{"id":7258,"date":"2012-07-20T22:16:17","date_gmt":"2012-07-20T21:16:17","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=7258"},"modified":"2018-02-05T14:23:15","modified_gmt":"2018-02-05T14:23:15","slug":"the-first-curly-arrows-lead-to-this","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7258","title":{"rendered":"The first curly arrows…lead to this?"},"content":{"rendered":"
\n

Little did I imagine, when I discovered<\/a> the original example of using curly arrows to express mechanism, that the molecule described there might be rather too anarchic to use in my introductory tutorials on organic chemistry. Why? It simply breaks the (it has to be said to some extent informal) rules<\/a>! Consider the dimerisation of nitrosomethane (in fact a well-known <\/a>equilibrium).<\/p>\n

\"\"<\/p>\n

Curly arrow pushing for this reaction results in two arrows heading in effect towards the same bond, at the same time! The result has two adjacent nitrogen atoms each with (formal) positive charges on them, and the N…N bond order goes from zero to two in a single step. Surely this cannot be allowed to happen as shown above? Well, the IRC (intrinsic reaction coordinate) computed<\/a> from a \u03c9B97XD\/6-311G(d,p) calculation is shown below. The barrier is small, the profile uneventful. I conclude that if I ever see a student exam script showing two curly arrows heading directly towards<\/a> the same bond at the same time, it might even deserve to be graded correct!\u00a0<\/p>\n

\"\"<\/p>\n

\"\"
\n \"\"<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

Little did I imagine, when I discovered the original example of using curly arrows to express mechanism, that the molecule described there might be rather too anarchic to use in my introductory tutorials on organic chemistry. Why? It simply breaks the (it has to be said to some extent informal) rules! Consider the dimerisation of […]<\/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":"","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":[2327],"tags":[879,843,373],"ppma_author":[2661],"class_list":["post-7258","post","type-post","status-publish","format-standard","hentry","category-curl-arrows","tag-dimer","tag-reaction-mechanism","tag-tutorial-material"],"yoast_head":"\nThe first curly arrows...lead to this? - 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=7258\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The first curly arrows...lead to this? - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"Little did I imagine, when I discovered the original example of using curly arrows to express mechanism, that the molecule described there might be rather too anarchic to use in my introductory tutorials on organic chemistry. Why? It simply breaks the (it has to be said to some extent informal) rules! Consider the dimerisation of […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7258\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2012-07-20T21:16:17+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2018-02-05T14:23:15+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/07\/dim_nitrosomethane.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=\"1 minute\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"The first curly arrows...lead to this? - Henry Rzepa's Blog","robots":{"index":"index","follow":"follow","max-snippet":"max-snippet:-1","max-image-preview":"max-image-preview:large","max-video-preview":"max-video-preview:-1"},"canonical":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7258","og_locale":"en_GB","og_type":"article","og_title":"The first curly arrows...lead to this? - Henry Rzepa's Blog","og_description":"Little did I imagine, when I discovered the original example of using curly arrows to express mechanism, that the molecule described there might be rather too anarchic to use in my introductory tutorials on organic chemistry. 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These are so-called high spin systems with unpaired electrons and the mechanism of C-H activation involves both double\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.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2020\/06\/TS.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":26523,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=26523","url_meta":{"origin":7258,"position":3},"title":"More examples of “double-headed” curly arrows: S and C Nucleophiles attacking acetyl chloride","author":"Henry Rzepa","date":"October 12, 2023","format":false,"excerpt":"In an earlier post on this topic,\u2021 I described how the curly-arrows describing the mechanism of a nucleophilic addition at a carbonyl group choreograph in two distinct ways, as seen in red or blue below. The arrows in red can be described as firstly addition to the carbonyl group to\u2026","rel":"","context":"In \"Interesting chemistry\"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=interesting-chemistry"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":28849,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=28849","url_meta":{"origin":7258,"position":4},"title":"Mechanism of the dimerisation of Nitrosobenzene.","author":"Henry Rzepa","date":"June 14, 2025","format":false,"excerpt":"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\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":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2025\/06\/trans.gif?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2025\/06\/trans.gif?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2025\/06\/trans.gif?resize=525%2C300&ssl=1 1.5x"},"classes":[]},{"id":20464,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=20464","url_meta":{"origin":7258,"position":5},"title":"The Graham reaction: Deciding upon a reasonable mechanism and curly arrow representation.","author":"Henry Rzepa","date":"February 18, 2019","format":false,"excerpt":"Students learning organic chemistry are often asked in examinations and tutorials to devise the mechanisms (as represented by curly arrows) for the core corpus of important reactions, with the purpose of learning skills that allow them to go on to improvise mechanisms for new reactions. A common question asked by\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":"","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\/7258","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=7258"}],"version-history":[{"count":10,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7258\/revisions"}],"predecessor-version":[{"id":19406,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7258\/revisions\/19406"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=7258"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=7258"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=7258"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=7258"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}