{"id":9917,"date":"2013-03-25T07:55:01","date_gmt":"2013-03-25T07:55:01","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=9917"},"modified":"2014-01-17T07:41:43","modified_gmt":"2014-01-17T07:41:43","slug":"concerted-vs-stepwise-meisenheimer-mechanisms-for-aromatic-nucleophilic-substitution","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9917","title":{"rendered":"Concerted vs stepwise (Meisenheimer) mechanisms for aromatic nucleophilic substitution."},"content":{"rendered":"
\n

My two previous explorations of aromatic substitutions have involved an electrophile (NO+<\/sup><\/a> or Li+<\/sup><\/a>). Time now to look at a nucleophile, representing nucleophilic aromatic substitution<\/em>. 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[1]<\/a><\/span>. I ask the same question as before; are there any circumstances under which the mechanism could instead be concerted, by-passing this intermediate?<\/p>\n

\"meis\"<\/p>\n

To start, I will adopt Nu = Cl(-), X=Y=H, and as the positive counter-ion I will use the guanidinium cation. As usual, wB97XD\/6-311G(d,p) with a continuum water model applied. A transition state is located at the half way stage[2]<\/a><\/span>, indicating no intermediate Meisenheimer! It represents in effect a direct Sn2 substitution at an aromatic sp2<\/sup> carbon. The barrier for this (unactivated) substitution is very high (an imaginary \u03bdi<\/i><\/sub> 507 cm-1<\/sup> for the asymmetric Cl-C-Cl stretch confirms it as a transition state).<\/p>\n\n\n\n
\"meis\"[3]<\/a><\/span><\/td>\n\"meis\"<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

With Nu=F(-), the barrier decreases significantly[4]<\/a><\/span> and the curvature of the potential at the transition state becomes much broader (\u03bdi<\/i><\/sub> 130 cm-1<\/sup>), a prelude perhaps to the mechanism transitioning from being concerted to a stepwise manifestation involving a discrete intermediate.<\/p>\n\n\n\n
\"meisf\"[5]<\/a><\/span><\/td>\n\"meisf\"<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

With X=p-NO2<\/sub>, Nu = Cl(-), a similar trend is seen[6]<\/a><\/span> with a barrier that is both lower and wider (\u03bdi<\/i><\/sub> 355 cm-1<\/sup>); the o-NO2<\/sub> isomer continues that trend (\u03bdi<\/i><\/sub>\u00a0333 cm-1<\/sup>)[7]<\/a><\/span><\/p>\n\n\n\n
\"meisno2\"[8]<\/a><\/span><\/td>\n\"meisno2\"<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Finally with X=Y=NO2<\/sub>, Nu = Cl(-), a proper Meisenheimer intermediate is now located; the asymmetric Cl-C-Cl stretch is no longer imaginary but real (\u03bd\u00a0385 cm-1<\/sup>)[9]<\/a><\/span>. This is closely related to a known crystal structure (Nu=OEt with Cs replacing the guanidinium cation)[10]<\/a><\/span>.<\/p>\n

\"Click

Click to view asymmetric Cl-C-Cl stretch.<\/p><\/div>\n

So we see here a mechanism which can be finely tuned by the substituents to exhibit either concerted mechanistic behaviour, or armed with groups that stabilise an intermediate ion-pair, to transition to a fully stepwise reaction.<\/p>\n

There is one other aspect I want to explore. In the Meisenheimer intermediate, the cyclic conjugation and hence the aromaticity of the original aryl ring is (at least partially) interrupted. But what of the concerted transition state; must it too loose the original aromaticity? In the structure diagram drawn at the top of this post, I hinted it might not! A NICS(0) probe place at the QTAIM-determined centroid of the aryl ring (X=Y=H, Nu=Cl) indicates a value of -9.0[11]<\/a><\/span> (benzene itself is about -10 ppm), indicating relatively little cyclic conjugation is actually lost in the transition state. The nature of the molecular orbital confirms this. Shown below is the most stable of the three\u00a0aromatic \u03c0-MOs, again resembling that of benzene very closely and the two C-Cl partially formed bonds participate fully in this conjugation. So we might call this strongly hyper-conjugated aromaticity<\/a>.<\/p>\n

\"Click

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

So I end by restating that this classical text-book mechanism, in which aromatic nucleophilic substitution is shown as proceeding through a Meisenheimer intermediate (the analogue of the electrophilic Wheland intermediate) may in fact only be true of aryl groups substituted with electron withdrawing groups. Without these, the mechanism converts to a concerted type, albeit with a much higher reaction barrier, possibly high enough that no actual examples of this type actually occur in reality. But it again reinforces that mechanisms may not always be what the text-books tell us.<\/p>\n

\n

Postscript:<\/strong> I append here the IRC for\u00a0X=Y=NO2<\/sub>, Nu = Cl(-), which as \u00a0I noted above has become a stepwise reaction:<\/p>\n\n\n\n
\"meis-trinitro\"[12]<\/a><\/span><\/td>\n\"meis-trinitro\"<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

References<\/h2>\n
  1. J. Meisenheimer, \"Ueber Reactionen aromatischer Nitrok\u00f6rper\", Justus Liebigs Annalen der Chemie<\/i>, vol. 323, pp. 205-246, 1902. https:\/\/doi.org\/10.1002\/jlac.19023230205<\/a>\n\n<\/li>\n
  2. H.S. Rzepa, \"Gaussian Job Archive for C7H11Cl2N3\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.658805<\/a>\n\n<\/li>\n
  3. H.S. Rzepa, \"Gaussian Job Archive for C7H11Cl2N3\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.658893<\/a>\n\n<\/li>\n
  4. H.S. Rzepa, \"Gaussian Job Archive for C7H11F2N3\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.658831<\/a>\n\n<\/li>\n
  5. H.S. Rzepa, \"Gaussian Job Archive for C7H11F2N3\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.658890<\/a>\n\n<\/li>\n
  6. H.S. Rzepa, \"Gaussian Job Archive for C7H10Cl2N4O2\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.658806<\/a>\n\n<\/li>\n
  7. H.S. Rzepa, \"Gaussian Job Archive for C7H10Cl2N4O2\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.658891<\/a>\n\n<\/li>\n
  8. H.S. Rzepa, \"Gaussian Job Archive for C7H10Cl2N4O2\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.658910<\/a>\n\n<\/li>\n
  9. H.S. Rzepa, \"Gaussian Job Archive for C7H8Cl2N6O6\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.658897<\/a>\n\n<\/li>\n
  10. R. Destro, C.M. Gramaccioli, and M. Simonetta, \"The crystal and molecular structure of the complexes of 2,4,6-trinitrophenetole with caesium or potassium ethoxide (Meisenheimer salts)\", Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry<\/i>, vol. 24, pp. 1369-1386, 1968. https:\/\/doi.org\/10.1107\/s0567740868004322<\/a>\n\n<\/li>\n
  11. H.S. Rzepa, \"Gaussian Job Archive for C7H11Cl2N3\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.658903<\/a>\n\n<\/li>\n
  12. H.S. Rzepa, \"Gaussian Job Archive for C7H8Cl2N6O6\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.659274<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    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. 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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":7344,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7344","url_meta":{"origin":9917,"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":9706,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9706","url_meta":{"origin":9917,"position":2},"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":20212,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=20212","url_meta":{"origin":9917,"position":3},"title":"Concerted Nucleophilic Aromatic Substitution Mediated by the PhenoFluor Reagent.","author":"Henry Rzepa","date":"September 20, 2018","format":false,"excerpt":"Recently, the 100th anniversary of the birth of the famous chemist Derek Barton was celebrated with a symposium. One of the many wonderful talks presented was by Tobias Ritter and entitled \"Late-stage fluorination for PET imaging\" and this resonated for me. The challenge is how to produce C-F bonds under\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\/2018\/09\/fluoridation.gif?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":9659,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9659","url_meta":{"origin":9917,"position":4},"title":"Understanding the electrophilic aromatic substitution of indole.","author":"Henry Rzepa","date":"March 3, 2013","format":false,"excerpt":"The electrophilic substitution of indoles is a staple of any course on organic chemistry. Indoles also hold a soft-spot for me, since I synthesized not a few as part of my Ph.D. studies., The preference for substitution in the 3-position is normally explained using the arrows shown below (position 3=green,2=blue,1=red).\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":"Molecular electrostatic potential. Click for 3D.","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/03\/indole-mep.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":2423,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=2423","url_meta":{"origin":9917,"position":5},"title":"The oldest reaction mechanism: updated!","author":"Henry Rzepa","date":"September 14, 2010","format":false,"excerpt":"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.\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":[]}],"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\/9917","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=9917"}],"version-history":[{"count":41,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/9917\/revisions"}],"predecessor-version":[{"id":11932,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/9917\/revisions\/11932"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=9917"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=9917"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=9917"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=9917"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}