{"id":10252,"date":"2013-04-29T11:08:09","date_gmt":"2013-04-29T10:08:09","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=10252"},"modified":"2013-04-29T11:45:37","modified_gmt":"2013-04-29T10:45:37","slug":"why-diphenyl-peroxide-does-not-exist","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10252","title":{"rendered":"Why diphenyl peroxide does not exist."},"content":{"rendered":"
\n

A few posts back, I explored the “benzidine rearrangement<\/a>” of diphenyl hydrazine. This reaction requires diprotonation to proceed readily, but we then discovered<\/a> that replacing one NH by an O as in N,O-diphenyl hydroxylamine required only monoprotonation to undergo an equivalent facile rearrangement. So replacing both NHs by O to form diphenyl peroxide (Ph-O-O-Ph) completes this homologous series. I had speculated <\/a>that PhNHOPh might exist if all traces of catalytic acid were removed, but could the same be done to PhOOPh? Not if it continues the trend and requires no prior protonation at all!<\/p>\n

\"PhOOPh\"<\/p>\n

Here is the results of a \u03c9B97XD\/6-311G(d,p)\/SCRF=water calculation. Now I should explain that the conventional explanation for the non-existence of PhOOPh is that the O-O bond homolyses very readily to form phenoxy radicals[1]<\/a><\/span>. But of course other peroxides such as t-Bu-O-O-t-Bu do exist (although they are rather fragile) and so the phenyl analogue is clearly special.<\/p>\n\n\n\n\n
\"PhOOPh\"\u00a0<\/td>\n\"PhOOPha1\"\u00a0<\/td>\n<\/tr>\n
\u00a0\"PhOOPh2\"<\/td>\n\"PhOOPha1\"\u00a0<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

You will notice from the IRC profiles shown above that even without any prior protonation, the barrier to O-O cleavage is really very small (~ 4 kcal\/mol). But the method I have used to calculate this is a closed shell DFT procedure. This does not allow the formation of the (open shell) biradical that two phenoxy radicals would represent. The barrier is low even without the formation of phenoxy radicals! Of course, as with the two previous examples, the actual initial product formed is the\u00a0\u03c0-complex as first suggested by Michael Dewar. The wavefunction of such a species requires special treatment, since it is best described <\/a>as a linear combination of two closed-shell configurations, what is called a multi-configuration or multi-reference wavefunction.\u2021<\/sup> So the single-configuration closed shell calculation that the above IRC represents must be an upper bound to a proper description of the energy transition state. In other words, if the description is improved, the barrier can only get even lower!\u00a0<\/p>\n

Notice in the above that the\u00a0\u03c0-complex formed in the first stage (of two) is actually lower in energy than the diphenyl peroxide itself, and that the barrier for this\u00a0\u03c0-complex to then collapse to form the C-C bond between the two 4-positions is also tiny. This\u00a0\u03c0-complex in other words is very transient indeed, probably not surviving for even one molecular vibration. To all intents and purposes, this really is a concerted [5,5] sigmatropic shift, as shown in the schematic at the top of this post. But the bottom line is that the homolysis argument need not be the only one (although it \u00a0is not necessarily incorrect). One can just as readily explain why PhOOPh does not exist by invoking facile formation of Dewar-like\u00a0\u03c0-complex instead.<\/p>\n

\n

\u2021<\/sup> Another deceptively simple little molecule that requires such a treatment is C2<\/sub>, the topic of much recent debate![2]<\/a><\/span>, [3]<\/a><\/span><\/p>\n

References<\/h2>\n
  1. R. Benassi, U. Folli, S. Sbardellati, and F. Taddei, \"Conformational properties and homolytic bond cleavage of organic peroxides. I. An empirical approach based upon molecular mechanics and <i>ab initio<\/i> calculations\", Journal of Computational Chemistry<\/i>, vol. 14, pp. 379-391, 1993. https:\/\/doi.org\/10.1002\/jcc.540140402<\/a>\n\n<\/li>\n
  2. S. Shaik, H.S. Rzepa, and R. Hoffmann, \"One Molecule, Two Atoms, Three Views, Four Bonds?\", Angewandte Chemie International Edition<\/i>, vol. 52, pp. 3020-3033, 2013. https:\/\/doi.org\/10.1002\/anie.201208206<\/a>\n\n<\/li>\n
  3. J.M. Matxain, F. Ruip\u00e9rez, I. Infante, X. Lopez, J.M. Ugalde, G. Merino, and M. Piris, \"Communication: Chemical bonding in carbon dimer isovalent series from the natural orbital functional theory perspective\", The Journal of Chemical Physics<\/i>, vol. 138, 2013. https:\/\/doi.org\/10.1063\/1.4802585<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    A few posts back, I explored the “benzidine rearrangement” of diphenyl hydrazine. This reaction requires diprotonation to proceed readily, but we then discovered that replacing one NH by an O as in N,O-diphenyl hydroxylamine required only monoprotonation to undergo an equivalent facile rearrangement. So replacing both NHs by O to form diphenyl peroxide (Ph-O-O-Ph) completes […]<\/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":[4],"tags":[1045,24,1046,591,1047,843],"ppma_author":[2661],"class_list":["post-10252","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-actual-initial-product","tag-energy","tag-energy-transition-state","tag-michael-dewar","tag-new-hampshire","tag-reaction-mechanism"],"yoast_head":"\nWhy diphenyl peroxide does not exist. - 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=10252\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Why diphenyl peroxide does not exist. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"A few posts back, I explored the “benzidine rearrangement” of diphenyl hydrazine. This reaction requires diprotonation to proceed readily, but we then discovered that replacing one NH by an O as in N,O-diphenyl hydroxylamine required only monoprotonation to undergo an equivalent facile rearrangement. So replacing both NHs by O to form diphenyl peroxide (Ph-O-O-Ph) completes […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10252\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2013-04-29T10:08:09+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2013-04-29T10:45:37+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/04\/PhOOPh.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":"Why diphenyl peroxide does not exist. - Henry Rzepa'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=10252","og_locale":"en_GB","og_type":"article","og_title":"Why diphenyl peroxide does not exist. - Henry Rzepa's Blog","og_description":"A few posts back, I explored the “benzidine rearrangement” of diphenyl hydrazine. This reaction requires diprotonation to proceed readily, but we then discovered that replacing one NH by an O as in N,O-diphenyl hydroxylamine required only monoprotonation to undergo an equivalent facile rearrangement. 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The monoprotonated mechanism.","author":"Henry Rzepa","date":"January 8, 2013","format":false,"excerpt":"Eagle-eyed footnote readers might have spotted one at the bottom of the post on the benzidine rearrangement. I was comparing the N-N bond lengths in crystal structures of known diprotonated hydrazines (~1.45\u00c5) with the computed N-N bond length at the start point of the intrinsic reaction coordinate for the [5,5]\u2026","rel":"","context":"In \"free energy barrier\"","block_context":{"text":"free energy barrier","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=free-energy-barrier"},"img":{"alt_text":"Transition state between p-complex and N-N diprotonated diphenyhydrazine. Click for 3D.","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/01\/pi-TS.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":9135,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9135","url_meta":{"origin":10252,"position":1},"title":"Why is N,O-diphenyl hydroxylamine (PhNHOPh) unknown?","author":"Henry Rzepa","date":"January 16, 2013","format":false,"excerpt":"If you search e.g. Scifinder for N,O-diphenyl hydroxylamine (RN\u00a024928-98-1) there is just one literature citation, to a 1962 patent. Nothing else; not even a calculation (an increasing proportion of the molecules reported in Chemical Abstracts have now only ever been subjected to calculation, not synthesis).\u00a0A search of Reaxys also offers\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":8961,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8961","url_meta":{"origin":10252,"position":2},"title":"The mechanism of the Benzidine rearrangement.","author":"Henry Rzepa","date":"January 6, 2013","format":false,"excerpt":"The benzidine rearrangement is claimed to be an example of the quite rare\u00a0[5,5] sigmatropic migration, which is a ten-electron homologation of the very common [3,3] sigmatropic reaction (e.g. the Cope or Claisen). Some benzidine rearrangements are indeed thought to go through the [3,3] route. The topic has been reviewed here.\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":"NCI surface. Click for 3D.","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/01\/benzidinenci.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":9105,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9105","url_meta":{"origin":10252,"position":3},"title":"The Benzidine rearrangement. Computed kinetic isotope effects.","author":"Henry Rzepa","date":"January 11, 2013","format":false,"excerpt":"Kinetic isotope effects have become something of a lost art when it comes to exploring reaction mechanisms. But in their heyday they were absolutely critical for establishing the mechanism of the benzidine rearrangement. This classic mechanism proceeds via bisprotonation of diphenyl hydrazine, but what happens next was the crux. Does\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":12329,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12329","url_meta":{"origin":10252,"position":4},"title":"More (blog) connections spotted. Something new about diphenyl magnesium?","author":"Henry Rzepa","date":"April 17, 2014","format":false,"excerpt":"I have just noticed unexpected links between two old posts, one about benzene, one about diphenyl magnesium\u00a0and\u00a0a link to August Kekul\u00e9.\u2020 The post about benzene dealt with the apparently simple issue of why all the C-C bonds are of equal length. The answer, in brief is purely because of the\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":9218,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9218","url_meta":{"origin":10252,"position":5},"title":"Aromaticity in the benzidine-like \u03c0-complex formed from PhNHOPh.","author":"Henry Rzepa","date":"January 19, 2013","format":false,"excerpt":"The transient \u03c0-complex formed during the \"[5,5]\" sigmatropic rearrangement of protonated N,O-diphenyl hydroxylamine can be (formally) represented as below, namely the interaction of a six-\u03c0-electron aromatic ring (the phenoxide anion 2) with a\u00a0four-\u03c0-electron phenyl dication-anion pair 1. Can one analyse this interaction in terms of aromaticity? I showed previously that\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":"pi-QTAIM","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/01\/pi-QTAIM.jpg?resize=350%2C200","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\/10252","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=10252"}],"version-history":[{"count":7,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/10252\/revisions"}],"predecessor-version":[{"id":10265,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/10252\/revisions\/10265"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=10252"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=10252"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=10252"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=10252"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}