{"id":22471,"date":"2020-06-09T06:51:53","date_gmt":"2020-06-09T05:51:53","guid":{"rendered":"https:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=22471"},"modified":"2020-06-09T07:18:19","modified_gmt":"2020-06-09T06:18:19","slug":"fascinating-stereoelectronic-control-in-metaldehyde-and-chloral","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22471","title":{"rendered":"Fascinating stereoelectronic control in Metaldehyde and Chloral."},"content":{"rendered":"
\n

Metaldehyde<\/strong> is an insecticide used to control slugs. When we unsuccessfully tried to get some recently, I discovered it is now deprecated in the UK<\/a>. So my immediate reaction was to look up its structure to see if that cast any light (below, R=CH3<\/sub>, shown as one stereoisomer).<\/p>\n

\"\"<\/a><\/p>\n

A X-ray crystal structure exists (DOI: 10.5517\/ccdc.csd.cc20n2pg<\/a>) and reveals it to be the tetramer of acetaldehyde, or (CH3<\/sub>CHO)4<\/sub>. One further structure came to light, another tetramer of trichloromethylacetaldehyde, known as chloral.[1]<\/a><\/span> This latter compound forms a hydrate, hence chloral hydrate. These two compounds, differing only in the methyl group, show very different conformations of the eight-membered rings. As to why this is, makes for a fascinating story.<\/p>\n

\"\"

Click to obtain 3D model<\/p><\/div>\n

Firstly, the approach I used. I optimised the structure from the crystal data using \u03c9B97XD\/Def2-TZVPP, using C4v<\/sub> symmetry in which all four of the methine hydrogens point in the same direction. The resulting four H…H contacts of 2.13\u00c5 are on the short side, and are certainly contributing to the stability by dispersion (London) attractions. The C-O distances are 1.399\u00c5. I then did an ELF (Electron localisation function) analysis to identify what are called the monosynaptic basins. Better known as lone pairs! These (along with the disynaptic basins along the C-O bonds) are shown in purple above. I have displayed the torsion or dihedral angles between each of the lone pairs on oxygen and the adjacent C-O bond (150.1 and 42.0\u00b0). This now reveals the so-called anomeric effects in the molecule. Basically one of the lone pairs on oxygen has eight sets of 150.1 angles and the other lone pair eight sets of 42.0\u00b0. Only the former lone pairs are close to being anti-periplanar to the adjacent C-O bond. In this geometry, this lone pair can donate into the\u00a0C-O\u03c3*<\/sup> orbital of the bond. One can quantify the strength of this interaction using NBO (natural bond orbital) analysis, which gives a so-called E(2) perturbation interaction energy of 19.7 kcal\/mol, in total eight of them. The other lone pair on each oxygen shows no discernible interaction.<\/p>\n

On to Chloral (X = CCl3<\/sub>). This shows an entirely different geometry with Ci<\/sub> symmetry. This has two distinctly different pairs of oxygen atoms, with a pair of 2.36\u00c5 H…H contacts and two pairs of C-O distances 1.406 and 1.378\u00c5; 1.406 and 1.380\u00c5. This asymmetry immediately implies chloral will be more reactive towards e.g.<\/i> hydrolysis, since one of the C-O bonds is already slightly lengthened. There are 16 distinct dihedral values between an oxygen lone pair and either an adjacent C-O or a C-CCl3<\/sub> bond. The largest has a torsion angle at C-O of 161.2\u00b0 with an NBO E(2) energy of 20.6 kcal\/mol for a C-O interaction; the other torsions are 149.9, 141.1, 127.5, 112.6, 112.0, 111.1, 74.6, 68.2, 54.1, 42.6, 38.7, 32.6, 20.7, 5.7 and 4.1. There is a new anomeric effect to the adjacent C-CCl3<\/sub> bond of 12.1 kcal\/mol, lower for this latter interaction because the angle (113°) is far from the ideal 180\u00b0. In this model, all eight oxygen lone pairs play a role in stabilising the molecule, whereas in metaldehyde only four lone pairs do this.<\/p>\n

\"\"

Click to obtain 3D model<\/p><\/div>\n

One can now transpose the symmetry of each molecule onto the other compound. Metaldehyde in Ci<\/sub> symmetry is +5.5 kcal\/mol higher in free energy and chloral in C4v<\/sub> symmetry is +3.9 kcal\/mol. The origins of these difference are probably dissipated across the multiple anomeric effects and H…H dispersion attractions.<\/p>\n

This technique of locating the centroids of lone pairs using ELF and then correlating the dihedral angle between the lone pair and any adjacent C-X bond (X = electronegative, which makes the C-X bond a good electron acceptor) is very useful in explaining instances of the anomeric effect and comparing them across isomers. <\/p>\n

References<\/h2>\n
  1. D. Hay, and M. Mackay, \"The crystal and molecular structure of metachloral, 2(e),4(e),6(e),8(e)-Tetrakis-(trichloromethy1)-1,3,5,7-tetraoxocan\", Australian Journal of Chemistry<\/i>, vol. 33, pp. 2249-2253, 1980. https:\/\/doi.org\/10.1071\/ch9802249<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    Metaldehyde is an insecticide used to control slugs. When we unsuccessfully tried to get some recently, I discovered it is now deprecated in the UK. So my immediate reaction was to look up its structure to see if that cast any light (below, R=CH3, shown as one stereoisomer). A X-ray crystal structure exists (DOI: 10.5517\/ccdc.csd.cc20n2pg) […]<\/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":[1745,4],"tags":[],"ppma_author":[2661],"class_list":["post-22471","post","type-post","status-publish","format-standard","hentry","category-crystal_structure_mining","category-interesting-chemistry"],"yoast_head":"\nFascinating stereoelectronic control in Metaldehyde and Chloral. - 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=22471\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Fascinating stereoelectronic control in Metaldehyde and Chloral. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"Metaldehyde is an insecticide used to control slugs. When we unsuccessfully tried to get some recently, I discovered it is now deprecated in the UK. So my immediate reaction was to look up its structure to see if that cast any light (below, R=CH3, shown as one stereoisomer). A X-ray crystal structure exists (DOI: 10.5517\/ccdc.csd.cc20n2pg) […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22471\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2020-06-09T05:51:53+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2020-06-09T06:18:19+00:00\" \/>\n<meta property=\"og:image\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2020\/06\/metaldehyde.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=\"3 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"Fascinating stereoelectronic control in Metaldehyde and Chloral. - 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=22471","og_locale":"en_GB","og_type":"article","og_title":"Fascinating stereoelectronic control in Metaldehyde and Chloral. - Henry Rzepa's Blog","og_description":"Metaldehyde is an insecticide used to control slugs. 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We met for the first time in person for more than 18 months and were treated to a splendid and diverse program about the subject.\u2026","rel":"","context":"In "Chiroptics"","block_context":{"text":"Chiroptics","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2644"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2021\/10\/NAIACE.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":11597,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11597","url_meta":{"origin":22471,"position":3},"title":"The subtle effect of dispersion forces on the shapes of molecules: benzyl magnesium bromide.","author":"Henry Rzepa","date":"November 10, 2013","format":false,"excerpt":"In the previous post I mentioned in passing the Grignard reagent benzyl magnesium bromide as having tetrahedral coordination at Mg. But I have now noticed, largely through spotting Steve Bachrach's post on \"Acene dimers \u2013 open or closed?\" another geometric effect perhaps worthy of note, certainly one not always noted\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":20333,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=20333","url_meta":{"origin":22471,"position":4},"title":"The history of Alizarin (and madder).","author":"Henry Rzepa","date":"October 18, 2018","format":false,"excerpt":"The Royal Society of Chemistry historical group (of which I am a member) organises two or three one day meetings a year. Yesterday the October meeting covered (amongst other themes) the fascinating history of madder and its approximately synthetic equivalent alizarin. Here I add a little to the talk given\u2026","rel":"","context":"In "Historical"","block_context":{"text":"Historical","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=565"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":26199,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=26199","url_meta":{"origin":22471,"position":5},"title":"Blue blood.","author":"Henry Rzepa","date":"August 7, 2023","format":false,"excerpt":"Respiratory pigments are metalloproteins that transport O2, the best known being the bright red\/crimson coloured hemoglobin in human blood. The colour derives from Fe2+ at the core of a tetraporphyrin ring. But less well known is blue blood, and here the colour derives from an oxyhemocyanin unit based on Cu1+\u2026","rel":"","context":"In "crystal_structure_mining"","block_context":{"text":"crystal_structure_mining","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1745"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/08\/Screenshot57.jpg?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/08\/Screenshot57.jpg?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/08\/Screenshot57.jpg?resize=525%2C300&ssl=1 1.5x, https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/08\/Screenshot57.jpg?resize=700%2C400&ssl=1 2x, https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/08\/Screenshot57.jpg?resize=1050%2C600&ssl=1 3x, https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2023\/08\/Screenshot57.jpg?resize=1400%2C800&ssl=1 4x"},"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\/22471","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=22471"}],"version-history":[{"count":24,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/22471\/revisions"}],"predecessor-version":[{"id":22500,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/22471\/revisions\/22500"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=22471"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=22471"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=22471"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=22471"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}