{"id":12115,"date":"2014-03-12T07:01:38","date_gmt":"2014-03-12T07:01:38","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=12115"},"modified":"2014-03-31T17:12:19","modified_gmt":"2014-03-31T16:12:19","slug":"aromatic-electrophilic-substitution-a-different-light-on-the-bromination-of-benzene","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12115","title":{"rendered":"Aromatic electrophilic substitution. A different light on the bromination of benzene."},"content":{"rendered":"
\n

My

\"br2+benzene\"<\/a>\"br2+benzene\"<\/a><\/p>\n

The “text-book” mechanism involves nucleophilic attack by the benzene on the bromine to form a “Wheland intermediate” (the blue arrows) followed in a clear second step by proton removal by the liberated bromide anion (the red arrows). But one group had other ideas[1]<\/a><\/span>, proposing in 2011 that the blue and red arrows conflate into a single concerted process which does NOT involve an explicit Wheland intermediate ion-pair. The text-books would have to be re-written! Paul Schleyer (a co-author of the above) recently contacted me about this reaction, noting that no explicit intrinsic reaction coordinate (IRC) had been reported in the 2011 article. Could I run one to establish that the course of this reaction really was concerted and “Whelandless<\/a>“?<\/p>\n

The level of theory used before[1]<\/a><\/span>\u00a0is r<\/strong>b3lyp\/6-311++G(2d,2p)\/SCRF=CCl4<\/sub>\u00a0(the r <\/strong>is added here, for reasons that will soon become apparent) and the animation[2]<\/a><\/span> is shown below, which is followed by repeating the calculation with addition of a D3-type dispersion correction to the core rb3lyp DFT method.[3]<\/a><\/span> Without dispersion, the final HBr becomes H-bonded to the other Br, but with dispersion it instead forms a\u00a0\u03c0-facial hydrogen bond to the aromatic ring. Even for such a small molecule, one can easily observe the effects of dispersion forces!<\/p>\n

\"Br2+benzene\"<\/a>\"Br2+benzene+D3\"<\/a><\/p>\n

\"br2-d3\"<\/a>\"br2+d3\"<\/a><\/p>\n

The reaction is indeed concerted, but it is also asynchronous<\/em><\/strong> as revealed by the characteristic feature at IRC ~3. We might conclude that the Wheland does make an appearance in this mechanism, but only as\u00a0a “hidden intermediate<\/em>“. It is a relay-race with the blue arrows above running first, and then without pause smoothly passing the baton of the reaction to the red arrows. The activation energy is high, commensurate with a reaction that in fact does not take place at normal temperatures.<\/p>\n

Boris Galabov (another co-author[1]<\/a><\/span>) then pointed out to me that the spin-restricted wavefunction (r<\/strong> above) at the transition state is unstable with respect to spin u<\/strong>nrestriction.[4]<\/a><\/span> This means that some open-shell biradical character is present at least at the transition state if not the entire pathway. So what would happen if the IRC were repeated using u<\/strong>b3lyp instead of rb3lyp? Would allowing for biradical character still retain the concerted nature?<\/p>\n

Before showing the results, I have to point out that the u<\/strong>IRC must be done in two stages,\u2021<\/sup> the first being the path to the transition state and the second the path down from it to products (the program I use to show the profiles is not capable\u00a0has errors when splicing the two together<\/del>). First the upward path[5]<\/a><\/span> (without dispersion) ending at the TS, followed by the path down.[6]<\/a><\/span><\/p>\n

\"urE\"<\/a><\/p>\n\n\n\n\n
IRC profile for spin-unrestricted pathway\u00a0<\/caption>\n
\"ufE\"<\/a><\/td>\n<\/tr>\n
\"ufG\"<\/a><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

On the approach path, the spin expectation operator\u00a0<S2<\/sup>> starts at zero but at IRC ~2.0 it becomes non-zero (biradical character forms) and this persists to the transition state and to IRC ~-2 beyond on the downward path before reverting again to a closed shell singlet. In this central region we have what amounts to a “hidden biradicaloid intermediate”. Since the C-Br bond formation and the subsequent C-H bond cleavage are NOT synchronous, we also retain the hidden Wheland characteristics. So this system is perhaps best described as having a “hidden biradicaloid Wheland intermediate<\/em>“; a double whammy in the vernacular. \u00a0The non zero value of \u00a0<S2<\/sup>> lowers the activation barrier from \u00a0~42 kcal\/mol to \u00a0~37 kcal\/mol, but it still remains a barrier which is insurmountable at room temperatures.<\/p>\n

The bottom line remains: according to this quantum model, the reaction is concerted, as originally claimed.[1]<\/a><\/span><\/p>\n

\n

\u2021<\/sup> The technical explanation is as follows. The IRC is started at the TS, and the SCF is converged using a broken-symmetry keyword guess(mix)<\/em>. As the IRC proceeds on the path down to reactant, each step uses the density matrix from the previous step as the initial SCF guess. This ensures that the unrestricted wavefunction remains symmetry broken if that is the lowest energy solution. Before the reactant is reached however, <S2<\/sup>> has collapsed to zero. Then the forward path is started, again from the TS. However, the program continues to use the last density matrix and hence <S2<\/sup>> continues to be zero for this entire path. Hence the reason for performing two separate IRC calculations, to ensure that the correct value of <S2<\/sup>> is achieved on both pathways.<\/p>\n

\n

References<\/h2>\n
  1. J. Kong, B. Galabov, G. Koleva, J. Zou, H.F. Schaefer, and P.V.R. Schleyer, \"The Inherent Competition between Addition and Substitution Reactions of Br<sub>2<\/sub> with Benzene and Arenes\", Angewandte Chemie International Edition<\/i>, vol. 50, pp. 6809-6813, 2011. https:\/\/doi.org\/10.1002\/anie.201101852<\/a>\n\n<\/li>\n
  2. H.S. Rzepa, \"Gaussian Job Archive for C6H6Br2\", 2014. https:\/\/doi.org\/10.6084\/m9.figshare.956223<\/a>\n\n<\/li>\n
  3. H.S. Rzepa, \"Gaussian Job Archive for C6H6Br2\", 2014. https:\/\/doi.org\/10.6084\/m9.figshare.956247<\/a>\n\n<\/li>\n
  4. M.J. Dewar, S. Olivella, and H.S. Rzepa, \"MNDO study of ozone and its decomposition into (O2 + 0)\", Chemical Physics Letters<\/i>, vol. 47, pp. 80-84, 1977. https:\/\/doi.org\/10.1016\/0009-2614(77)85311-6<\/a>\n\n<\/li>\n
  5. H.S. Rzepa, \"Gaussian Job Archive for C6H6Br2\", 2014. https:\/\/doi.org\/10.6084\/m9.figshare.958784<\/a>\n\n<\/li>\n
  6. H.S. Rzepa, \"Gaussian Job Archive for C6H6Br2\", 2014. https:\/\/doi.org\/10.6084\/m9.figshare.958785<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    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 attack by the benzene on […]<\/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":[4,1086],"tags":[693,152,190,1189,1190,1169,1188,899,1133,74,1170],"ppma_author":[2661],"class_list":["post-12115","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","category-reaction-mechanism-2","tag-activation-energy","tag-animation","tag-aromatic","tag-boris-galabov","tag-co-author","tag-electrophilic","tag-lowest-energy-solution","tag-op-director-of-aromatic-electrophilic-substitution","tag-paul-schleyer","tag-pence","tag-substitution"],"yoast_head":"\nAromatic electrophilic substitution. A different light on the bromination of benzene. - 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=12115\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Aromatic electrophilic substitution. A different light on the bromination of benzene. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"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 attack by the benzene on […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12115\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2014-03-12T07:01:38+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2014-03-31T16:12:19+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/03\/br2+benzene.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=\"4 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"Aromatic electrophilic substitution. 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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 attack by the benzene on […]","og_url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12115","og_site_name":"Henry Rzepa's Blog","article_published_time":"2014-03-12T07:01:38+00:00","article_modified_time":"2014-03-31T16:12:19+00:00","og_image":[{"url":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/03\/br2+benzene.svg","type":"","width":"","height":""}],"author":"Henry Rzepa","twitter_card":"summary_large_image","twitter_misc":{"Written by":"Henry Rzepa","Estimated reading time":"4 minutes"},"schema":{"@context":"https:\/\/schema.org","@graph":[{"@type":"Article","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12115#article","isPartOf":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12115"},"author":{"name":"Henry Rzepa","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281"},"headline":"Aromatic electrophilic substitution. 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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.\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\/09\/wheland.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":7344,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7344","url_meta":{"origin":12115,"position":1},"title":"The first curly arrows. The d\u00e9nouement.","author":"Henry Rzepa","date":"July 23, 2012","format":false,"excerpt":"Recollect, Robinson was trying to explain why the nitroso group appears to be an o\/p director of aromatic electrophilic substitution. Using \u03c3\/\u03c0 orthogonality, I suggested that the (first ever) curly arrows as he drew them could not be the complete story, and that a transition state analysis would be needed.\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\/2012\/07\/p-wheland.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":16563,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16563","url_meta":{"origin":12115,"position":2},"title":"Exploring the electrophilic directing influence of heteroaromatic rings using crystal structure data mining.","author":"Henry Rzepa","date":"June 21, 2016","format":false,"excerpt":"This is a follow-up to the post on\u00a0exploring the directing influence of (electron donating) substituents on benzene with the focus on heteroaromatic rings such indoles, pyrroles and group 16 analogues (furans, thiophenes etc). The search query is shown above\u00a0(and is available here). As before, the distance is compared\u00a0from an electrophile,\u2026","rel":"","context":"In "crystal_structure_mining"","block_context":{"text":"crystal_structure_mining","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1745"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":12056,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12056","url_meta":{"origin":12115,"position":3},"title":"The mechanism of diazo coupling: more hidden mechanistic intermediates.","author":"Henry Rzepa","date":"March 8, 2014","format":false,"excerpt":"The diazo-coupling reaction dates back to the 1850s (and a close association with Imperial College via the first professor of chemistry there, August von Hofmann) and its mechanism was much studied in the heyday of physical organic chemistry. 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":9706,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9706","url_meta":{"origin":12115,"position":4},"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":9917,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9917","url_meta":{"origin":12115,"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\/12115","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=12115"}],"version-history":[{"count":28,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/12115\/revisions"}],"predecessor-version":[{"id":12178,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/12115\/revisions\/12178"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=12115"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=12115"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=12115"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=12115"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}