{"id":11512,"date":"2013-11-05T06:57:47","date_gmt":"2013-11-05T06:57:47","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=11512"},"modified":"2013-11-09T17:01:39","modified_gmt":"2013-11-09T17:01:39","slug":"kinetic-vs-thermodynamic-enolization","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11512","title":{"rendered":"Kinetic vs thermodynamic enolization."},"content":{"rendered":"
\n

The concept of kinetic vs<\/em> thermodynamic control of a reaction is often taught in the context of the enolisation of e.g.<\/em> 1-methylcyclohexanone as induced by a base. The story goes that at low temperatures (-78\u00b0C), the rate of the sterically more hindered thermodynamic enolisation does not compete with the faster kinetic product but that at higher temperatures when an equilibrium is possible, the thermodynamically more stable tetrasubstituted enol is formed. I set out to see if this result can be modelled.<\/p>\n

\"enol\"<\/a><\/p>\n

I started with the scheme shown above, R=X=H (i.e.<\/em> two water molecules inducing the enolisation).\u00a0<\/p>\n\n\n\n\n
\"kinetic\"<\/td>\n<\/tr>\n
\u00a0\"kineticE\"<\/a><\/td>\n\u00a0\"kineticG\"<\/a><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Points to note:<\/p>\n

    \n
  1. The transition state is very late<\/a> (i.e.<\/em>\u00a0product, or enol-like). The cleaving C-H and the forming H-O (base) bond lengths are respectively 1.724 and 1.042\u00c5 (\u03c9B97XD\/6-311G(d,p)\/SCRF=acetone).<\/li>\n
  2. The activation barrier is high (~33 kcal\/mol). This is because water is a very weak base.<\/li>\n
  3. The difference in \u0394G298<\/sub>\u2021<\/sup> shows the thermodynamic[1]<\/a><\/span> transition state is 0.6 kcal\/mol lower than the kinetic[2]<\/a><\/span><\/li>\n
  4. OK, what about \u0394G220<\/sub>\u2021<\/sup> then? The difference is still 0.5 kcal\/mol.<\/li>\n
  5. The reaction is endothermic by about 7 kcal\/mol. It is known that enols are less stable than ketones.<\/li>\n
  6. The tetrasubstituted “thermodynamic” enol product is 1.7 kcal\/mol more stable than the trisubstituted “kinetic” form.<\/li>\n<\/ol>\n

    So, to avoid the transition state inheriting too much of the product-stability, it has to be an early one rather than a late one! How about guanidine as a stronger base[3]<\/a><\/span>? The C-H and H-N lengths are now 1.414 and 1.291\u00c5, and the barrier goes down to ~16 kcal\/mol. The transition state is certainly more reactant like, and indeed the \u0394G298<\/sub>\u2021<\/sup> difference is now 0.0 kcal\/mol! Notice also that with the stronger base, the initial product is in fact an ion-pair, comprising the guanidinium cation and the enolate anion.<\/p>\n

    \"kinetic-guan\"
    \"kinetic-guanE\"<\/p>\n

    \u00a0Something more is clearly needed in the model to convert the kinetic enol to a low-temperature winner. A stronger base probably, but perhaps also a hindered base? If I find such a model, I will report back here.<\/p>\n

    Postscript:<\/b> My closing remark above was that I was going to try to locate transition states using a stronger, perhaps more hindered base<\/a>. The obvious one to try is LDA (lithium di-isopropylamide, or LiNi<\/sup>Pr2<\/sub>). This indeed results in an earlier and (possibly) more hindered transition state[4]<\/a><\/span> (C-H 1.227\u00c5). \u00a0\u0394\u0394G298<\/sub>\u2021<\/sup> now is \u00a02.4 kcal\/mol in favour of the kinetic over the thermodynamic[5]<\/a><\/span> isomer, which is enough to ensure a ratio of ~60:1 in favour of that outcome at 298K, or ~250:1 at -78\u00b0C. The activation barrier is now really very small \u00a0(~4 kcal\/mol) which matches the supposition that it is a very early transition state indeed.<\/p>\n

    \"Click

    Click for 3D<\/p><\/div>\n

    An NCI (non-covalent-interactions) analysis of the kinetic transition state is shown below. It shows distinct \u00a0(green = weakly stabilising) zones between the isopropyl groups of the base and the cyclohexanone. So the effect might not be actually “steric-hindrance”, but in fact originate from optimal (dispersion) attractions (shown in blue to green, whilst destabilising steric hindrance would be shown in yellow to red) directing the formation of the kinetic isomer[6]<\/a><\/span>.<\/p>\n

    \"Click

    Click for 3D<\/p><\/div>\n

    Li is a complex element, and of course oligomeric structures<\/a> abound. I am currently exploring these, and if I find anything interesting, there will no doubt be a postscript to the postscript.<\/p>\n

    Postscript to the postscript<\/b>.\u00a0The next model up includes adding one\u00a0thf<\/strong> as coordinating solvent to the Li.\u00a0\u0394\u0394G220<\/sub>\u2021<\/sup>\u00a0is now attenuated to 1.1 \u00a0kcal\/mol in favour of the kinetic[7]<\/a><\/span> over the thermodynamic,[8]<\/a><\/span> leading to a 12:1 ratio at 220K.<\/p>\n

    \"Click

    Click for 3D<\/p><\/div>\n

    References<\/h2>\n
    1. H.S. Rzepa, \"Gaussian Job Archive for C7H16O3\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.840464<\/a>\n\n<\/li>\n
    2. H.S. Rzepa, \"Gaussian Job Archive for C7H16O3\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.840465<\/a>\n\n<\/li>\n
    3. H.S. Rzepa, \"Gaussian Job Archive for C8H17N3O\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.840490<\/a>\n\n<\/li>\n
    4. H.S. Rzepa, \"Gaussian Job Archive for C13H26LiNO\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.841757<\/a>\n\n<\/li>\n
    5. H.S. Rzepa, \"Gaussian Job Archive for C13H26LiNO\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.842572<\/a>\n\n<\/li>\n
    6. S. Ehrlich, H.F. Bettinger, and S. Grimme, \"Dispersion\u2010Driven Conformational Isomerism in \u03c3\u2010Bonded Dimers of Larger Acenes\", Angewandte Chemie International Edition<\/i>, vol. 52, pp. 10892-10895, 2013. https:\/\/doi.org\/10.1002\/anie.201304674<\/a>\n\n<\/li>\n
    7. H.S. Rzepa, \"Gaussian Job Archive for C17H34LiNO2\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.845578<\/a>\n\n<\/li>\n
    8. H.S. Rzepa, \"Gaussian Job Archive for C17H34LiNO2\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.845579<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

      The concept of kinetic vs thermodynamic control of a reaction is often taught in the context of the enolisation of e.g. 1-methylcyclohexanone as induced by a base. The story goes that at low temperatures (-78\u00b0C), the rate of the sterically more hindered thermodynamic enolisation does not compete with the faster kinetic product but that at […]<\/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":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[1086],"tags":[1150],"ppma_author":[2661],"class_list":["post-11512","post","type-post","status-publish","format-standard","hentry","category-reaction-mechanism-2","tag-faster-kinetic-product"],"yoast_head":"\nKinetic vs thermodynamic enolization. - 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=11512\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Kinetic vs thermodynamic enolization. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"The concept of kinetic vs thermodynamic control of a reaction is often taught in the context of the enolisation of e.g. 1-methylcyclohexanone as induced by a base. The story goes that at low temperatures (-78\u00b0C), the rate of the sterically more hindered thermodynamic enolisation does not compete with the faster kinetic product but that at […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11512\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2013-11-05T06:57:47+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2013-11-09T17:01:39+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/11\/enol.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=\"3 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"Kinetic vs thermodynamic enolization. - 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=11512","og_locale":"en_GB","og_type":"article","og_title":"Kinetic vs thermodynamic enolization. - Henry Rzepa's Blog","og_description":"The concept of kinetic vs thermodynamic control of a reaction is often taught in the context of the enolisation of e.g. 1-methylcyclohexanone as induced by a base. 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Subversive thoughts for electrophilic substitution of Indole.","author":"Henry Rzepa","date":"March 10, 2013","format":false,"excerpt":"I mentioned in the last post that one can try to predict the outcome of electrophilic aromatic substitution by approximating the properties of the transition state from those of either the reactant or the (presumed Wheland) intermediate by invoking Hammond's postulate. A third option is readily available nowadays; calculate the\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":"Click for 3D.","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/03\/3-NO-indole-ESP.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":8246,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8246","url_meta":{"origin":11512,"position":1},"title":"Thalidomide. The role of water in the mechanism of its aqueous racemisation.","author":"Henry Rzepa","date":"November 10, 2012","format":false,"excerpt":"Thalidomide is a chiral molecule, which was sold in the 1960s as a sedative in its (S,R)-racemic form. The tragedy was that the (S)-isomer was tetragenic, and only the (R) enantiomer acts as a sedative. What was not appreciated at the time is that interconversion of the (S)- and (R)\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.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/11\/thal1.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":15505,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=15505","url_meta":{"origin":11512,"position":2},"title":"Kinetic isotope effect models as a function of ring substituent for indole-3-carboxylic acids and indolin-2-ones.","author":"Henry Rzepa","date":"January 20, 2016","format":false,"excerpt":"The original strategic objective of my PhD researches in 1972-74 was to explore how primary kinetic hydrogen isotope effects might be influenced by the underlying structures of the transition states involved. Earlier posts dealt with how\u00a0one can\u00a0construct quantum-chemical\u00a0models of these transition states that fit the known properties of the reactions.\u2026","rel":"","context":"In "reaction mechanism"","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":[]},{"id":17771,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17771","url_meta":{"origin":11512,"position":3},"title":"Reaction coordinates vs Dynamic trajectories as illustrated by an example reaction mechanism.","author":"Henry Rzepa","date":"March 20, 2017","format":false,"excerpt":"The example a few posts back of how methane might invert its configuration by transposing two hydrogen atoms illustrated the reaction mechanism by locating a transition state and following it down in energy using an intrinsic reaction coordinate\u00a0(IRC). Here I explore an alternative method based instead on computing a molecular\u2026","rel":"","context":"In "reaction mechanism"","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":[]},{"id":7192,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7192","url_meta":{"origin":11512,"position":4},"title":"Origins of the Regioselectivity of Cyclopropylcarbinyl Ring Opening Reactions.","author":"Henry Rzepa","date":"July 20, 2012","format":false,"excerpt":"Twenty years are acknowledged to be a long time in Internet\/Web terms. In the early days (in 1994), it was a taken that the passage of 1 Web day in the Internet time-warp was ~\u2261 7 for the rest of the world (the same factor as applied to the lives\u2026","rel":"","context":"In \"computational chemistry\"","block_context":{"text":"computational chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=computational-chemistry"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/07\/Motherwell_orbitals.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":3621,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=3621","url_meta":{"origin":11512,"position":5},"title":"The thermodynamic energies of left and right handed DNA.","author":"Henry Rzepa","date":"March 5, 2011","format":false,"excerpt":"In this earlier post, I noted some aspects of the calculated structures of both Z- and B-DNA duplexes. These calculations involved optimising the positions of around 250-254 atoms, for d(CGCG)2 and d(ATAT)2, an undertaking which has taken about two months of computer time! The geometries are finally optimised to the\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":"","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","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\/11512","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=11512"}],"version-history":[{"count":34,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/11512\/revisions"}],"predecessor-version":[{"id":11596,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/11512\/revisions\/11596"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=11512"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=11512"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=11512"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=11512"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}