{"id":12546,"date":"2014-05-18T16:38:40","date_gmt":"2014-05-18T15:38:40","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=12546"},"modified":"2014-05-18T16:56:50","modified_gmt":"2014-05-18T15:56:50","slug":"benzene-as-you-have-never-seen-it-represented-before","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12546","title":{"rendered":"Benzene. As you have never seen it represented before!"},"content":{"rendered":"
\n

Continuing my european visits, here are two photos from Bonn. First, a word about how the representation of benzene evolved, attributed to Kekul\u00e9.<\/p>\n

\"The<\/a>

The sausage formula<\/p><\/div>\n

Above is his first effort, made in 1865.<\/p>\n

\"The<\/a>

The bent bond formula<\/p><\/div>\n

This one above is better, offered in 1866. But whilst what we now know as the double bond (C=C) is perhaps understandably kinked (and we now call these banana bonds, since nature tends to abhor kinks in electron density), so too are the single bonds!<\/p>\n

\"The<\/a>

The plinth<\/p><\/div>\n

So when it came to erecting a statue in his honour around 1890, the kinks were straightened out! The figure on the right (female) represents science (and purity). The two chaps on the left are workers representing industry. Note that they have still not quite gotten the lengths of the putative single and double bonds in proportion. By 1872 of course, Kekul\u00e9 had proposed[1]<\/a><\/span>,[2]<\/a><\/span> his oscillating model, the one that is taught to this day.<\/p>\n

I feel I should add one modern interpretation to this concept. An oscillation implies a frequency. Kekul\u00e9 could only know that this oscillation was fast on what might be called the “laboratory scale” (in other words, no-one had been able to isolate the individual isomers of substituted benzenes, which implies that the rate constant inter-converting them was probably faster than k = 10-3<\/sup> s-1<\/sup> or a few minutes half-life).\u2021<\/sup> We now know that this oscillation is ~1014<\/sup> s-1<\/sup>, the timescale of a molecular vibration! By the way, transition state theory tells us that Ln<\/i>(k\/T) = 23.76 – \u0394G\u2021<\/sup>\/RT<\/b> where k is the unimolecular rate constant, \u0394G\u2021<\/sup> the free energy barrier, T the temperature and R the gas constant. Setting \u0394G\u2021<\/sup> to zero gives us a rate constant of ~1014<\/sup> s-1<\/sup> (the barrier must be zero, or very close to it).<\/p>\n

\"The<\/a>

The Statue<\/p><\/div>\n

Here is the statue atop the plinth above. Apparently it is honoured by the students with robes and other accoutrements on special occasions.<\/p>\n

\n

\u2021<\/sup>Another famous timescale inference was by Beckmann in 1889[3]<\/a><\/span> when he deduced the existence of a transient unseen intermediate in the (what he thought was) racemisation of menthone. That intermediate of course was the enol.<\/p>\n

References<\/h2>\n
  1. A. Kekul\u00e9, \"Ueber einige Condensationsproducte des Aldehyds\", Justus Liebigs Annalen der Chemie<\/i>, vol. 162, pp. 77-124, 1872. https:\/\/doi.org\/10.1002\/jlac.18721620110<\/a>\n\n<\/li>\n
  2. A. Kekul\u00e9, \"Ueber einige Condensationsproducte des Aldehyds\", Justus Liebigs Annalen der Chemie<\/i>, vol. 162, pp. 309-320, 1872. https:\/\/doi.org\/10.1002\/jlac.18721620211<\/a>\n\n<\/li>\n
  3. E. Beckmann, \"Untersuchungen in der Campherreihe\", Justus Liebigs Annalen der Chemie<\/i>, vol. 250, pp. 322-375, 1889. https:\/\/doi.org\/10.1002\/jlac.18892500306<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    Continuing my european visits, here are two photos from Bonn. First, a word about how the representation of benzene evolved, attributed to Kekul\u00e9. Above is his first effort, made in 1865. This one above is better, offered in 1866. But whilst what we now know as the double bond (C=C) is perhaps understandably kinked (and […]<\/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":[565],"tags":[357,206,1226],"ppma_author":[2661],"class_list":["post-12546","post","type-post","status-publish","format-standard","hentry","category-historical","tag-rt","tag-free-energy-barrier","tag-gas-constant"],"yoast_head":"\nBenzene. 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Generations of chemists have learnt early on in their studies of the subject that these two representations of where the electron pairs in benzene might be located (formally called electronic resonance or valence bond forms) each contribute ~50% to 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":"The Kekule structures of benzene.","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/06\/benzene.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":21925,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=21925","url_meta":{"origin":12546,"position":1},"title":"The singlet and open shell higher-spin states of [4], [6] and [8]-annulenes and their Kekul\u00e9 vibrational modes","author":"Henry Rzepa","date":"March 11, 2020","format":false,"excerpt":"In 2001, Shaik and co-workers published the first of several famous review articles on the topic\u00a0A Different Story of \u03c0-Delocalization. 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Another prototypical reaction, and again one where benzene is too inactive for the reaction to occur easily, is the catalyst-free bromination of benzene to give bromobenzene and HBr.\u00a0 The \"text-book\" mechanism involves nucleophilic\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":"br2+benzene","src":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/03\/br2+benzene.svg","width":350,"height":200},"classes":[]},{"id":23044,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=23044","url_meta":{"origin":12546,"position":3},"title":"Is cyanogen chloride (fluoride) a source of C\u2a78N(+)?","author":"Henry Rzepa","date":"November 28, 2020","format":false,"excerpt":"In 2010 I recounted the story of an organic chemistry tutorial, in which I asked the students the question \"how would you synthesize 3-nitrobenzonitrile\". The expected answer was to generate a nitronium ion to nitrate benzonitrile, but can one invert this by generating a C\u2a78N+ ion to cyanate nitrobenzene? The\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\/2020\/11\/TS-CN-254x300.gif?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":18555,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=18555","url_meta":{"origin":12546,"position":4},"title":"Dispersion “bonds”: a new example with an ultra-short H…H distance.","author":"Henry Rzepa","date":"June 26, 2017","format":false,"excerpt":"About 18 months ago, there was much discussion\u00a0on this blog about a system reported by Bob Pascal and co-workers containing a short H...H contact of ~1.5\u00c5. In this system, the hydrogens were both attached to Si as Si-H...H-Si and compressed together by rings. Now a new report and\u00a0commented upon by\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":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2017\/06\/201-1024x987.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":2502,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=2502","url_meta":{"origin":12546,"position":5},"title":"Secrets of a university admissions interviewer","author":"Henry Rzepa","date":"September 19, 2010","format":false,"excerpt":"Many university chemistry departments, and mine is no exception, like to invite applicants to our courses to show them around. Part of the activities on the day is an \"interview\" in which the candidate is given a chance to shine. 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