{"id":45,"date":"2009-04-03T08:58:09","date_gmt":"2009-04-03T07:58:09","guid":{"rendered":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=45"},"modified":"2026-06-17T16:28:15","modified_gmt":"2026-06-17T15:28:15","slug":"pericyclic-assistance-for-sn-1-solvolysis","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=45","title":{"rendered":"Pericyclic assistance for SN-1 solvolysis"},"content":{"rendered":"<div class=\"kcite-section\" kcite-section-id=\"45\">\n<div id=\"attachment_46\" style=\"width: 363px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-46\" class=\"size-full wp-image-46\" title=\"p23\" onclick=\"jmolApplet([400,400],'load wp-content\/uploads\/2009\/04\/cat-251.log;frame 49; vectors on; color vectors yellow;vectors scale 5.0; vibration 6;','c1');\" src=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/04\/p23.gif\" alt=\"Pericylically assisted solvolysis. Click above to see model.\" width=\"353\" height=\"96\" \/><p id=\"caption-attachment-46\" class=\"wp-caption-text\">Click on diagram to see model.<\/p><\/div>\n<p>The reaction above is ostensibly a very simple pericyclic ring opening  of a cyclopropyl carbocation to an allyl cation, preceeded by a preparatory step involving SN-1 solvolysis. As a 2-electron thermal process, the second step proceeds with disrotation of the terminii. Can this stereochemistry be illustrated with a computed model for the transition state for this process? Well, starting with a naked cation, its actually quite tricky to find such a transition state. In reality such cations are always solvated in <em>real reactions<\/em>, and a gas phase model is actually somewhat artificial. A much better bet is to also include in <a href=\"http:\/\/www.ch.ic.ac.uk\/local\/organic\/pericyclic\/p1_electro_17.html\" target=\"new\" rel=\"noopener\">the model<\/a> the SN-1 step that is presumed to preceed the actual pericyclic ring opening. Here we have included a simple protonated water molecule as the leaving group, rather than the tosyl group shown in the diagram above. Now when attempts are made to locate the ring opening transition state, it turns out that the SN-1 reaction and this ring opening are strongly coupled together. The ring opening helps to evict the  water, or alternatively, one can look on it as the departing water helping to open the ring.<\/p>\n<p><a href=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/04\/cyclopropyl-ring-opening_tot_ener-1.svg\"><img decoding=\"async\" src=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/04\/cyclopropyl-ring-opening_tot_ener-1.svg\" alt=\"\" class=\"alignnone size-full wp-image-25503\" width=\"500\"\/><\/a><br \/>\nSuch an intimate coupling of one mechanistic type (a pericyclic reaction) with another (SN-1 solvolysis) is a relatively unrecognized aspect of reaction mechanisms (although of course it is one of the characteristics of enzyme-catalysed reactions). I will cover several other examples of such synchrony in mechanisms in future entries in the blog.<\/p>\n<hr \/>\n<span id=\"cite_ITEM-45-0\" name=\"citation\"><a href=\"#ITEM-45-0\">[1]<\/a><\/span>\n<hr \/>\n<h2>References<\/h2>\n    <ol class=\"kcite-bibliography csl-bib-body\"><li id=\"ITEM-45-0\">H. Rzepa, \"Pericyclic assistance for SN-1 solvolysis\", <i>Imperial College London<\/i>, 2022. <a href=\"https:\/\/doi.org\/10.14469\/hpc\/11096\">https:\/\/doi.org\/10.14469\/hpc\/11096<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> <!-- kcite-section 45 -->","protected":false},"excerpt":{"rendered":"<p>\u00a0 The reaction above is ostensibly a very simple pericyclic ring opening of a cyclopropyl carbocation to an allyl cation, preceeded by a preparatory step involving SN-1 solvolysis. As a 2-electron thermal process, the second step proceeds with disrotation of the terminii. Can this stereochemistry be illustrated with a computed model for the transition state [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"activitypub_content_warning":"","activitypub_content_visibility":"","activitypub_max_image_attachments":5,"activitypub_interaction_policy_quote":"anyone","activitypub_status":"federated","footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[4],"tags":[22,2648],"ppma_author":[2661],"class_list":["post-45","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-gas-phase-model","tag-interesting-chemistry"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v28.3 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Pericyclic assistance for SN-1 solvolysis - Henry Rzepa&#039;s Blog<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=45\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Pericyclic assistance for SN-1 solvolysis - Henry Rzepa&#039;s Blog\" \/>\n<meta property=\"og:description\" content=\"\u00a0 The reaction above is ostensibly a very simple pericyclic ring opening of a cyclopropyl carbocation to an allyl cation, preceeded by a preparatory step involving SN-1 solvolysis. As a 2-electron thermal process, the second step proceeds with disrotation of the terminii. Can this stereochemistry be illustrated with a computed model for the transition state [&hellip;]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=45\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa&#039;s Blog\" \/>\n<meta property=\"article:published_time\" content=\"2009-04-03T07:58:09+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2026-06-17T15:28:15+00:00\" \/>\n<meta property=\"og:image\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/04\/p23.gif\" \/>\n\t<meta property=\"og:image:width\" content=\"706\" \/>\n\t<meta property=\"og:image:height\" content=\"193\" \/>\n\t<meta property=\"og:image:type\" content=\"image\/gif\" \/>\n<meta name=\"author\" content=\"Henry Rzepa\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Henry Rzepa\" \/>\n\t<meta name=\"twitter:label2\" content=\"Estimated reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"2 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"Pericyclic assistance for SN-1 solvolysis - Henry Rzepa&#039;s Blog","robots":{"index":"index","follow":"follow","max-snippet":"max-snippet:-1","max-image-preview":"max-image-preview:large","max-video-preview":"max-video-preview:-1"},"canonical":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=45","og_locale":"en_GB","og_type":"article","og_title":"Pericyclic assistance for SN-1 solvolysis - Henry Rzepa&#039;s Blog","og_description":"\u00a0 The reaction above is ostensibly a very simple pericyclic ring opening of a cyclopropyl carbocation to an allyl cation, preceeded by a preparatory step involving SN-1 solvolysis. 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Perisolvolysis of a cyclopropyl chloride.","author":"Henry Rzepa","date":"December 13, 2011","format":false,"excerpt":"There are many treasures in Woodward and Hoffmann's (WH)\u00a0classic monograph. One such is acetolysis of \u00a0the endo chloride (green), which is much much faster than that of the exo isomer (red). The explanation given in their article (p 805) confines itself to succinctly stating that only loss of the endo\u2026","rel":"","context":"In &quot;Interesting chemistry&quot;","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/12\/cpendo.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":17692,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17692","url_meta":{"origin":45,"position":1},"title":"What is the (calculated) structure of a norbornyl cation anion-pair in water?","author":"Henry Rzepa","date":"April 1, 2017","format":false,"excerpt":"In a comment appended to an earlier post, I mused about the magnitude of the force constant relating to the interconversion between a classical and a non-classical structure for the norbornyl cation. Most calculations indicate the force constant for an \"isolated\" symmetrical cation\u00a0is +ve, which means it is a true\u2026","rel":"","context":"In &quot;crystal_structure_mining&quot;","block_context":{"text":"crystal_structure_mining","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1745"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2017\/03\/b3lypd3bj.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":63,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=63","url_meta":{"origin":45,"position":2},"title":"The SN-1 Reaction live!","author":"Henry Rzepa","date":"April 3, 2009","format":false,"excerpt":"The ionization of a C-X bond (X=halogen) to form what we call a carbocation and which is known as the SN-1 reaction goes way back in the history of chemistry. Julius Steglitz was probably the first person to suggest such an ionization, back in 1899 (Steglitz, J.; Am. Chem. J.,\u2026","rel":"","context":"In &quot;Interesting chemistry&quot;","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"SN-1 Reaction. Click on image to see  3D model","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/04\/sn1.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":1135,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1135","url_meta":{"origin":45,"position":3},"title":"The SN1 Reaction- revisited","author":"Henry Rzepa","date":"November 11, 2009","format":false,"excerpt":"In an earlier post I wrote about the iconic SN1 solvolysis reaction, and presented a model for the transition state involving 13 water molecules. Here, I follow this up with an improved molecule containing 16 water molecules, and how the barrier for this model compares with experiment. This latter is\u2026","rel":"","context":"In &quot;Interesting chemistry&quot;","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"Transition state for  Sn1 solvolysis of  tert-butyl chloride","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/11\/sn1.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":3392,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=3392","url_meta":{"origin":45,"position":4},"title":"Do electrons prefer to move in packs of 4, 6 or 8 during proton exchange in a calixarene?","author":"Henry Rzepa","date":"January 7, 2011","format":false,"excerpt":"This story starts with a calixarene, a molecule (suitably adorned with substituents) frequently used as a host to entrap a guest and perchance make the guest do something interesting. Such a calixarene was at the heart of a recent story where an attempt was made to induce it to capture\u2026","rel":"","context":"In &quot;Interesting chemistry&quot;","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/01\/calixarene.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":20560,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=20560","url_meta":{"origin":45,"position":5},"title":"Smoke and mirrors. All is not what it seems with this Sn2 reaction!","author":"Henry Rzepa","date":"April 4, 2019","format":false,"excerpt":"Previously, I explored the Graham reaction to form a diazirine. The second phase of the reaction involved an Sn2' displacement of N-Cl forming C-Cl. Here I ask how facile the simpler displacement of C-Cl by another chlorine might be and whether the mechanism is Sn2 or the alternative Sn1. The\u2026","rel":"","context":"In &quot;reaction mechanism&quot;","block_context":{"text":"reaction mechanism","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1086"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]}],"jetpack_likes_enabled":false,"authors":[{"term_id":2661,"user_id":1,"is_guest":0,"slug":"admin","display_name":"Henry Rzepa","avatar_url":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g","author_category":"1","first_name":"Henry","last_name":"Rzepa","user_url":"https:\/\/orcid.org\/0000-0002-8635-8390","job_title":"","description":"Henry Rzepa is Emeritus Professor of Computational Chemistry at Imperial College London."}],"jetpack_featured_media_url":"","_links":{"self":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/45","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=45"}],"version-history":[{"count":17,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/45\/revisions"}],"predecessor-version":[{"id":31652,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/45\/revisions\/31652"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=45"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=45"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=45"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=45"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}