{"id":2423,"date":"2010-09-14T11:27:32","date_gmt":"2010-09-14T10:27:32","guid":{"rendered":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=2423"},"modified":"2014-04-14T08:37:11","modified_gmt":"2014-04-14T07:37:11","slug":"the-oldest-reaction-mechanism-updated","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=2423","title":{"rendered":"The oldest reaction mechanism: updated!"},"content":{"rendered":"
\n

Unravelling reaction mechanisms is thought to be a 20th century phenomenon, coincident more or less with the development of electronic theories of chemistry. Hence electronic\u00a0arrow pushing<\/a><\/em> as a term. But here I argue that the true origin of this immensely powerful technique in chemistry goes back to the 19th century. In 1890, Henry Armstrong proposed what amounts to close to the modern mechanism for the process we now know as aromatic electrophilic substitution <\/strong>[1]<\/a><\/span>. Beyond doubt, he invented what is now known as the Wheland Intermediate<\/strong> (about 50 years before Wheland wrote about it, and hence I argue here it should really be called the Armstrong\/Wheland intermediate). This is illustrated (in modern style) along the top row of the diagram.
\n

\"\"<\/a>

The mechanism of aromatic electrophilic substitution<\/p><\/div><\/p>\n

In 1887, Armstrong had tabulated the well known\u00a0ortho\/meta\/para<\/em> directing properties of substituents already on the ring towards this reaction[2]<\/a><\/span>. He even offered an explanation, which is not entirely wrong, given that in 1890, the electron had not yet been discovered! That did not stop Armstrong, who invented an entity he called the affinity<\/strong> for the purpose of developing his theories (in this theory, benzene had an inner circle of six affinities, which had a tendency to resist disruption). Armstrong’s description of the properties of the affinity matches that of the (yet to be discovered) electron very closely! But that is enough of history. The mechanism shown above emerged in its present representation (and naming) during the heyday of physical organic chemistry between 1926 – 1940, and of course is an absolute staple of all text books on organic chemistry. But, sacrilege, is it correct? Could what is referred to as an intermediate instead be a transition state<\/strong>? (shown in the bottom pathway of the scheme).<\/p>\n

Consider instead the following, in which X is replaced by an acetic acid motif;<\/p>\n

\"\"<\/a>

Transition state alternative to the Wheland<\/p><\/div>\n

The two steps, a bond formation between the benzene and E, and the proton abstraction from the benzene by X, are now synchronized into a single step, and the intermediate is now transformed into a transition state. Time to put this theory to the test. X is going to be made trifluoroacetate (R=CF3<\/sub>) and we are going to test it with E= NO+<\/sup> and F+ <\/sup> (yes, trifluoroacetyl hypofluorite is a known chemical, and it really does fluorinate1<\/sup> aromatic compounds at -78C). Firstly, E= NO+<\/sup>. A B3LYP\/6-311G(d,p) calculation[3]<\/a><\/span>\u00a0\u00a0run in a solvent simulated as dichloromethane, reveals the mid point to indeed be a transition state and NOT<\/strong> an intermediate![4]<\/a><\/span>.<\/p>\n

\"\"

Wheland as a transition state. Click image for animation<\/p><\/div>\n

There is one crucial aspect to this transition state that Armstrong himself made a point of. In the Wheland intermediate proper, the aromaticity of the benzene ring must be disrupted. As a transition state, it need not be (at least not completely). Thus the two bonds labeled as a<\/strong><\/em> have calculated lengths of ~1.415\u00c5, only slightly longer than the aromatic length, and certainly not single bonds as implied by the Wheland intermediate! Notice also the significant motion by the hydrogen, which implies the reaction would be subject to a kinetic isotope effect (this would normally be interpreted in terms of the second stage of the stepwise reaction shown along the top a being rate limiting, but this result shows this need not be so). Thus, if the structure is favourable, this veritable old mechanism can be redesigned to give a new, 21st century look to a 19th century staple<\/a>! By the way, the free energy of activation for this reaction is calculated as ~22 kcal\/mol, a perfectly viable thermal reaction. No doubt, by suitable design of the group X, this might be reduced.<\/p>\n

Now on to E=F+<\/sup>[5]<\/a><\/span>. This looks a little different. F+<\/sup> is now a much more voracious electrophile than the nitrosonium cation, and it therefore jumps ahead of the second mechanistic step, with no motion of the hydrogen being involved at this stage (one might also imagine making X a better base to swing things the other way).<\/p>\n\n\n\n
\n
\"\"

Transition state E=F+ leading to Wheland Intermediate. Click for \u00a03D model.<\/p><\/div>\n<\/td>\n

\n
\"\"

Genuine Wheland intermediate for E=F+ Click for 3D model<\/p><\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Now a full blown Armstrong\/Wheland intermediate<\/strong> does indeed form (10042\/to-5174<\/a>); an intimate ion pair if you will, even in the relatively non polar dichloromethane as modelled solvent. The structure \u00a0(shown above) is rather unexpected. \u00a0This reaction has \u0394G\u2020<\/sup> of ~5 kcal\/mol, \u00a0which is significantly lower than for the E=NO+<\/sup> system.<\/p>\n

Chemistry is full of surprises, and it is always a wonder how a slightly different take on even the oldest of reactions can reveal something new.<\/p>\n

Reference.<\/strong><\/p>\n

<<\/p>\n

p>1.\u00a0Umemoto, T.; Mukono, T.. 1-Acylamido-2-fluoro-4-acylbenzenes.<\/strong> Jpn. Kokai Tokkyo Koho\u00a0 (1986), Patent number JP61246156.<\/p>\n

References<\/h2>\n
  1. \"Proceedings of the Chemical Society, Vol. 6, No. 85\", Proceedings of the Chemical Society (London)<\/i>, vol. 6, pp. 95, 1890. https:\/\/doi.org\/10.1039\/pl8900600095<\/a>\n\n<\/li>\n
  2. H.E. Armstrong, \"XXVIII.\u2014An explanation of the laws which govern substitution in the case of benzenoid compounds\", J. Chem. Soc., Trans.<\/i>, vol. 51, pp. 258-268, 1887. https:\/\/doi.org\/10.1039\/ct8875100258<\/a>\n\n<\/li>\n
  3. S.R. Gwaltney, S.V. Rosokha, M. Head-Gordon, and J.K. Kochi, \"Charge-Transfer Mechanism for Electrophilic Aromatic Nitration and Nitrosation via the Convergence of (ab Initio) Molecular-Orbital and Marcus\u2212Hush Theories with Experiments\", Journal of the American Chemical Society<\/i>, vol. 125, pp. 3273-3283, 2003. https:\/\/doi.org\/10.1021\/ja021152s<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    Unravelling reaction mechanisms is thought to be a 20th century phenomenon, coincident more or less with the development of electronic theories of chemistry. Hence electronic\u00a0arrow pushing as a term. But here I argue that the true origin of this immensely powerful technique in chemistry goes back to the 19th century. In 1890, Henry Armstrong proposed […]<\/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":[4],"tags":[284,152,16,27,40,165,2651,2648,335],"ppma_author":[2661],"class_list":["post-2423","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-acetic-acid","tag-animation","tag-aromaticity","tag-chemical","tag-free-energy","tag-henry-armstrong","tag-historical","tag-interesting-chemistry","tag-wheland-intermediate"],"yoast_head":"\nThe oldest reaction mechanism: updated! - 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=2423\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The oldest reaction mechanism: updated! - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"Unravelling reaction mechanisms is thought to be a 20th century phenomenon, coincident more or less with the development of electronic theories of chemistry. 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Nick Greeves, purveyor of the excellent ChemTube3D site, contacted me about the\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":"cis-diazo","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/03\/cis-diazo.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":12115,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12115","url_meta":{"origin":2423,"position":1},"title":"Aromatic electrophilic substitution. A different light on the bromination of benzene.","author":"Henry Rzepa","date":"March 12, 2014","format":false,"excerpt":"My previous post related to the aromatic electrophilic substitution of benzene using as electrophile phenyl diazonium chloride. Another prototypical reaction, and again one where benzene is too inactive for the reaction to occur easily, is the catalyst-free bromination of benzene to give bromobenzene and HBr.\u00a0 The \"text-book\" mechanism involves nucleophilic\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":"br2+benzene","src":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/03\/br2+benzene.svg","width":350,"height":200},"classes":[]},{"id":11757,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11757","url_meta":{"origin":2423,"position":2},"title":"Does forming a Wheland intermediate disrupt all aromaticity?","author":"Henry Rzepa","date":"December 6, 2013","format":false,"excerpt":"Text books will announce that during aromatic electrophilic substitution, aromaticity is lost by the formation of a Wheland intermediate (and regained by eliminating a proton). Is that entirely true? I will start by considering the simplest of all such intermediates, the NMR of which was first reported by Olah and\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\/12\/wheland-NNM.jpeg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":9706,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9706","url_meta":{"origin":2423,"position":3},"title":"Kinetic vs Thermodynamic control. 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":15048,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=15048","url_meta":{"origin":2423,"position":4},"title":"I’ve started so I’ll finish. The mechanism of diazo coupling to indoles – forty (three) years on!","author":"Henry Rzepa","date":"December 24, 2015","format":false,"excerpt":"The BBC TV quiz series Mastermind\u00a0was first broadcast in the UK in 1972,\u00a0the same time\u00a0I was starting to investigate\u00a0the mechanism of diazocoupling to substituted indoles as part of my Ph.D. researches. The BBC program became known\u00a0for the\u00a0catch phrase\u00a0I've started so I'll finish;\u00a0here I will try to follow this precept with\u2026","rel":"","context":"In "Historical"","block_context":{"text":"Historical","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=565"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":9917,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9917","url_meta":{"origin":2423,"position":5},"title":"Concerted vs stepwise (Meisenheimer) mechanisms for aromatic nucleophilic substitution.","author":"Henry Rzepa","date":"March 25, 2013","format":false,"excerpt":"My two previous explorations of aromatic substitutions have involved an electrophile (NO+ or Li+). Time now to look at a nucleophile, representing nucleophilic aromatic substitution. The mechanism of this is thought to pass through an intermediate analogous to the Wheland for an electrophile, this time known as the Meisenheimer complex.\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\/trinitro.jpg?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\/2423","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=2423"}],"version-history":[{"count":2,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/2423\/revisions"}],"predecessor-version":[{"id":12305,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/2423\/revisions\/12305"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=2423"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=2423"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=2423"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=2423"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}