Today’s activity (ChemActivity 34R) used potential surfaces to assign charges to hydrogen atoms in ethylene and acetylene. The charge trend goes like this (I’ve added ethane for good measure):<\/p>\n
(least +)<\/b> H in ethane < H in ethylene < H in acetylene << H in water (most +)<\/b><\/p>\n
<\/p>\n
This trend can be rationalized by thinking about the energies of the overlapping atomic orbitals.<\/p>\n
\n
A detailed analysis of MOs for polar bonds was provided for HF in the study guide for Chapter 3<\/a><\/b> (see page 4) and pictures of the BMO and ABMO for HF were displayed in lecture #8, Sept 18<\/b><\/a> (see slide #6 – “Unequal AO Energies”).<\/p>\n As you probably found while working today’s ChemActivity, the energy trend among carbon’s orbitals is: 2s << sp < sp2 < sp3 < 2p<\/i>. The energy of hydrogen’s 1s<\/i> orbital is slightly higher than the energies of carbon’s hybrid orbitals, so the MO for a CH bond are always built from orbitals of different energy (C < H).<\/p>\n The greatest inequality<\/i> appears when we overlap H 1s<\/i> and C sp<\/i>. The MO is weighted towards the lower energy orbital (sp<\/i>) and the resulting bond is polarized towards carbon. Smaller inequalities occur when C sp2<\/i> and sp3<\/i> orbitals overlap with H 1s<\/i>, so these bonds are not as polar.<\/p>\n","protected":false},"excerpt":{"rendered":" Today’s activity (ChemActivity 34R) used potential surfaces to assign charges to hydrogen atoms in ethylene and acetylene. The charge trend goes like this (I’ve added ethane for good measure):(least +) H in ethane < H in ethylene < H in…<\/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-155","post","type-post","status-publish","format-standard","hentry","category-post-lecture"],"_links":{"self":[{"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/posts\/155","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=155"}],"version-history":[{"count":2,"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/posts\/155\/revisions"}],"predecessor-version":[{"id":5140,"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/posts\/155\/revisions\/5140"}],"wp:attachment":[{"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/media?parent=155"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/categories?post=155"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/tags?post=155"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}