{"id":14492,"date":"2015-08-26T14:17:05","date_gmt":"2015-08-26T13:17:05","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=14492"},"modified":"2016-06-10T08:51:59","modified_gmt":"2016-06-10T07:51:59","slug":"mesomeric-resonance-in-substituted-benzenes-a-crystallographic-reality-check","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14492","title":{"rendered":"Mesomeric resonance in substituted benzenes: a crystallographic reality check."},"content":{"rendered":"<div class=\"kcite-section\" kcite-section-id=\"14492\">\n<p>\n\tPreviously, I showed how conjugation in dienes and diaryls can be visualised by inspecting bond lengths as a function of torsions. Here is another illustration, this time of the mesomeric resonance on a benzene ring induced by an electron donating substituent (an amino group) or an electron withdrawing substituent (cyano).\n<\/p>\n<p>\n\t<img decoding=\"async\" alt=\"Scheme\" src=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2015\/08\/benzenes.svg\" width=\"400\" \/>\n<\/p>\n<p>\n\tIn both cases, you can see this resonance showing as a lengthening of the C(ipso)-C(ortho) and C(meta)-C(para) bonds, and a contracting of the C(ortho)-C(meta) bonds. Does this reflect in the measured structures? The usual&nbsp;search is applied (R &lt; 5%, no disorder, no errors) and qualified with the following:\n<\/p>\n<ol>\n<li>\n\t\tThe amino has three bonds, and can bear either H, or 4-bonded carbon only.\n\t<\/li>\n<li>\n\t\tR on the ring can be either H or C.\n\t<\/li>\n<li>\n\t\tThree distances are defined.\n\t<\/li>\n<\/ol>\n<p>\n\t<img decoding=\"async\" alt=\"Scheme\" src=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2015\/08\/search1.jpg\" width=\"440\" \/>\n<\/p>\n<p>\n\tThe results of a search are shown below; the hotspot shows the C-C(ortho) distance is close to 1.40&Aring;, whilst the corresponding value for C(ortho)-C(meta) is 1.38&Aring;, a contraction of ~0.02&Aring;. The&nbsp;contraction is smaller for phenols (~0.01&Aring;).\n<\/p>\n<p>\n\t<img decoding=\"async\" alt=\"Scheme\" src=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2015\/08\/aminophenyl.jpg\" width=\"440\" \/>\n<\/p>\n<p>\n\tThe C(ortho)-C(meta) vs C(meta)-C(para) amino plot shows a cluster of hotspots for which the former (1.38&Aring;) is &nbsp;shorter than the latter (~1.39&Aring;) but the effect is less clear cut as the distance from the substituent increases.\n<\/p>\n<p>\n\t<img decoding=\"async\" alt=\"Scheme\" src=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2015\/08\/amino-m-p-phenyl.jpg\" width=\"440\" \/>\n<\/p>\n<p>\n\tFor an electron withdrawing cyano substituent, C(ipso)-C(ortho) at 1.395&Aring; is longer than C(ortho)-C(meta) at 1.385&Aring;, although the difference seems smaller than for the amino substituent. The (ortho)-C(meta) to C(meta)-C(para) comparison is similar.\n<\/p>\n<p>\n\t<img decoding=\"async\" alt=\"Scheme\" src=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2015\/08\/cyanophenyl.jpg\" width=\"440\" \/>\n<\/p>\n<p>\n\t<img decoding=\"async\" alt=\"Scheme\" src=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2015\/08\/cyano-m-p-phenyl.jpg\" width=\"440\" \/>\n<\/p>\n<p>\n\tThese searches take but a few minutes to perform, and do serve as a reality check on the oft-seen mesomeric &pi;-resonance shown in all organic text books.<\/p>\n<!-- kcite active, but no citations found -->\n<\/div> <!-- kcite-section 14492 -->","protected":false},"excerpt":{"rendered":"<p>Previously, I showed how conjugation in dienes and diaryls can be visualised by inspecting bond lengths as a function of torsions. Here is another illustration, this time of the mesomeric resonance on a benzene ring induced by an electron donating substituent (an amino group) or an electron withdrawing substituent (cyano). In both cases, you can [&hellip;]<\/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":[2,1745],"tags":[557,1563,1566,1564,1565,1443,1567],"ppma_author":[2661],"class_list":["post-14492","post","type-post","status-publish","format-standard","hentry","category-chemical-it","category-crystal_structure_mining","tag-chemical-bonding","tag-chemical-nomenclature","tag-hammett-equation","tag-mesomeric-effect","tag-substituent","tag-substitution-reactions","tag-usual-search"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.5 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Mesomeric resonance in substituted benzenes: a crystallographic reality check. - 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=14492\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Mesomeric resonance in substituted benzenes: a crystallographic reality check. - Henry Rzepa&#039;s Blog\" \/>\n<meta property=\"og:description\" content=\"Previously, I showed how conjugation in dienes and diaryls can be visualised by inspecting bond lengths as a function of torsions. Here is another illustration, this time of the mesomeric resonance on a benzene ring induced by an electron donating substituent (an amino group) or an electron withdrawing substituent (cyano). In both cases, you can [&hellip;]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14492\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa&#039;s Blog\" \/>\n<meta property=\"article:published_time\" content=\"2015-08-26T13:17:05+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2016-06-10T07:51:59+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2015\/08\/benzenes.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":"Mesomeric resonance in substituted benzenes: a crystallographic reality check. - 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=14492","og_locale":"en_GB","og_type":"article","og_title":"Mesomeric resonance in substituted benzenes: a crystallographic reality check. - Henry Rzepa&#039;s Blog","og_description":"Previously, I showed how conjugation in dienes and diaryls can be visualised by inspecting bond lengths as a function of torsions. Here is another illustration, this time of the mesomeric resonance on a benzene ring induced by an electron donating substituent (an amino group) or an electron withdrawing substituent (cyano). In both cases, you can [&hellip;]","og_url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14492","og_site_name":"Henry Rzepa&#039;s Blog","article_published_time":"2015-08-26T13:17:05+00:00","article_modified_time":"2016-06-10T07:51:59+00:00","og_image":[{"url":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2015\/08\/benzenes.svg","type":"","width":"","height":""}],"author":"Henry Rzepa","twitter_card":"summary_large_image","twitter_misc":{"Written by":"Henry Rzepa","Estimated reading time":"1 minute"},"schema":{"@context":"https:\/\/schema.org","@graph":[{"@type":"Article","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14492#article","isPartOf":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14492"},"author":{"name":"Henry Rzepa","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281"},"headline":"Mesomeric resonance in substituted benzenes: a crystallographic reality check.","datePublished":"2015-08-26T13:17:05+00:00","dateModified":"2016-06-10T07:51:59+00:00","mainEntityOfPage":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14492"},"wordCount":301,"commentCount":0,"image":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14492#primaryimage"},"thumbnailUrl":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2015\/08\/benzenes.svg","keywords":["chemical bonding","Chemical nomenclature","Hammett equation","Mesomeric effect","Substituent","Substitution reactions","usual search"],"articleSection":["Chemical IT","crystal_structure_mining"],"inLanguage":"en-GB","potentialAction":[{"@type":"CommentAction","name":"Comment","target":["https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14492#respond"]}]},{"@type":"WebPage","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14492","url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14492","name":"Mesomeric resonance in substituted benzenes: a crystallographic reality check. - 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Introductory organic chemistry tells us that electron donating substituents promote the ortho and para positions over the meta. Here I try to\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":16573,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16573","url_meta":{"origin":14492,"position":1},"title":"How does an OH or NH group approach an aromatic ring to hydrogen bond with its \u03c0-face?","author":"Henry Rzepa","date":"June 22, 2016","format":false,"excerpt":"I previously used data mining of crystal structures to explore the directing influence of substituents on aromatic and heteroaromatic rings. Here\u00a0I explore, quite literally, a different angle to the hydrogen bonding interactions between a benzene ring and\u00a0OH or NH groups. I start by defining a benzene ring with a centroid.\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":"aromatic-pi-query","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2016\/06\/aromatic-pi-query-1-e1466580253270.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":17122,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17122","url_meta":{"origin":14492,"position":2},"title":"Long C=C bonds.","author":"Henry Rzepa","date":"December 1, 2016","format":false,"excerpt":"Following on from a search for long C-C bonds, here is the same repeated for C=C double bonds. The query restricts the search to each carbon having just two non-metallic substituents. To avoid conjugation with these, they each are 4-coordinated; the carbons themselves are three-coordinated. Further constraints are the usual\u2026","rel":"","context":"In &quot;crystal_structure_mining&quot;","block_context":{"text":"crystal_structure_mining","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1745"},"img":{"alt_text":"sq","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2016\/12\/sq-1024x415.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":29121,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=29121","url_meta":{"origin":14492,"position":3},"title":"Why an Electron-Withdrawing Group is an o, m-Director rather than m-Director in Electrophilic Aromatic Substitution: The example of CN vs NC.","author":"Henry Rzepa","date":"July 22, 2025","format":false,"excerpt":"In the previous post I followed up on an article published on the theme \"Physical Organic Chemistry: Never Out of Style\". Paul Rablen presented the case that the amount of o (ortho) product in electrophilic substitution of a phenyl ring bearing an EWG (electron withdrawing group) is often large enough\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":"","width":0,"height":0},"classes":[]},{"id":19550,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=19550","url_meta":{"origin":14492,"position":4},"title":"A record polarity for a neutral compound?","author":"Henry Rzepa","date":"April 13, 2018","format":false,"excerpt":"In several posts a year or so ago I considered various suggestions for the most polar neutral molecules, as measured by the dipole moment. A record had been claimed for a synthesized molecule of ~14.1\u00b10.7D. I pushed this to a calculated 21.7D for an admittedly hypothetical and unsynthesized molecule. Here\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":14395,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14395","url_meta":{"origin":14492,"position":5},"title":"The structure of naphthalene: 1890-1925, and a modern twist.","author":"Henry Rzepa","date":"July 18, 2015","format":false,"excerpt":"This is a little historical essay into the electronic structure of naphthalene, presented as key dates (and also collects comments made which were appended to other posts). 1890: Henry Armstrong presents the following structure of naphthalene. Three words need translation into modern usage. Where he uses the word nuclei the\u2026","rel":"","context":"In &quot;Historical&quot;","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\/14492","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=14492"}],"version-history":[{"count":9,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/14492\/revisions"}],"predecessor-version":[{"id":16514,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/14492\/revisions\/16514"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=14492"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=14492"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=14492"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=14492"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}