{"id":26523,"date":"2023-10-12T12:57:24","date_gmt":"2023-10-12T11:57:24","guid":{"rendered":"https:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=26523"},"modified":"2023-10-12T13:03:31","modified_gmt":"2023-10-12T12:03:31","slug":"more-examples-of-double-headed-curly-arrows-s-and-c-nucleophiles-attacking-acetyl-chloride","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=26523","title":{"rendered":"More examples of &#8220;double-headed&#8221; curly arrows: S and C Nucleophiles attacking acetyl chloride"},"content":{"rendered":"<div class=\"kcite-section\" kcite-section-id=\"26523\">\n<p>In an earlier post on this topic,<span id=\"cite_ITEM-26523-0\" name=\"citation\"><a href=\"#ITEM-26523-0\">[1]<\/a><\/span><sup>\u2021<\/sup> 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 form either a transient intermediate (a two-step process) or instead a formal transition state state as a concerted single-step mechanism. The blue arrows do the reverse; firstly elimination and then followed by addition. I will use the shorthand <strong>AE<\/strong> for the first type and <strong>EA<\/strong> for the second type. Here I explore some more nucleophiles to see which of these two mechanisms they follow. Data for these results can be found at <a href=\"https:\/\/doi.org\/10.14469\/hpc\/13171\">10.14469\/hpc\/13171<\/a><br \/>\n<a href=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/10\/double-headed4.svg\"><img decoding=\"async\" class=\"aligncenter size-full wp-image-26546\" src=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/10\/double-headed4.svg\" alt=\"\" width=\"450\" \/><\/a><strong>N- carbon ylid:<\/strong> This is a very facile (low-barrier) reaction with a C-O bond length response that initially increases steeply, followed by a more modest decline and hence corresponds to an <strong>AE<\/strong> mechanism.<\/p>\n<p><img decoding=\"async\" class=\"aligncenter size-medium wp-image-26534\" src=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/10\/acetyl-chlorine-C-Nuc_tot_ener.gif\" alt=\"\" width=\"540\" \/><\/p>\n<p><img decoding=\"async\" class=\"aligncenter size-medium wp-image-26530\" src=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/10\/acetyl-chlorine-C-Nuc_tot_ener.svg\" alt=\"\" width=\"450\" \/><\/p>\n<p><img decoding=\"async\" class=\"aligncenter size-medium wp-image-26535\" src=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/10\/acetyl-chlorine-add-C-nuc-DM.svg\" alt=\"\" width=\"450\" \/><\/p>\n<p><img decoding=\"async\" class=\"aligncenter size-medium wp-image-26536\" src=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/10\/acetyl-chlorine-C-Nuc-BL.svg\" alt=\"\" width=\"450\" \/><\/p>\n<p><strong>P carbon-Ylid:<\/strong>\u00a0 Essentially identical to the previous example, and again an <strong>AE<\/strong> mechanism.<\/p>\n<p><img decoding=\"async\" class=\"aligncenter size-medium wp-image-26527\" src=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/10\/acetyl-chloride-P-ylid.gif\" alt=\"\" width=\"540\" \/><\/p>\n<p><a href=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/10\/acetyl-chloride-P-ylid_tot_ener.svg\"><img decoding=\"async\" class=\"aligncenter size-full wp-image-26524\" src=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/10\/acetyl-chloride-P-ylid_tot_ener.svg\" alt=\"\" width=\"450\" \/><\/a><\/p>\n<p><img decoding=\"async\" class=\"aligncenter size-medium wp-image-26525\" src=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/10\/acetyl-chloride-P-ylid_DM.svg\" alt=\"\" width=\"450\" \/><\/p>\n<p><img decoding=\"async\" class=\"aligncenter size-medium wp-image-26526\" src=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/10\/acetyl-chloride-P-ylid_BL12.svg\" alt=\"\" width=\"450\" \/><\/p>\n<p><strong>S carbon-ylid: <\/strong>Again, an AE mechanism.<\/p>\n<p><img decoding=\"async\" class=\"aligncenter size-medium wp-image-26533\" src=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/10\/acetyl-chloride-S-ylid.gif\" alt=\"\" width=\"540\" \/><\/p>\n<p><img decoding=\"async\" class=\"aligncenter size-medium wp-image-26529\" src=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/10\/acetyl-chloride-S-ylid_tot_ener.svg\" alt=\"\" width=\"450\" \/><\/p>\n<p><img decoding=\"async\" class=\"aligncenter size-medium wp-image-26531\" src=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/10\/acetyl-chloride-S-ylid_DM.svg\" alt=\"\" width=\"450\" \/><\/p>\n<p><img decoding=\"async\" class=\"aligncenter size-medium wp-image-26532\" src=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/10\/acetyl-chloride-S-ylid_BL12.svg\" alt=\"\" width=\"450\" \/><\/p>\n<p><strong>S-nucleophile:\u00a0<\/strong> This one is different, showing a larger barrier and initial small decrease in the C-O length followed by a larger increase. This one is an <strong>EA<\/strong> mechanism.<\/p>\n<p><img decoding=\"async\" class=\"aligncenter size-medium wp-image-26537\" src=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/10\/acetyl-chloride-MeSH_tot_ener.gif\" alt=\"\" width=\"540\" \/><\/p>\n<p><img decoding=\"async\" class=\"aligncenter size-medium wp-image-26528\" src=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/10\/acetyl-chloride-MeSH_tot_ener.svg\" alt=\"\" width=\"450\" \/><\/p>\n<p><img decoding=\"async\" class=\"aligncenter size-medium wp-image-26538\" src=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/10\/acetyl-chloride-MeSH_DM.svg\" alt=\"\" width=\"450\" \/><\/p>\n<p><img decoding=\"async\" class=\"aligncenter size-medium wp-image-26539\" src=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/10\/acetyl-chloride-MeSH_BL12.svg\" alt=\"\" width=\"450\" \/><\/p>\n<p>As I noted previously, it would be useful to have two double headed curly arrows available in palletes of these; <strong style=\"color: red;\">&lt;\u2014&gt;<\/strong> (AE) and <strong style=\"color: blue;\">&gt;\u2014&lt;<\/strong> (EA) to illustrate the difference between the two mechanistic types.<\/p>\n<hr \/>\n<p><sup>\u2021<\/sup>This is the first instance where I cite a blog using a CrossRef DOI generated for it. Previous such citations used a DataCite DOI, which the bibliographic software used here to add them to the post (Kcite) does not support.<\/p>\n<hr \/>\n<h2>References<\/h2>\n    <ol class=\"kcite-bibliography csl-bib-body\"><li id=\"ITEM-26523-0\">H. Rzepa, \"The &quot;double-headed&quot; curly arrow as used in mechanistic representations.\", 2023. <a href=\"https:\/\/doi.org\/10.59350\/f00wf-5tq46\">https:\/\/doi.org\/10.59350\/f00wf-5tq46<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> <!-- kcite-section 26523 -->","protected":false},"excerpt":{"rendered":"<p>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 form either a transient intermediate [&hellip;]<\/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_feature_clip_id":0,"_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},"jetpack_post_was_ever_published":false},"categories":[],"tags":[2648],"ppma_author":[2661],"class_list":["post-26523","post","type-post","status-publish","format-standard","hentry","tag-interesting-chemistry"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.8 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>More examples of &quot;double-headed&quot; curly arrows: S and C Nucleophiles attacking acetyl chloride - Henry Rzepa&#039;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=26523\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"More examples of &quot;double-headed&quot; curly arrows: S and C Nucleophiles attacking acetyl chloride - Henry Rzepa&#039;s Blog\" \/>\n<meta property=\"og:description\" content=\"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 form either a transient intermediate [&hellip;]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=26523\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa&#039;s Blog\" \/>\n<meta property=\"article:published_time\" content=\"2023-10-12T11:57:24+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2023-10-12T12:03:31+00:00\" \/>\n<meta property=\"og:image\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/10\/double-headed4.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=\"5 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"More examples of \"double-headed\" curly arrows: S and C Nucleophiles attacking acetyl chloride - Henry Rzepa&#039;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=26523","og_locale":"en_GB","og_type":"article","og_title":"More examples of \"double-headed\" curly arrows: S and C Nucleophiles attacking acetyl chloride - Henry Rzepa&#039;s Blog","og_description":"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. 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I am now going to explore the consequences of changing one of the actors by adding\u2026","rel":"","context":"In &quot;Curly arrows&quot;","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":26340,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=26340","url_meta":{"origin":26523,"position":1},"title":"The &#8220;double-headed&#8221; curly arrow as used in mechanistic representations.","author":"Henry Rzepa","date":"August 29, 2023","format":false,"excerpt":"The schematic representation of a chemical reaction mechanism is often drawn using a palette of arrows connecting or annotating the various molecular structures involved. These can be selected from a chemical arrows palette, taken for this purpose from the commonly used structure drawing program Chemdraw. Explanations of how to apply\u2026","rel":"","context":"In &quot;Curly arrows&quot;","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\/2023\/08\/IMG_3157-131x300.jpeg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":16361,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16361","url_meta":{"origin":26523,"position":2},"title":"What is the approach trajectory of enhanced (super?) nucleophiles towards a carbonyl group?","author":"Henry Rzepa","date":"May 11, 2016","format":false,"excerpt":"I have previously commented on the B\u00fcrgi\u2013Dunitz angle, this being the preferred approach trajectory of a nucleophile towards the electrophilic carbon of a carbonyl group. Some special types of nucleophile such as hydrazines (R2N-NR2) are supposed to have enhanced reactivity due to what might be described as\u00a0buttressing of adjacent lone\u2026","rel":"","context":"In &quot;Chemical IT&quot;","block_context":{"text":"Chemical IT","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":26272,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=26272","url_meta":{"origin":26523,"position":3},"title":"Pre-mechanism for the Swern Oxidation: formation of chlorodimethylsulfonium chloride.","author":"Henry Rzepa","date":"August 25, 2023","format":false,"excerpt":"The Swern oxidation is a class of \"activated\" dimethyl sulfoxide (DMSO) reaction in which the active species is a chlorodimethylsulfonium chloride salt. The mechanism of this transformation as shown in e.g. Wikipedia is illustrated below.\u2021 However, an interesting and important aspect of chemistry is not apparent in this schematic mechanism\u2026","rel":"","context":"In &quot;Curly arrows&quot;","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":27114,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=27114","url_meta":{"origin":26523,"position":4},"title":"The 100th Anniversary year of Curly Arrows.","author":"Henry Rzepa","date":"June 14, 2024","format":false,"excerpt":"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\u2026","rel":"","context":"In &quot;Interesting chemistry&quot;","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":26523,"position":5},"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 &quot;reaction mechanism&quot;","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":[]}],"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","author_category":"1","first_name":"Henry","last_name":"Rzepa","user_url":"https:\/\/orcid.org\/0000-0002-8635-8390","job_title":"","description":"Henry Rzepa is Emeritus Professor of Computational Chemistry at Imperial College London."}],"_links":{"self":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/26523","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=26523"}],"version-history":[{"count":11,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/26523\/revisions"}],"predecessor-version":[{"id":26558,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/26523\/revisions\/26558"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=26523"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=26523"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=26523"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=26523"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}