{"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":"2015-08-26T14:17:05","modified_gmt":"2015-08-26T13:17:05","slug":"mesomeric-resonance-in-substituted-benzenes-a-crystallographic-reality-check","status":"publish","type":"post","link":"https:\/\/www.rzepa.net\/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>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).<\/p>\n<p><img decoding=\"async\" src=\"https:\/\/www.rzepa.net\/blog\/wp-content\/uploads\/2015\/08\/benzenes.svg\" alt=\"Scheme\" width=\"400\" \/><\/p>\n<p>In 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\u00a0search is applied (R &lt; 5%, no disorder, no errors) and qualified with the following:<\/p>\n<ol>\n<li>The amino has three bonds, and can bear either H, or 4-bonded carbon only.<\/li>\n<li>R on the ring can be either H or C.<\/li>\n<li>Three distances are defined.<\/li>\n<\/ol>\n<p><img data-recalc-dims=\"1\" decoding=\"async\" src=\"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2015\/08\/search1.jpg?w=440\" alt=\"Scheme\"  \/><\/p>\n<p>The results of a search are shown below; the hotspot shows the C-C(ortho) distance is close to 1.40\u00c5, whilst the corresponding value for C(ortho)-C(meta) is 1.38\u00c5, a contraction of ~0.02\u00c5. The\u00a0contraction is smaller for phenols (~0.01\u00c5).<\/p>\n<p><img data-recalc-dims=\"1\" decoding=\"async\" src=\"https:\/\/i0.wp.com\/www.rzepa.net\/blog\/wp-content\/uploads\/2015\/08\/aminophenyl.jpg?w=440&#038;ssl=1\" alt=\"Scheme\"  \/><\/p>\n<p>The C(ortho)-C(meta) vs C(meta)-C(para) amino plot shows a cluster of hotspots for which the former (1.38\u00c5) is \u00a0shorter than the latter (~1.39\u00c5) but the effect is less clear cut as the distance from the substituent increases.<\/p>\n<p><img data-recalc-dims=\"1\" decoding=\"async\" src=\"https:\/\/i0.wp.com\/www.rzepa.net\/blog\/wp-content\/uploads\/2015\/08\/amino-m-p-phenyl.jpg?w=440&#038;ssl=1\" alt=\"Scheme\"  \/><\/p>\n<p>For an electron withdrawing cyano substituent, C(ipso)-C(ortho) at 1.395\u00c5 is longer than C(ortho)-C(meta) at 1.385\u00c5, although the difference seems smaller than for the amino substituent. The (ortho)-C(meta) to C(meta)-C(para) comparison is similar.<\/p>\n<p><img data-recalc-dims=\"1\" decoding=\"async\" src=\"https:\/\/i0.wp.com\/www.rzepa.net\/blog\/wp-content\/uploads\/2015\/08\/cyanophenyl.jpg?w=440&#038;ssl=1\" alt=\"Scheme\"  \/><\/p>\n<p><img data-recalc-dims=\"1\" decoding=\"async\" src=\"https:\/\/i0.wp.com\/www.rzepa.net\/blog\/wp-content\/uploads\/2015\/08\/cyano-m-p-phenyl.jpg?w=440&#038;ssl=1\" alt=\"Scheme\"  \/><\/p>\n<p>These searches take but a few minutes to perform, and do serve as a reality check on the oft-seen mesomeric \u03c0-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,"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":[3,1754],"tags":[510,1565,1578,1594,1612,1509,1616],"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"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p1gPyz-3LK","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/14492","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=14492"}],"version-history":[{"count":0,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/14492\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=14492"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=14492"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=14492"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}