{"id":8540,"date":"2012-12-04T08:56:52","date_gmt":"2012-12-04T08:56:52","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=8540"},"modified":"2012-12-08T08:02:05","modified_gmt":"2012-12-08T08:02:05","slug":"the-mechanism-of-the-birch-reduction-part-3-reduction-of-benzene","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8540","title":{"rendered":"The mechanism of the Birch reduction. Part 3: reduction of benzene"},"content":{"rendered":"<div class=\"kcite-section\" kcite-section-id=\"8540\">\n<p>Birch reduction of benzene itself results in 1,4-cyclohexadiene rather than the more stable (conjugated) 1,3-cyclohexadiene. Why is this?<\/p>\n<p><img decoding=\"async\" class=\"aligncenter size-full wp-image-8541\" title=\"birch3\" src=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/12\/birch3.svg\" alt=\"\" width=\"450\" \/><\/p>\n<p>The mechanism, as elaborated in the previous <a href=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=8508\" target=\"_blank\">two posts<\/a>, involves a one-electron transfer from a sodium atom to form the radical anion, which is then protonated in a second step, and this is again reduced to form a pentadienyl anion in the penultimate step.<span id=\"cite_ITEM-8540-0\" name=\"citation\"><a href=\"#ITEM-8540-0\">[1]<\/a><\/span>\u00a0The question now becomes why does this anion protonate to give predominantly the less stable diene product? The answer involves the actual structure of this anion. A <a href=\"http:\/\/hdl.handle.net\/10.6084\/m9.figshare.101004\" target=\"_blank\">calculation<\/a> at the\u00a0\u03c9B97XD\/6-311+G(d,p)\/SCRF=acetonitrile level for the ion pair comprising the cyclohexadienyl anion and a Na(NH<sub>3<\/sub>)<sub>3<\/sub><sup>+<\/sup> counterion is shown below.<\/p>\n<div id=\"attachment_8544\" style=\"width: 289px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-8544\" class=\" wp-image-8544 \" title=\"birch-ip\" onclick=\"jmolInitialize('..\/Jmol\/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content\/uploads\/2012\/12\/benzene-ip.cml;measure 1 2;measure 4 5; ');\" src=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/12\/birch-ip.jpg\" alt=\"\" width=\"279\" height=\"295\" \/><p id=\"caption-attachment-8544\" class=\"wp-caption-text\">Structure of the cyclohexadienyl ion pair. Click for 3D.<\/p><\/div>\n<p>From this, it appears that the sodium cation is \u03b7<sup>2<\/sup> coordinated to each of two relatively localised double bonds (1.37\u00c5), resulting in the negative charge accumulating on just the one carbon (red arrow), this being the carbon that then exclusively receives a final proton. The highest energy (-0.115 au) natural bond orbital (NBO) also emerges as being located on this carbon (the next two highest energy NBOs only come in at -0.303 au, and reside on each of the localised alkene bonds).<\/p>\n<div id=\"attachment_8547\" style=\"width: 250px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-8547\" class=\" wp-image-8547 \" title=\"birch-ip-nbo\" onclick=\"jmolInitialize('..\/Jmol\/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content\/uploads\/2012\/12\/ip_mo42.cub.xyz;isosurface color orange blue wp-content\/uploads\/2012\/12\/ip_mo42.cub.jvxl translucent;zoom 80;');\" src=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/12\/birch-ip-nbo.jpg\" alt=\"\" width=\"240\" height=\"240\" \/><p id=\"caption-attachment-8547\" class=\"wp-caption-text\">The highest energy NBO orbital. Click for 3D.<\/p><\/div>\n<p>The molecular electrostatic potential in effect integrates over all the electrons (not just those in the highest orbital), resulting in a function that measures the attractiveness of any point to a proton (red). It too shows that the most attractive region (red) for a proton is again on this carbon.<\/p>\n<div id=\"attachment_8551\" style=\"width: 279px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-8551\" class=\" wp-image-8551 \" title=\"birch-ip-mep\" onclick=\"jmolInitialize('..\/Jmol\/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content\/uploads\/2012\/12\/ip_mo42.cub.xyz;isosurface color red green wp-content\/uploads\/2012\/12\/ip_esp.cub.jvxl translucent;zoom 80;');\" src=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/12\/birch-ip-mep.jpg\" alt=\"\" width=\"269\" height=\"274\" \/><p id=\"caption-attachment-8551\" class=\"wp-caption-text\">Molecular electrostatic potential. Click for 3D.<\/p><\/div>\n<p>There is even evidence from crystal structures that this sort of motif is possible. Thus the dianion of 1,4-diphenylbenzene (with two\u00a0Na(thf)<sub>3<\/sub><sup>+<\/sup> counter-ions) reveals<span id=\"cite_ITEM-8540-1\" name=\"citation\"><a href=\"#ITEM-8540-1\">[2]<\/a><\/span> this type of coordination. \u00a0The buckling seen in the above mono-anion is inhibited by the presence of cations on both sides of the di-anion, but the pattern of short\/long bonds seen above also manifests in the crystal structure.<\/p>\n<div id=\"attachment_8556\" style=\"width: 311px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-8556\" class=\"size-full wp-image-8556\" title=\"BIRCH-XRAY\" onclick=\"jmolInitialize('..\/Jmol\/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content\/uploads\/2012\/12\/BABKOE1.cif;measure 2 3;measure 28 26;');\" src=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/12\/BIRCH-XRAY1.jpg\" alt=\"\" width=\"301\" height=\"361\" \/><p id=\"caption-attachment-8556\" class=\"wp-caption-text\">Crystal model. Click for 3D.<\/p><\/div>\n<p>So the take home message is that the counter-ion (solvated sodium cations)\u00a0in the Birch reduction \u00a0of benzene itself may coordinate to the anionic intermediates in the reductive process, and the resulting geometry of this ion-pair determines the eventual product of protonation.<\/p>\n<h2>References<\/h2>\n    <ol class=\"kcite-bibliography csl-bib-body\"><li id=\"ITEM-8540-0\">H.E. Zimmerman, and P.A. Wang, \"Regioselectivity of the Birch reduction\", <i>Journal of the American Chemical Society<\/i>, vol. 112, pp. 1280-1281, 1990. <a href=\"https:\/\/doi.org\/10.1021\/ja00159a078\">https:\/\/doi.org\/10.1021\/ja00159a078<\/a>\n\n<\/li>\n<li id=\"ITEM-8540-1\">J.H. Noordik, H.M. Doesburg, and P.A.J. Prick, \"Structures of the sodium\u2013&lt;i&gt;p&lt;\/i&gt;-terphenyl ion pairs: disodium terphenylide\u2013tetrahydrofuran (1\/6) and disodium diterphenylide terphenyl\u20131,2-dimethoxyethane (1\/6)\", <i>Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry<\/i>, vol. 37, pp. 1659-1663, 1981. <a href=\"https:\/\/doi.org\/10.1107\/s0567740881006833\">https:\/\/doi.org\/10.1107\/s0567740881006833<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> <!-- kcite-section 8540 -->","protected":false},"excerpt":{"rendered":"<p>Birch reduction of benzene itself results in 1,4-cyclohexadiene rather than the more stable (conjugated) 1,3-cyclohexadiene. Why is this? The mechanism, as elaborated in the previous two posts, involves a one-electron transfer from a sodium atom to form the radical anion, which is then protonated in a second step, and this is again reduced to form [&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_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":[951,24,953,952,843,373],"ppma_author":[2661],"class_list":["post-8540","post","type-post","status-publish","format-standard","hentry","tag-birch-reduction","tag-energy","tag-eventual-product","tag-less-stable-diene-product","tag-reaction-mechanism","tag-tutorial-material"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.6 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>The mechanism of the Birch reduction. Part 3: reduction of benzene - 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=8540\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The mechanism of the Birch reduction. Part 3: reduction of benzene - Henry Rzepa&#039;s Blog\" \/>\n<meta property=\"og:description\" content=\"Birch reduction of benzene itself results in 1,4-cyclohexadiene rather than the more stable (conjugated) 1,3-cyclohexadiene. Why is this? The mechanism, as elaborated in the previous two posts, involves a one-electron transfer from a sodium atom to form the radical anion, which is then protonated in a second step, and this is again reduced to form [&hellip;]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8540\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa&#039;s Blog\" \/>\n<meta property=\"article:published_time\" content=\"2012-12-04T08:56:52+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2012-12-08T08:02:05+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/12\/birch3.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=\"2 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"The mechanism of the Birch reduction. 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Sequel to benzene reduction.","author":"Henry Rzepa","date":"December 5, 2012","format":false,"excerpt":"I noted briefly in discussing why Birch reduction of benzene gives 1,4-cyclohexadiene (diagram below) that the geometry of the end-stage pentadienyl anion was distorted in the presence of the sodium cation to favour this product. This distortion actually has some pedagogic value, and so I elaborate this here. The starting\u2026","rel":"","context":"In \"antiaromaticity\"","block_context":{"text":"antiaromaticity","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=antiaromaticity"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/12\/benzene-22.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":8508,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8508","url_meta":{"origin":8540,"position":1},"title":"The mechanism of the Birch reduction. Part 2: a transition state model.","author":"Henry Rzepa","date":"December 3, 2012","format":false,"excerpt":"I promised that the follow-up to on the topic of Birch reduction would focus on the proton transfer reaction between the radical anion of anisole and a proton source, as part of analysing whether the mechanistic pathway proceeds O or M. To add some context, Hammond's postulate\u00a0\u00a0states that \"the structure\u2026","rel":"","context":"In \"Birch reduction\"","block_context":{"text":"Birch reduction","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=birch-reduction"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/12\/birch-mts.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":8452,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8452","url_meta":{"origin":8540,"position":2},"title":"The mechanism of the Birch reduction. Part 1: reduction of anisole.","author":"Henry Rzepa","date":"December 1, 2012","format":false,"excerpt":"The Birch reduction is a classic method for partially reducing e.g. aryl ethers using electrons (from sodium dissolved in ammonia) as the reductant rather than e.g. dihydrogen. As happens occasionally in chemistry, a long debate broke out over the two alternative mechanisms labelled O (for ortho protonation of the initial\u2026","rel":"","context":"In \"dielectric\"","block_context":{"text":"dielectric","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=dielectric"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/12\/birch1.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":26272,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=26272","url_meta":{"origin":8540,"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":3723,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=3723","url_meta":{"origin":8540,"position":4},"title":"Chemicalizing a blog.","author":"Henry Rzepa","date":"March 30, 2011","format":false,"excerpt":"I am at the ACS meeting, attending a session on chemistry and the Internet. This post was inspired by Chemicalize, a service offered by ChemAxon, which scans a post like this one, and identifies molecules named. I had previously used generic post taggers, which frankly did not work well in\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":10237,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10237","url_meta":{"origin":8540,"position":5},"title":"How to predict the regioselectivity of epoxide ring opening.","author":"Henry Rzepa","date":"April 28, 2013","format":false,"excerpt":"I recently got an email from a student asking about the best way of rationalising epoxide ring opening using some form of molecule orbitals. This reminded me of the famous experiment involving propene epoxide. In the presence of 0.3% NaOH, propene epoxide reacts with ethanol at the unsubstituted carbon (~82%\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":[]}],"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\/8540","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=8540"}],"version-history":[{"count":21,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/8540\/revisions"}],"predecessor-version":[{"id":8559,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/8540\/revisions\/8559"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=8540"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=8540"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=8540"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=8540"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}