{"id":3003,"date":"2010-12-11T23:11:54","date_gmt":"2010-12-11T22:11:54","guid":{"rendered":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=3003"},"modified":"2026-01-02T14:42:50","modified_gmt":"2026-01-02T14:42:50","slug":"janus-mechanisms-the-past-and-the-future-reactions-of-the-diazonium-cation","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=3003","title":{"rendered":"Janus mechanisms (the past and the future): Reactions of the diazonium cation."},"content":{"rendered":"
\n

Janus was the mythological Roman god<\/a> depicted as having two heads facing opposite directions, looking simultaneously into the past and the future. Some of the most ancient (i.e. <\/em>19th century) known reactions can be considered part of a chemical mythology; <\/a>perhaps it is time for\u00a0a Janus-like look into their future.<\/p>\n

\"\"<\/a>

Reaction of the diazonium cation with cyanide.<\/p><\/div>\n

The phenyl diazonium ion is often introduced early in most chemistry teaching; it is used to produce spectacularly coloured solutions from colourless starting materials and makes an immediate impression.1<\/sup> The reaction of this species with cyanide salts often appears in introductory courses of aromatic chemistry as a means of producing aryl cyanides. It entered the text books around a century ago as the\u00a0Sandmeyer reaction<\/a> (using copper(I)cyanide, but it is also reported as occurring using more ionic cyanide salts as well).2<\/sup> The mechanism of the ionic reaction however has been given little attention recently. One common representation is as a unimolecular reaction to lose nitrogen gas forming an arene cation, which is mechanistically then followed by fast quenching with cyanide anion to replace the diazo group with the cyano group.<\/p>\n

Computational modelling of such ion-pair reactions has now become possible,3<\/sup> and is going to be used here to peek into the future. A B3LYP\/6-311G(d,p)\/SCRF calculation shows a transition state involving C-N cleavage, with an adjacent cyanide ion doing rather more than merely spectating. The dipole moment of the transition state is 11D (in acetonitrile as solvent). The structure shows the ion-pair endeavouring to minimise the charge separation, with the cyanide approaching at a rather different angle from the departing diazo group. This sort of SN<\/sub>2 displacement at an sp2<\/sup> (as opposed to sp3<\/sup>) carbon centre is mechanistically quite unusual.4<\/sup> The free energy of activation for this mechanism is calculated<\/a> as 24.9 kcal\/mol, which is slightly worryingly high for what is considered a room-temperature reaction (the same method gave quite reasonable barriers for another ion-pair mechanism3<\/sup>).<\/p>\n

\"\"

Phenyldiazonium cation + cyanide anion; substitution mechanism. Click for 3D<\/p><\/div>\n

So time to see if all is what it might seem. There are many other mechanisms that might be explored; below is what seems quite a reasonable one, the elimination of the diazo-group with accompanying proton abstraction to form a benzyne<\/a>. This transition state has an activation free energy of 17.8 kcal\/mol, a much more reasonable value for a room temperature reaction. The dipole moment is 17.1D (the reactant ion-pair is 19.7D).<\/p>\n

\"\"

Benzyne mechanism, in acetonitrile solvent. Click for 3D<\/p><\/div>\n

So could it be that this veritable reaction actually proceeds via<\/em> a different mechanism from that in the text books? Benzyne would be formed as a very reactive intermediate, and presumably in the presence of cyanide anions, it would react by nucleophilic addition to form benzonitrile, the same product as before. How could this be verified? Well, if the carbon atom carrying the diazonium group were to be labelled as say 14<\/sup>C, the original mechanism would carry all that label at one carbon in the benzonitrile product. But the benzyne mechanism would scramble the label between two carbons. Janus therefore sees the future in the shape of a useful experiment which could be done to distinguish the two alternative mechanisms.<\/p>\n

It is also noteworthy that the two alternative transition states have different dipole moments, and so are affected differently by solvent polarity. Thus in water, the activation free energies are respectively (substitution\/elimination) 25.1 and 17.9, whilst in benzene as solvent they are much higher: 48.7 and 39.0 kcal\/mol. The effect of the solvent upon the structure of the transition state is also considerable. Below is shown the benzyne elimination mechanism as calculated in the non polar benzene as solvent. Note how the proton transfer is much more advanced, and the C…N cleavage is less advanced than in acetonitrile as solvent.<\/p>\n

\"\"

Benzyne transition state, in benzene solvent. Click for 3D<\/p><\/div>\n

We are seeing something of a revolution here. Gradually, the mechanisms of the reaction library built up over the last 100 years or so are increasingly being explored using quantitative calculations. It seems entirely likely that more surprises will crop up.<\/p>\n

\n
    \n
  1. At the age of ~12 I was introduced to chemistry via<\/em> this reaction, an exposure at least in part why almost 50 years later I am still doing chemistry and why I write this blog.<\/li>\n
  2. Kazitsyna, L. A.; Gruzdneva, V. N. Vestnik Moskovskogo Universiteta, Seriya 2: Khimiya<\/em>, 1975<\/strong>, 16<\/em>, 331-7.<\/li>\n
  3. The ion-pair mechanism of the racemisation of iso-bornyl chloride, another ancient and almost mythological reaction, has recently been studied in this manner.[1]<\/a><\/span><\/li>\n
  4. Z. Wu and R. Glaser, “Ab Initio Study of the SN1<\/sub>Ar and SN2<\/sub>Ar Reactions of Benzenediazonium Ion with Water. On the Conception of \u201cUnimolecular Dediazoniation\u201d in Solvolysis Reactions”[2]<\/a><\/span><\/li>\n

    <\/strong><\/ol>\n

    <\/strong><\/p>\n


    \n<\/strong><\/p>\n


    \n<\/strong><\/p>\n

    <\/strong><\/p>\n

    \n

    This post has DOI: 10.59350\/g18gn-rra49<\/a><\/p>\n

    References<\/h2>\n
    1. J. Kong, P.V.R. Schleyer, and H.S. Rzepa, \"Successful Computational Modeling of Isobornyl Chloride Ion-Pair Mechanisms\", The Journal of Organic Chemistry<\/i>, vol. 75, pp. 5164-5169, 2010. https:\/\/doi.org\/10.1021\/jo100920e<\/a>\n\n<\/li>\n
    2. Z. Wu, and R. Glaser, \"Ab Initio Study of the S<sub>N<\/sub>1Ar and S<sub>N<\/sub>2Ar Reactions of Benzenediazonium Ion with Water. On the Conception of \u201cUnimolecular Dediazoniation\u201d in Solvolysis Reactions\", Journal of the American Chemical Society<\/i>, vol. 126, pp. 10632-10639, 2004. https:\/\/doi.org\/10.1021\/ja047620a<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

      Janus was the mythological Roman god depicted as having two heads facing opposite directions, looking simultaneously into the past and the future. Some of the most ancient (i.e. 19th century) known reactions can be considered part of a chemical mythology; perhaps it is time for\u00a0a Janus-like look into their future. The phenyl diazonium ion is […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_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":"federated","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},"jetpack_post_was_ever_published":false},"categories":[4],"tags":[352,350,348,349,235,353,347,40,202,2651,254,354,351],"ppma_author":[2661],"class_list":["post-3003","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-activation-free-energy","tag-benzonitrile-product","tag-chemical-mythology","tag-coloured-solutions","tag-cyano","tag-diazo","tag-diazonium","tag-free-energy","tag-henry-rzepa","tag-historical","tag-ion-pair-mechanisms","tag-l-a","tag-nitrogen-gas"],"yoast_head":"\nJanus mechanisms (the past and the future): Reactions of the diazonium cation. - 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=3003\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Janus mechanisms (the past and the future): Reactions of the diazonium cation. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"Janus was the mythological Roman god depicted as having two heads facing opposite directions, looking simultaneously into the past and the future. 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The ionisation mechanism and kinetic isotope effects for 1,3-dimethylindolin-2 one","author":"Henry Rzepa","date":"January 7, 2016","format":false,"excerpt":"This is the third and final study deriving from my Ph.D.. The first two topics dealt with the mechanism of heteroaromatic electrophilic attack using either a diazonium cation or a proton as electrophile, followed by either proton abstraction or carbon dioxide loss from the resulting Wheland intermediate. This final study\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":12056,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12056","url_meta":{"origin":3003,"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":6750,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6750","url_meta":{"origin":3003,"position":4},"title":"Mechanism of the diazomethane alkylation of a carboxylic acid.","author":"Henry Rzepa","date":"May 19, 2012","format":false,"excerpt":"Many reaction mechanisms involve a combination of bond formation\/cleavage between two non-hydrogen atoms and those involving reorganisation of proximate hydrogens. The Baeyer-Villiger discussed previously illustrated a complex dance between the two types. Here I take a look at another such mechanism, the methylation of a carboxylic acid by diazomethane. Text-books\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\/2012\/05\/diazomethane12.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":3003,"position":5},"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":[]}],"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\/3003","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=3003"}],"version-history":[{"count":12,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/3003\/revisions"}],"predecessor-version":[{"id":30710,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/3003\/revisions\/30710"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=3003"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=3003"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=3003"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=3003"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}