{"id":27114,"date":"2024-06-14T16:46:18","date_gmt":"2024-06-14T15:46:18","guid":{"rendered":"https:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=27114"},"modified":"2024-06-16T09:49:55","modified_gmt":"2024-06-16T08:49:55","slug":"the-100th-anniversary-year-of-curly-arrows","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=27114","title":{"rendered":"The 100th Anniversary year of Curly Arrows."},"content":{"rendered":"
\n

Chemists now use the term “curly arrows” as a language to describe the electronic rearrangements that occur when a (predominately organic) molecule transforms to another – the so called chemical reaction. It is also used to infer, via<\/em> valence bond or resonance theory, what the mechanistic implications of that reaction are. It was in this latter context that the very first such usage occured in 1924[1]<\/a><\/span> taking the form of a letter by Robert Robinson to the secretary of the Chemical Society and “read” on December 18th 1924. The following diagram was included:<\/p>\n

\"\"

First curly arrows<\/p><\/div>\n

I have commented previously on this diagram[cite10.59350\/qqwk3-dgj13[\/cite] and will not discuss it further here. To commemorate the 100th anniversary of their invention, I include shots of two “modern” sets of curly arrows, taken from a lecture I give to university students at the end of their first university year.<\/p>\n

    \n
  1. The first was a new take on the peracid epoxidation of an alkene[2]<\/a><\/span> in which quantum mechanical calculations have revealed that the classic take on the curly arrow mechanism for this reaction can be split into two sets, five<\/b> for the first stage of the reaction up to the transition state and two<\/b> for the final stage<\/li>\n<\/ol>\n
    \"\"

    Four becomes seven<\/p><\/div>\n

      \n
    1. The second was also discussed here[3]<\/a><\/span> and involves what is arguably a new type of arrow to join the existing stable – the dashed arrow (in red below). This electron transfer arrow can take place over long distances (15\u00c5 or more) and adds the concept that an arrow can have the properties of an (approximate) length as well as direction, start+points and perhaps even “curlyness”.<\/li>\n<\/ol>\n
      \"\"

      Proton coupled electron transfers<\/p><\/div>\n

      As a “language” describing mechanism and reactivity in molecules, curly arrows are still in common use, but as chemistry itself evolves into new areas, will curly arrows themselves morph into new forms, or will their use gradually decline?<\/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, \"Experimental evidence for "hidden intermediates"? Epoxidation of ethene by peracid.\", 2013. https:\/\/doi.org\/10.59350\/fdy9j-9fp48<\/a>\n\n<\/li>\n
      3. H. Rzepa, \"Curly arrows in the 21st Century. Proton-coupled electron transfers.\", 2020. https:\/\/doi.org\/10.59350\/rj90z-mxh96<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

        Chemists now use the term “curly arrows” as a language to describe the electronic rearrangements that occur when a (predominately organic) molecule transforms to another – the so called chemical reaction. It is also used to infer, via valence bond or resonance theory, what the mechanistic implications of that reaction are. It was in this […]<\/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],"tags":[],"ppma_author":[2661],"class_list":["post-27114","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry"],"yoast_head":"\nThe 100th Anniversary year of Curly Arrows. - 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=27114\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The 100th Anniversary year of Curly Arrows. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"Chemists now use the term “curly arrows” as a language to describe the electronic rearrangements that occur when a (predominately organic) molecule transforms to another – the so called chemical reaction. It is also used to infer, via valence bond or resonance theory, what the mechanistic implications of that reaction are. It was in this […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=27114\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2024-06-14T15:46:18+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2024-06-16T08:49:55+00:00\" \/>\n<meta property=\"og:image\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2024\/06\/Screenshot-334.jpg\" \/>\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=\"3 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"The 100th Anniversary year of Curly Arrows. - 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=27114","og_locale":"en_GB","og_type":"article","og_title":"The 100th Anniversary year of Curly Arrows. - Henry Rzepa's Blog","og_description":"Chemists now use the term “curly arrows” as a language to describe the electronic rearrangements that occur when a (predominately organic) molecule transforms to another – the so called chemical reaction. 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Those of Ingold in 1926.","author":"Henry Rzepa","date":"August 22, 2018","format":false,"excerpt":"In 2012, I wrote a story of the first ever reaction curly arrows, attributed to Robert Robinson in 1924. At the time there was a great rivalry between him and another UK chemist, Christopher Ingold, with the latter also asserting his claim for their use. As part of the move\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":28849,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=28849","url_meta":{"origin":27114,"position":1},"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":7234,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7234","url_meta":{"origin":27114,"position":2},"title":"The first ever curly arrows.","author":"Henry Rzepa","date":"July 20, 2012","format":false,"excerpt":"I was first taught curly arrow pushing in 1968, and have myself taught it to many a generation of student since. But the other day, I learnt something new. Nick Greeves was kind enough to send me this link\u2021to the origin of curly arrow pushing in organic chemistry, where the\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":[]},{"id":22445,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22445","url_meta":{"origin":27114,"position":3},"title":"Curly arrows in the 21st Century. Proton-coupled electron transfers.","author":"Henry Rzepa","date":"June 10, 2020","format":false,"excerpt":"One of the most fascinating and important articles dealing with curly arrows I have seen is that by Klein and Knizia on the topic of C-H bond activations using an iron catalyst. 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":11065,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11065","url_meta":{"origin":27114,"position":4},"title":"Experimental evidence for “hidden intermediates”? Epoxidation of ethene by peracid.","author":"Henry Rzepa","date":"August 25, 2013","format":false,"excerpt":"The concept of a \"hidden intermediate\" in a reaction pathway has been promoted by Dieter Cremer and much invoked on this blog. When I used this term in a recent article of ours, a referee tried to object, saying it was not in common use in chemistry. The term clearly\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":"peracid+alkene1","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/08\/peracid%2Balkene1.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":19807,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=19807","url_meta":{"origin":27114,"position":5},"title":"A Theoretical Method for Distinguishing X\u2010H Bond Activation Mechanisms.","author":"Henry Rzepa","date":"July 25, 2018","format":false,"excerpt":"Consider the four reactions. The first two are taught in introductory organic chemistry as (a) a proton transfer,\u00a0often abbreviated PT,\u00a0from X to B (a base) and (b) a hydride transfer from X to A (an acid). The third example is taught as a hydrogen atom transfer or HAT from X\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\/07\/024-1024x350.jpg?resize=350%2C200&ssl=1","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\/27114","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=27114"}],"version-history":[{"count":19,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/27114\/revisions"}],"predecessor-version":[{"id":27151,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/27114\/revisions\/27151"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=27114"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=27114"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=27114"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=27114"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}