/11\/models-of-chemical-bonding.html"},"modified":"2014-03-18T10:13:06","modified_gmt":"2014-03-18T17:13:06","slug":"models-of-chemical-bonding","status":"publish","type":"post","link":"https:\/\/blogs.reed.edu\/chem201202\/2008\/11\/models-of-chemical-bonding\/","title":{"rendered":"Models of Chemical Bonding"},"content":{"rendered":"<p>Today I decided to re-trace my steps on Monday and provide a more complete description of chemical bonds through the eyes of molecular orbital theory. A list of take-home lessons from this <a href=\"https:\/\/blogs.reed.edu\/chem201202\/files\/bin\/L26.111208%20Chemical%20Bonding%20revisited.pdf\"><b>presentation<\/b><\/a>:<\/p>\n<ul>\n<li>MO models contain new orbitals (molecular orbitals), while VB (valence bond) models do not<\/li>\n<li>MOs can be mentally (de)constructed as combinations of atomic orbitals<\/li>\n<\/ul>\n<p>So far not much new stuff &#8230; the previous points had already been made in the context of the &#8220;localized MO&#8221; model presented at the start of the semester.<br \/><!--more-->But now we add these points:<\/p>\n<ul>\n<li>MO energies depend on molecular geometry &amp; the # of orbital nodes<\/li>\n<li>Molecular<br \/>\ngeometry influences MO energy by changing the shape of the orbital.<br \/>\nThis affects where electrons hang out and how they move, but we can<br \/>\nsimplify all that by simply watching how geometry affects the way the<br \/>\natomic orbital &#8220;pieces&#8221; overlap.<\/li>\n<ul>\n<li>Constructive overlap of these pieces stabilizes an MO (bonding MO)<\/li>\n<li>Destructive overlap destabilizes an MO (antibonding MO)<\/li>\n<li>Loss of overlap as atoms move apart makes MO energy equivalent to energy of atomic orbital &#8220;pieces&#8221;<\/li>\n<\/ul>\n<\/ul>\n<p>Where<br \/>\nthis gets really interesting (disturbing) is when we look at any system<br \/>\nin which antibonding orbitals are occupied, e.g., He + He or F-F. The<br \/>\nVB model contains lots of lone pairs. The MO model, on the other hand,<br \/>\ncontains no lone pairs, just bonding and antibonding electrons. Whoa.<br \/>\nCheck out the <a href=\"https:\/\/blogs.reed.edu\/chem201202\/files\/bin\/L26.111208%20Chemical%20Bonding%20revisited.pdf\"><b>PowerPoint slides<\/b><\/a>.<\/p>\n<p><b>Radical reactions.<\/b><br \/>\nThe lecture was completed by looking at a typical radical reaction:<br \/>\nradical-initiated anti-Markovnikov addition of HBr to an alkene. The<br \/>\nmechanism involved three different types of processes: <b>initiation<\/b>, <b>propagation<\/b>, and <b>termination<\/b>.<\/p>\n<p>The heat of reaction for these processes can be calculated using bond energies, <b>DH<sup>o<\/sup><\/b>. The general rule is this: an exothermic reaction breaks a weaker bond (smaller DH<sup>o<\/sup>) and makes a stronger bond (larger DH<sup>o<\/sup>).<br \/>\nCompare this wording to the general rule for acid-base reactions: a<br \/>\nfavorable reaction consumes a stronger acid-base pair and makes a<br \/>\nweaker acid-base pair.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Today I decided to re-trace my steps on Monday and provide a more complete description of chemical bonds through the eyes of molecular orbital theory. A list of take-home lessons from this presentation:MO models contain new orbitals (molecular orbitals), while&#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-76","post","type-post","status-publish","format-standard","hentry","category-post-lecture"],"_links":{"self":[{"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/posts\/76","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=76"}],"version-history":[{"count":2,"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/posts\/76\/revisions"}],"predecessor-version":[{"id":5205,"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/posts\/76\/revisions\/5205"}],"wp:attachment":[{"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/media?parent=76"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/categories?post=76"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/tags?post=76"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}