{"id":4196,"date":"2011-05-29T09:11:49","date_gmt":"2011-05-29T09:11:49","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=4196"},"modified":"2011-05-29T09:11:49","modified_gmt":"2011-05-29T09:11:49","slug":"the-inner-secrets-of-an-ion-pair-isobornyl-chloride-rearrangements","status":"publish","type":"post","link":"https:\/\/www.rzepa.net\/blog\/?p=4196","title":{"rendered":"The inner secrets of an ion-pair: Isobornyl chloride rearrangements."},"content":{"rendered":"
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Observation of the slow racemization of isobornyl chloride in a polar solvent in 1923-24 by Meerwein led\u00a0to the recognition that mechanistic interpretation is the key to understanding chemical reactivity. The hypothesis of ion pairs in which a chloride anion<\/strong> is partnered by a carbocation<\/strong> long ago entered the standard textbooks (see DOI\u00a010.1021\/ed800058c<\/a> and 10.1021\/jo100920e<\/a> for background reading). But the intimate secrets of such ion-pairs are still perhaps not fully recognised. Here, to tease some of them them out, I use the NCI method, which has been the subject of several recent posts.<\/p>\n

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NCI analysis of the iod-pair transition state for methyl migration in isobornyl chloride. Click for 3D.<\/p><\/div>To remind, the colour coding of the NCI surface is blue=strongly attractive<\/span>, red=strongly repulsive<\/span>, green=weakly attractive<\/span>, yellow=weakly repulsive<\/span>. Shown above is the ion-pair transition state<\/a> for [1,2]methyl migration. Note how the hydrogen bonds between the chloride anion and the water molecules are clearly blue. Only slightly weaker (with a turquoise tint) is a pair of hydrogen bonds between the oxygen atoms and H-C bonds in the isobornyl cation. Such C-H…O bonding in ion-pairs seems to be particularly important. There are other blue regions, between an H…H pair, and a C-H bond and the carbon of the migrating methyl group. Also noteworthy is that many atom pairs have multi-coloured NCI regions, suggesting the interaction is not homogenous, and can be both attractive AND repulsive between any pair of atoms.<\/p>\n

The NCI plot below shows the competing 1,6-hydride shift in isobornyl chloride, again involving an ion-pair transition state.<\/p>\n

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NCI surfaces for the 1,6 hydride migration transition state in isobornyl chloride. Click for 3D.<\/p><\/div>Notice in this example how the migrating hydrogen supports an attractive hydrogen bond to the chloride anion (ostensibly between a hydride atom and an anionic chloride?), and again how there are a number of blue<\/strong> regions elsewhere.<\/p>\n

Modelling is increasingly focusing on these weaker interactions, that probably mediate much (stereo)selectivity in organic reactions. How long before such approaches themselves enter the text-books?<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

Observation of the slow racemization of isobornyl chloride in a polar solvent in 1923-24 by Meerwein led\u00a0to the recognition that mechanistic interpretation is the key to understanding chemical reactivity. The hypothesis of ion pairs in which a chloride anion is partnered by a carbocation long ago entered the standard textbooks (see DOI\u00a010.1021\/ed800058c and 10.1021\/jo100920e for […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[6],"tags":[513,200,531,216,543,615],"class_list":["post-4196","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-chemical-reactivity","tag-ion-pair","tag-isobornyl","tag-julia-contreras-garcia","tag-nnon-covalent-interactions","tag-watoc11"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p1gPyz-15G","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4196","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=4196"}],"version-history":[{"count":0,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4196\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=4196"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=4196"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=4196"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}