{"id":8452,"date":"2012-12-01T22:26:07","date_gmt":"2012-12-01T22:26:07","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=8452"},"modified":"2013-01-03T16:41:39","modified_gmt":"2013-01-03T16:41:39","slug":"the-mechanism-of-the-birch-reduction-part-1","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8452","title":{"rendered":"The mechanism of the Birch reduction. Part 1: reduction of anisole."},"content":{"rendered":"
\n

The Birch reduction<\/a> is a classic method for partially reducing e.g.<\/em> aryl ethers using electrons<\/strong> (from sodium dissolved in ammonia) as the reductant rather than e.g.<\/em> dihydrogen<\/a>. As happens occasionally in chemistry, a long debate broke out over the two alternative mechanisms labelled O<\/span><\/strong> (for ortho protonation of the initial radical anion intermediate) or M<\/span><\/strong> (for meta protonation). Text books seem to have settled down of late in favour of O<\/strong><\/span>. Here I take a look at the issue myself.<\/p>\n

\"\"<\/p>\n

Readers of my blog will note that I promote the use of models which are as reasonably complete as one can make them. In this case, if the intermediate is an anion, then I argue that the model should include the positive counter-ion. This is very often simply not included<\/a>, on the grounds that it “probably does not influence things”. Well, not on this blog! My model is methoxybenzene, a sodium atom solvated by 3NH3<\/sub> (the reaction itself is done in liquid ammonia with some added t-butanol) and continuum solvent (not ammonia itself, but acetonitrile which has a similar dielectric to liquid ammonia with some added butanol).\u00a0<\/p>\n

The start point is a solvated sodium atom (loosely) coordinated to anisole (methoxybenzene). The calculation<\/a> (\u03c9B97XD\/6-311+G(d,p)\/SCRF=acetonitrile) on this neutral system shows the spin density arising from the single unpaired electron is mostly (0.851) on the sodium, although a little spin density has crept onto the anisole. The dipole moment (12.0D) shows that solvation cannot just be ignored.\u00a0<\/p>\n

\"\"

Start point, with select spin densities. Click for 3D<\/p><\/div>\n

The next stage involves an electron transfer from the sodium to the anisole ring, and indeed the spin densities transfer from the Na to the two ortho-<\/span> and two meta-<\/span>positions on the ring (the residual value on Na is -0.02). This suggests that the valence bond representations in the diagram above are incomplete (they imply spin density on only two rather than four carbon atoms). The geometry of the anisole ring<\/a> now shows bond alternation, with two long bonds (1.45 – 1.46\u00c5) and four short bonds (1.39-1.41\u00c5). This could be viewed as the result of a pseudo-Jahn-Teller distortion<\/a> resulting from placing an electron into one of the degenerate LUMO molecular orbitals of the benzene-like set.\u00a0The free energy \u0394G298\u00a0<\/sub>of this charge-transferred product is 11.1 kcal\/mol exothermic compared to the reactant and it has a dipole moment of 11.6D, similar to the precursor despite being an ion pair!<\/p>\n

\"\"

The contact ion-pair resulting from electron transfer. Click for 3D.<\/p><\/div>\n

I start the analysis of how this species will protonate by inspecting the four highest energy NBOs (natural bond orbitals). Their energies are -0.144, -0.152, -0.167 and -0.167 au. The first of these with the highest energy might be expected to be the most basic. It corresponds to M<\/strong><\/span> in the above scheme (below). The next however is O<\/strong><\/span> and the last two are the remaining O\/M positions.<\/p>\n

\"\"

Highest energy (-0.144) NBO orbital on M. Click for 3D.<\/p><\/div>
\n

\"\"

Next highest energy (-0.152) \u00a0NBO on O. Click for 3D.<\/p><\/div><\/p>\n

Another measure of basicity is the molecular electrostatic potential (a measure of how attractive any point in space is towards a proton). It is shown below (as a green surface) only as the -ve potential (that part that is attractive to a proton). On the face bearing the proton donors (the ammonia groups attached to the Na) there is a clear preference for O<\/strong><\/span> (marked with a magenta arrow, but not the same O as predicted by the NBO), but with a slightly smaller basin corresponding to M <\/strong>(again, not the M from the NBO analysis and marked with an orange arrow).<\/span><\/span><\/p>\n

\"\"

Molecular electrostatic potential (-ve phase). Click for 3D<\/p><\/div>\n

Viewed from the other side of the anisole ring (and rendered at a higher threshold), the electrostatic potential seems to favour O<\/strong><\/span>, but only very slightly over\u00a0M<\/span><\/strong>. There really is not much in it.<\/p>\n

\"\"

Molecular electrostatic potential (-ve phase, other face).\u00a0Click for 3D.<\/p><\/div>\n

All these properties are measures of the radical-anion-ion-pair. It is clear these different measures do not agree with one another! What we really need is the transition state for the proton transfer. I will go off and hunt for these. If I find them, I will report back here.\u00a0And beyond the transition state are the dynamic trajectories for the protonation, which ultimately may be the only way of finally resolving this conundrum.\u00a0<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

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 radical anion intermediate) or M […]<\/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":[431,24,40,843,373],"ppma_author":[2661],"class_list":["post-8452","post","type-post","status-publish","format-standard","hentry","tag-dielectric","tag-energy","tag-free-energy","tag-reaction-mechanism","tag-tutorial-material"],"yoast_head":"\nThe mechanism of the Birch reduction. Part 1: reduction of anisole. - 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=8452\" \/>\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 1: reduction of anisole. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"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 radical anion intermediate) or M […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8452\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2012-12-01T22:26:07+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2013-01-03T16:41:39+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/12\/birch.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=\"4 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"The mechanism of the Birch reduction. 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Part 3: reduction of benzene","author":"Henry Rzepa","date":"December 4, 2012","format":false,"excerpt":"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\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-ip.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":8570,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8570","url_meta":{"origin":8452,"position":1},"title":"The mechanism of the Birch reduction. 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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":10237,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10237","url_meta":{"origin":8452,"position":3},"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 "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":21982,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=21982","url_meta":{"origin":8452,"position":4},"title":"The mechanism of Michael 1,4-Nucleophilic addition: a computationally derived reaction pathway.","author":"Henry Rzepa","date":"March 25, 2020","format":false,"excerpt":"In 2013, I created an iTunesU library of 115\u00a0mechanistic types in organic and organometallic chemistry, illustrated using video animations of the intrinsic reaction coordinate (IRC) computed using a high level quantum mechanical procedure. Many of those examples first derived from posts here. That collection\u00a0 is still available and is viewable\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":"","width":0,"height":0},"classes":[]},{"id":23522,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=23522","url_meta":{"origin":8452,"position":5},"title":"A computational mechanism for the aqueous hydrolysis of a ketal to a ketone and alcohol.","author":"Henry Rzepa","date":"April 1, 2021","format":false,"excerpt":"The previous post was about an insecticide and made a point that the persistence of both insecticides and herbicides is an important aspect of their environmental properties. Water hydrolysis will degrade them, a typical residency time being in the order of a few days. I noted in passing a dioxepin-based\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\/2021\/03\/R-1024x699.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\/8452","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=8452"}],"version-history":[{"count":50,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/8452\/revisions"}],"predecessor-version":[{"id":8861,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/8452\/revisions\/8861"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=8452"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=8452"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=8452"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=8452"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}