{"id":6262,"date":"2012-02-06T13:17:20","date_gmt":"2012-02-06T13:17:20","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=6262"},"modified":"2012-02-06T13:17:20","modified_gmt":"2012-02-06T13:17:20","slug":"an-exothermic-e2-elimination-an-unusual-intrinsic-reaction-coordinate","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6262","title":{"rendered":"An exothermic E2 elimination: an unusual intrinsic reaction coordinate."},"content":{"rendered":"
\n

The previous post<\/a> explored why E2 elimination reactions occur with an antiperiplanar<\/em> geometry for the transition state. Here I have tweaked the initial reactant to make the overall reaction exothermic<\/em> rather than endothermic<\/em> as it was before. The change is startling.<\/p>\n

\"\"<\/a>
\nThe exothermicity is of course due to the aromatisation of the ring.\u00a0The IRC is however quite different from before.
\n\"\"<\/a><\/p>\n

\"\"

IRC for E2 elimination. Click for 3D<\/p><\/div>\n

    \n
  1. The transition state (IRC=0.0) is reached early, and the initial movement is of the chlorine on the right. This in fact resembles an E1 elimination to form an intermediate carbocation. This, being a resonance stabilised (Wheland) type of cation, is particularly favoured.<\/li>\n
  2. At this point, the H-C bond has scarcely started breaking (1.13\u00c5).<\/li>\n
  3. When IRC of -5 is reached, the C-Cl bond is essentially broken (3.05\u00c5). At this point the energy is still higher than the reactant.<\/li>\n
  4. A sudden abrupt change occurs resulting from rapid proton transfer at IRC -6, and the energy plummets to become exothermic.<\/li>\n<\/ol>\n

    So, like the SN<\/sub>1 reaction<\/a> discussed in another post, this E2 reaction occurs in distinct stages, the first resembling an E1 mechanism, followed by a second phase leading to elimination. It is still a concerted reaction, but the proton transfer occurs only AFTER the transition state. The simple designation \u00a0E2\/E1 \u00a0is clearly not a fully adequate description of such mechanisms.<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    The previous post explored why E2 elimination reactions occur with an antiperiplanar geometry for the transition state. Here I have tweaked the initial reactant to make the overall reaction exothermic rather than endothermic as it was before. The change is startling. The exothermicity is of course due to the aromatisation of the ring.\u00a0The IRC is […]<\/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,"activitypub_content_warning":"","activitypub_content_visibility":"","activitypub_max_image_attachments":5,"activitypub_interaction_policy_quote":"anyone","activitypub_status":"","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}},"categories":[],"tags":[17,24,373],"ppma_author":[2661],"class_list":["post-6262","post","type-post","status-publish","format-standard","hentry","tag-conformational-analysis","tag-energy","tag-tutorial-material"],"yoast_head":"\nAn exothermic E2 elimination: an unusual intrinsic reaction coordinate. - Henry Rzepa'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=6262\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"An exothermic E2 elimination: an unusual intrinsic reaction coordinate. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"The previous post explored why E2 elimination reactions occur with an antiperiplanar geometry for the transition state. Here I have tweaked the initial reactant to make the overall reaction exothermic rather than endothermic as it was before. The change is startling. The exothermicity is of course due to the aromatisation of the ring.\u00a0The IRC is […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6262\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2012-02-06T13:17:20+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/02\/E2-benzo-d.svg\" \/>\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=\"1 minute\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"An exothermic E2 elimination: an unusual intrinsic reaction coordinate. - Henry Rzepa'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=6262","og_locale":"en_GB","og_type":"article","og_title":"An exothermic E2 elimination: an unusual intrinsic reaction coordinate. - Henry Rzepa's Blog","og_description":"The previous post explored why E2 elimination reactions occur with an antiperiplanar geometry for the transition state. 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What can quantum mechanics teach us about the mechanism of the reaction? The diagram below shows us the IRC, or intrinsic reaction coordinate for the process (for definitions, see\u2026","rel":"","context":"In \"alkene\"","block_context":{"text":"alkene","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=alkene"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/09\/hydration.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":6279,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6279","url_meta":{"origin":6262,"position":1},"title":"E2 elimination vs ring contraction: anti-periplanarity in action.","author":"Henry Rzepa","date":"February 20, 2012","format":false,"excerpt":"The anti-periplanar principle permeates organic reactivity. Here I pick up on an example of the antiperiplanar E2 elimination (below, blue) by comparing it to a competing reaction involving a [1,2] antiperiplanar migration (red). The relative rates of these two processes will depend on several factors such as the ability of\u2026","rel":"","context":"In \"conformational analysis\"","block_context":{"text":"conformational analysis","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=conformational-analysis"},"img":{"alt_text":"","src":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/02\/Ring.svg","width":350,"height":200},"classes":[]},{"id":1812,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1812","url_meta":{"origin":6262,"position":2},"title":"Dial a molecule: Can new reactions be designed by computer?","author":"Henry Rzepa","date":"March 13, 2010","format":false,"excerpt":"One future vision for chemistry over the next 20 years or so is the concept of having machines into which one dials a molecule, and as if by magic, the required specimen is ejected some time later. This is in some ways an extrapolation of the existing peptide and nucleotide\u2026","rel":"","context":"In "Interesting chemistry"","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\/2010\/03\/sultine.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":6205,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6205","url_meta":{"origin":6262,"position":3},"title":"An orbital analysis of the stereochemistry of the E2 elimination reaction","author":"Henry Rzepa","date":"February 4, 2012","format":false,"excerpt":"The so-called E2 elimination mechanism is another one of those mainstays of organic chemistry. It is important because it introduces the principle that anti-periplanarity of the reacting atoms is favoured over other orientations such as the syn-periplanar form; Barton used this principle to great effect in developing the theory of\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/02\/E2.svg","width":350,"height":200},"classes":[]},{"id":5087,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5087","url_meta":{"origin":6262,"position":4},"title":"cis-Butene: a reaction coordinate dissected and methyl flags.","author":"Henry Rzepa","date":"October 12, 2011","format":false,"excerpt":"In two previous posts, I have looked at why cis-butene adopts conformation (a) rather than (b). I suggested it boiled down to electronic interactions between the methyl groups and the central alkene resulting in the formation of a H...H \"topological\" bond, rather than attraction between the H...H region to form\u2026","rel":"","context":"In "General"","block_context":{"text":"General","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/10\/cis-butene2.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":5500,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5500","url_meta":{"origin":6262,"position":5},"title":"The peroxidation of alkynes: things are not always what they seem.","author":"Henry Rzepa","date":"November 16, 2011","format":false,"excerpt":"The epoxidation of an alkene to give an oxirane is taught in introductory organic chemistry. Formulating an analogous mechanism for such reaction of an alkyne\u00a0sounds straightforward, but one gradually realises that it requires raiding knowledge from several other areas of (perhaps slightly more advanced) chemistry to achieve a joined up\u2026","rel":"","context":"In "Curly arrows"","block_context":{"text":"Curly arrows","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2327"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/11\/1.gif?resize=350%2C200","width":350,"height":200},"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","0":null,"1":"","2":"","3":"","4":"","5":"","6":"","7":"","8":""}],"_links":{"self":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6262","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=6262"}],"version-history":[{"count":12,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6262\/revisions"}],"predecessor-version":[{"id":6277,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6262\/revisions\/6277"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=6262"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=6262"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=6262"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=6262"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}