{"id":28849,"date":"2025-06-14T15:31:30","date_gmt":"2025-06-14T14:31:30","guid":{"rendered":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=28849"},"modified":"2025-12-06T08:45:24","modified_gmt":"2025-12-06T08:45:24","slug":"mechanism-of-the-dimerisation-of-nitrosobenzene","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=28849","title":{"rendered":"Mechanism of the dimerisation of Nitrosobenzene."},"content":{"rendered":"
\n

I am in the process of revising my annual lecture to first year university students on the topic of “curly arrows”. I like to start my story in 1924, when Robert Robinson published the very first example[1]<\/a><\/span> as an illustration of why nitrosobenzene undergoes electrophilic bromination in the para position of the benzene ring. I follow this up by showing how “data mining” can be used to see if this supports his assertion. I have used the very latest version of the CSD crystal structure database to update the version originally posted here in 2020.[2]<\/a><\/span><\/p>\n

\"\"<\/p>\n

I then discuss some possible reasons why Robinson might have thought that bromination goes in the para position, including the observation[3]<\/a><\/span> that nitrosobenzene is in equilibrium with its dimer, and that such a dimer might be expected to more reactive towards electrophiles than the “deactivated” monomer.<\/p>\n

Not part of the main lecture, but held in reserve for any questions at the end, is the following curly arrow pushing for the dimerisation.<\/p>\n

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

This raises a simple question – do both the red and blue arrows shown below participate at the same time, or do they go sequentially? Time then for some calculation to answer this last question. An ωB97XD\/Def2-TZVPP\/SCRF=chloroform calculation[4]<\/a><\/span> using a closed shell wavefunction (to correspond to two-electron curly arrows) appears to show a smooth reaction profile. The N-N bond length also converges from no bond to a double bond shortly after the transition state (NN = 1.3Å) without anything intermediate (this for the (Z)-stereochemical isomer, not the one shown above and which will be discussed later). The reported activation free energy for this process ΔG198<\/sub> is 15.7 kcal\/mol[3]<\/a><\/span> whilst the calculated value by this method is 25.5 kcal\/mol. Even allowing for a concentration effect (1M) and quasi-harmonic corrections to the free energy, it is still 23.9 kcal\/mol. <\/p>\n

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

\"\"<\/p>\n

In a previous post[5]<\/a><\/span> when an overly large barrier was computed, one reason is that the wavefunction might have “biradical” character and that the appropriate curly arrows might not be the appropriate two electron variety at all, but instead one-electron ones, as shown below. <\/p>\n

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

\n

The degree of biradical character is given by the spin-expectation operator <S2<\/sup>>, which has a value of 0.0 for no biradical character and 1.0 for a pure biradical. This time the transition state for the dimerisation is calculated to have a value of <S2<\/sup>> = 0.5418 and ΔG198<\/sub> is now calculated as 21.8 kcal\/mol (20.3 with quasi-harmonic corrections). <\/p>\n

The energy and N-N bond length profiles for the reaction coordinate using “one-electron” curly arrows are shown below, the former being around 4 kcal\/mol lower than for the two-electron arrows.<\/p>\n

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

The dihedral at the central C-N-N-C bond shows it almost entirely twisted at the transition state (as might befit a biradical) and then a smooth rotation to co-planarity (as befits a double bond) as the second bond forms.<\/p>\n

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

Because the system has C2<\/sub>-symmetry and importantly no plane of symmetry, the π and σ electrons are now allowed to mix together and this can be seen in the two (equivalent) orbital overlap models below at the transition state, each nitrogen lone pair managing to overlap constructively (blue with purple, red with orange, click on the diagram to load the orbitals) with the N-O π*<\/sup> orbital of the second nitrosobenzene.‡<\/sup><\/p>\n

\"\"<\/p>\n

\"\"<\/p>\n

Why is this simple system better described (energetically) by the use of one-electron arrows rather than two electron ones? A simple explanation might be that the electrons like to move consecutively simply to reduce the electron repulsion that the two-electron model would impose on it (reducing the electron correlation incurred in the process). It’s probably more complicated than this, but it shows a rare example where two-electron arrows are not the most appropriate for describing a chemical reaction.<\/p>\n

\n

Postscript<\/b>. The 1-electron transition state (<S2<\/sup>> = 0.981) for formation of the trans stereochemical (E) isomer is higher than the cis (Z) by ~4 kcal\/mol.
\n\"\"
\n\"\"<\/a>
\n\"\"<\/p>\n

\n

Roald Hoffmann has alerted me to an important early paper of his describing exactly this phenomenon.[6]<\/a><\/span><\/p>\n

References<\/h2>\n
  1. \"Forthcoming events\", Journal of the Society of Chemical Industry<\/i>, vol. 43, pp. 1295-1298, 1924. https:\/\/doi.org\/10.1002\/jctb.5000435208<\/a>\n\n<\/li>\n
  2. H. Rzepa, \"The first ever curly arrows. Revisited with some crystal structure mining.\", 2020. https:\/\/doi.org\/10.59350\/c6thp-wqe69<\/a>\n\n<\/li>\n
  3. K.G. Orrell, V. \u0160ik, and D. Stephenson, \"Study of the monomer\u2010dimer equilibrium of nitrosobenzene using multinuclear one\u2010 and two\u2010dimensional NMR techniques\", Magnetic Resonance in Chemistry<\/i>, vol. 25, pp. 1007-1011, 1987. https:\/\/doi.org\/10.1002\/mrc.1260251118<\/a>\n\n<\/li>\n
  4. H. Rzepa, \"The mechanism of nitrosobenzene dimerisation\", 2025. https:\/\/doi.org\/10.14469\/hpc\/15278<\/a>\n\n<\/li>\n
  5. H. Rzepa, \"Mechanism of the Masamune-Bergman reaction. Part 4. Why was the DFT energy barrier too high for the Calicheamicin reaction?\", 2024. https:\/\/doi.org\/10.59350\/k4340-t6971<\/a>\n\n<\/li>\n
  6. R. Hoffmann, R. Gleiter, and F.B. Mallory, \"Non-least-motion potential surfaces. Dimerization of methylenes and nitroso compounds\", Journal of the American Chemical Society<\/i>, vol. 92, pp. 1460-1466, 1970. https:\/\/doi.org\/10.1021\/ja00709a002<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    I am in the process of revising my annual lecture to first year university students on the topic of “curly arrows”. I like to start my story in 1924, when Robert Robinson published the very first example as an illustration of why nitrosobenzene undergoes electrophilic bromination in the para position of the benzene ring. I […]<\/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":"federated","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":[],"ppma_author":[2661],"class_list":["post-28849","post","type-post","status-publish","format-standard","hentry","category-reaction-mechanism-2"],"yoast_head":"\nMechanism of the dimerisation of Nitrosobenzene. - 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=28849\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Mechanism of the dimerisation of Nitrosobenzene. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"I am in the process of revising my annual lecture to first year university students on the topic of “curly arrows”. 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One of the properties of this molecule is that the equilibrium between the monomer and dimer can be detected,\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":[]},{"id":22404,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22404","url_meta":{"origin":28849,"position":1},"title":"The first ever curly arrows. Revisited with some crystal structure mining.","author":"Henry Rzepa","date":"May 27, 2020","format":false,"excerpt":"With the current global lockdown, and students along with everyone else staying at home, I have noticed some old posts of mine are getting more attention than normal. One of these is an analysis I did in 2012 of Robinson's original curly arrow illustration. That and the fact that I\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":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2020\/05\/Screenshot-52-1024x622.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":30890,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30890","url_meta":{"origin":28849,"position":2},"title":"Valence bond representations with +ve charges on adjacent atoms? An odd titanium complex analysed.","author":"Henry Rzepa","date":"March 8, 2026","format":false,"excerpt":"A few posts back, I contemplated the curly arrows appropriate for the formation of nitrosobenzene dimer from nitrosobenzene, and commented on the odd nature of the N=N double bond formed in this process.. Odd, because the valence bond representation of this dimer (1 below) has two formally positive adjacent nitrogen\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":[]},{"id":12115,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12115","url_meta":{"origin":28849,"position":3},"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":22578,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22578","url_meta":{"origin":28849,"position":4},"title":"The Willgerodt-Kindler Reaction: mechanistic reality check 2.","author":"Henry Rzepa","date":"August 14, 2020","format":false,"excerpt":"Continuing an exploration of the mechanism of this reaction, an alternative new mechanism was suggested in 1989 (having been first submitted to the journal ten years earlier!). Here the key intermediate proposed is a thiirenium cation (labelled 8 in the article) and labelled\u00a0Int3 below. The model chosen is the same\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":29410,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=29410","url_meta":{"origin":28849,"position":5},"title":"Energy decomposition analysis of hindered alkenes: Tetra t-butylethene and others.","author":"Henry Rzepa","date":"August 13, 2025","format":false,"excerpt":"In the previous post, I introduced the N=N double bond in nitrosobenzene dimer, arguing that even though it was a formal double bond, its bond dissociation energy made it nonetheless a very weak double bond! This was backed up by a technique known as energy decomposition analysis or EDA. Here\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\/28849","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=28849"}],"version-history":[{"count":49,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/28849\/revisions"}],"predecessor-version":[{"id":30262,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/28849\/revisions\/30262"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=28849"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=28849"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=28849"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=28849"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}