{"id":92,"date":"2008-12-05T16:35:06","date_gmt":"2008-12-06T00:35:06","guid":{"rendered":"http:\/\/wordpress.reed.edu\/chem201202\/2008\/12\/sp-electrophiles-crown-ethers.html"},"modified":"2014-03-18T10:13:06","modified_gmt":"2014-03-18T17:13:06","slug":"sp-electrophiles-crown-ethers","status":"publish","type":"post","link":"https:\/\/blogs.reed.edu\/chem201202\/2008\/12\/sp-electrophiles-crown-ethers\/","title":{"rendered":"S\/P electrophiles & crown ethers"},"content":{"rendered":"

The PowerPoint slides from today’s lecture can be downloaded here<\/b><\/a>. A couple of comments on each slide …<\/p>\n

Slide 1<\/b> – The potential maps show why S and P make more inviting targets for a weak nucleophile (like ROH) than C. However, bond energy is another important factor. S-Cl and P-Br bonds are much weaker than C-Br. This difference helps explain why nucleophilic attack on S and P is more efficient.<\/p>\n

Slide 2<\/b> – I hope you remember this phrase: like charges repel<\/u>. Take another look at the crown ether map and notice where “like charges” are located. The six O surrounding the cavity, right? These atoms actually repel each other. Consequently, 18-crown-6 does not adopt the shape shown on this slide until a positively charged ion wanders into the cavity and provides strong attractive forces to offset O-O repulsion.<\/p>\n

Slide 3<\/b> – Three points to make here:<\/p>\n

    \n
  1. The color-energy scale for neutral 18-crown-6 (-180 to +180) is completely different from the scale used for the cations (0 to +1500).<\/li>\n
  2. Although all of the cations have the same charge, they do not create identical potentials. Potential and ion radius are inversely related. Li+ (small, +1600) vs. Na+ (medium, +1200) vs. K+ ( large, +900). This makes sense. Since Li is small, another ion can approach it more closely (larger potential). On the other hand, since K is large, another ion cannot approach as closely (smaller potential).<\/li>\n
  3. When a crown ether forms a complex with K+, it does not change the size of K+. The potential near K+ is smaller in the complex (+460) because there are offsetting contributions from K+ (very positive) and the ether (very negative). Since the potential near K+ is much smaller in the complex, the ability of complex K+ to form ionic bonds is greatly compromised.<\/li>\n<\/ol>\n<\/blockquote>\n","protected":false},"excerpt":{"rendered":"

    The PowerPoint slides from today's lecture can be downloaded here. A couple of comments on each slide ……<\/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-92","post","type-post","status-publish","format-standard","hentry","category-post-lecture"],"_links":{"self":[{"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/posts\/92","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=92"}],"version-history":[{"count":2,"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/posts\/92\/revisions"}],"predecessor-version":[{"id":5200,"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/posts\/92\/revisions\/5200"}],"wp:attachment":[{"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/media?parent=92"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/categories?post=92"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blogs.reed.edu\/chem201202\/wp-json\/wp\/v2\/tags?post=92"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}