{"id":16619,"date":"2017-03-19T13:07:59","date_gmt":"2017-03-19T13:07:59","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=16619"},"modified":"2017-03-22T09:25:09","modified_gmt":"2017-03-22T09:25:09","slug":"pyrophoric-metals-the-mechanism-of-thermal-decomposition-of-magnesium-oxalate","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16619","title":{"rendered":"Pyrophoric metals + the mechanism of thermal decomposition of magnesium oxalate."},"content":{"rendered":"
\n

A pyrophoric metal is one that burns spontaneously in oxygen; I came across this phenomenon as a teenager doing experiments at home. Pyrophoric iron<\/a> for example is prepared by heating anhydrous iron (II) oxalate in a sealed test tube (i.e.<\/em> to 600\u00b0 or higher). When the tube is broken open and the contents released, a shower of sparks forms. Not all metals do this; early group metals such as calcium undergo a different reaction releasing carbon monoxide and forming calcium carbonate and not the metal itself. Here as a prelude to the pyrophoric reaction proper, I take a look at this alternative mechanism using calculations.<\/p>\n

\"\"<\/a>
\n <\/a>There are ~60\u00a0crystal structures of metal oxalates, of which several are naturally occurring minerals (Fe, humboldtine[1]<\/a><\/span>, Ca, Weddellite[2]<\/a><\/span>, Li[3]<\/a><\/span>, Na[4]<\/a><\/span>, K[5]<\/a><\/span>, Cs[6]<\/a><\/span>. The natural geometry of the oxalate di-anion is planar (torsion 0 or 180\u00b0) but a small number are twisted such as the caesium oxalate.<\/p>\n

\"\"<\/p>\n

The kinetics of pyrolysis of a number of metal\u00a0\u00a0oxalates were studied some years ago (Ca[7]<\/a><\/span>, Li[8]<\/a><\/span>) indicating barriers ranging from 53-68 kcal\/mol. One proposed mechanism is as shown in this article.[7]<\/a><\/span><\/p>\n

\"\"<\/p>\n

It was surmised from the kinetic analysis that the k<\/em>1<\/sub> activation step (rotation about the C-C bond from planar to twisted) was ~12\u00a0\u00b1 20 kcal\/mol, whilst steps k<\/em>2<\/sub>\u00a0or k<\/em>3<\/sub>\u00a0had the much higher activation energy noted above. A search (of Scifinder) for quantum mechanical “reality checks”\u00a0of this mechanism revealed a blank and so I apply such a check\u00a0here using Mg as the metal.<\/p>\n

The carbonyl extrusion step (\u03c9B97XD\/Def2-TZVPPD\/SCRF=water, DOI: 10.14469\/hpc\/2320<\/a>) was studied with a water solvent field applied in an effort to mimic the solid state crystal structure of the species as a better representation of the ionic lattice than a pure vacuum calculation.\"\"\"\"<\/a>An IRC (intrinsic reaction coordinate, DOI: 10.14469\/hpc\/2324<\/a>) reveals the start-point geometry\u00a0still has a very small negative force constant (-38 cm-1<\/sup>, DOI:\u00a010.14469\/hpc\/2321<\/a>)\u00a0which now corresponds to a small rotation about the C-C bond to give a C2<\/sub>-symmetric conformation:<\/p>\n

\"\"But the barrier for this process is tiny and nothing like the\u00a0~12\u00a0\u00b1 20 kcal\/mol inferred from the kinetic analysis. Perhaps most of the incentive to pack into a totally planar geometry comes from the interactions in the ionic lattice.\u00a0The calculated free energy barrier (\u0394G298<\/sub>\u2021<\/sup>\u00a054.7 kcal\/mol, \u0394G755<\/sub>\u2021<\/sup>\u00a055.1 kcal\/mol) is within\u00a0the reported measured range.<\/p>\n

\"\"<\/p>\n

The mechanism for production of pyrophoric metal itself is likely to be far more complex, involving (inter alia<\/em>) electron transfer from oxygen to metal. If I find anything I will report back here.<\/p>\n

References<\/h2>\n
  1. T. Echigo, and M. Kimata, \"Single-crystal X-ray diffraction and spectroscopic studies on humboldtine and lindbergite: weak Jahn\u2013Teller effect of Fe2+ ion\", Physics and Chemistry of Minerals<\/i>, vol. 35, pp. 467-475, 2008. https:\/\/doi.org\/10.1007\/s00269-008-0241-7<\/a>\n\n<\/li>\n
  2. C. Sterling, \"Crystal structure analysis of weddellite, CaC2O4.(2+x)H2O\", Acta Crystallographica<\/i>, vol. 18, pp. 917-921, 1965. https:\/\/doi.org\/10.1107\/s0365110x65002219<\/a>\n\n<\/li>\n
  3. G.A. Jeffrey, and G.S. Parry, \"The Crystal Structure of Sodium Oxalate\", Journal of the American Chemical Society<\/i>, vol. 76, pp. 5283-5286, 1954. https:\/\/doi.org\/10.1021\/ja01650a007<\/a>\n\n<\/li>\n
  4. Dinnebier, R.E.., Vensky, S.., Panthofer, M.., and Jansen, M.., \"CCDC 192180: Experimental Crystal Structure Determination\", 2003. https:\/\/doi.org\/10.5517\/cc6fzcy<\/a>\n\n<\/li>\n
  5. Dinnebier, R.E.., Vensky, S.., Panthofer, M.., and Jansen, M.., \"CCDC 192182: Experimental Crystal Structure Determination\", 2003. https:\/\/doi.org\/10.5517\/cc6fzf0<\/a>\n\n<\/li>\n
  6. F.E. Freeberg, K.O. Hartman, I.C. Hisatsune, and J.M. Schempf, \"Kinetics of calcium oxalate pyrolysis\", The Journal of Physical Chemistry<\/i>, vol. 71, pp. 397-402, 1967. https:\/\/doi.org\/10.1021\/j100861a029<\/a>\n\n<\/li>\n
  7. D. Dollimore, and D. Tinsley, \"The thermal decomposition of oxalates. Part XII. The thermal decomposition of lithium oxalate\", Journal of the Chemical Society A: Inorganic, Physical, Theoretical<\/i>, pp. 3043, 1971. https:\/\/doi.org\/10.1039\/j19710003043<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    A pyrophoric metal is one that burns spontaneously in oxygen; I came across this phenomenon as a teenager doing experiments at home. Pyrophoric iron for example is prepared by heating anhydrous iron (II) oxalate in a sealed test tube (i.e. to 600\u00b0 or higher). When the tube is broken open and the contents released, a […]<\/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":true,"_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":[1745,1086],"tags":[2079,169,2077,1432,1395,2080,2078,1431,157,2081,1832,2082,2076,1834],"ppma_author":[2661],"class_list":["post-16619","post","type-post","status-publish","format-standard","hentry","category-crystal_structure_mining","category-reaction-mechanism-2","tag-aluminium","tag-calculated-free-energy-barrier","tag-carbon-monoxide","tag-chemical-elements","tag-chemistry","tag-higher-activation-energy","tag-iron","tag-matter","tag-metal","tag-metal-oxalates","tag-oxide","tag-pyrophoric-metal","tag-pyrophoricity","tag-reducing-agents"],"yoast_head":"\nPyrophoric metals + the mechanism of thermal decomposition of magnesium oxalate. - 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=16619\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Pyrophoric metals + the mechanism of thermal decomposition of magnesium oxalate. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"A pyrophoric metal is one that burns spontaneously in oxygen; I came across this phenomenon as a teenager doing experiments at home. 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This classic experiment conducted by Cotton (of quadruple bond fame) and Calderazzo in 1962 dates from an era when chemists conducted extensive kinetic analyses to back up any mechanistic speculations. Their suggested\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.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/11\/CO%2Bethene.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":25270,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=25270","url_meta":{"origin":16619,"position":1},"title":"Dioxane tetraketone – an ACS molecule of the week with a mystery.","author":"Henry Rzepa","date":"June 22, 2022","format":false,"excerpt":"I have long been fascinated by polymers of either carbon dioxide,\u2020 or carbon monoxide, or combinations of both.\u00a0One such molecule, referred to as dioxane tetraketone when it was featured on the ACS molecule-of-the-week site and also known as the anhydride of oxalic acid, or more formally 1,4-dioxane-2,3,5,6-tetraone, has been speculated\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\/2022\/06\/C4O6.gif?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":3546,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=3546","url_meta":{"origin":16619,"position":2},"title":"Triple metal delight","author":"Henry Rzepa","date":"March 1, 2011","format":false,"excerpt":"Much like chocolate, some of us metallaholics cannot get enough. So WUQXIP proved an irresistible frolic (DOI: 10.1021\/om020789h). Let us start with benzene. \u00a0It can have metals added in two ways, whilst preserving its essential aromaticity. Making a metal sandwich is of course very well known, ferrocene being the first\u2026","rel":"","context":"In "Hypervalency"","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/03\/metallocene.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":4751,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4751","url_meta":{"origin":16619,"position":3},"title":"Computational “reality checks” for mechanistic speculations.","author":"Henry Rzepa","date":"September 1, 2011","format":false,"excerpt":"I have mentioned Lewis a number of times in these posts; his suggestion of the shared electron covalent bond still underpins much chemical thinking. Take for example mechanistic speculations on the course of a reaction, a very common indulgence in almost all articles reporting such, and largely based on informed\u00a0\u00a0arrow\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.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/09\/nsch001.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":4814,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4814","url_meta":{"origin":16619,"position":4},"title":"Mindless chemistry or creative science?","author":"Henry Rzepa","date":"September 3, 2011","format":false,"excerpt":"The (hopefully tongue-in-cheek) title Mindless chemistry was given to an article reporting an automated stochastic search procedure for locating all possible minima with a given composition using high-level quantum mechanical calculations. \"Many new structures, often with nonintuitive geometries, were found\". Well, another approach is to follow unexpected hunches. One such\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.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/09\/fe2.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":8216,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8216","url_meta":{"origin":16619,"position":5},"title":"Secrets revealed for conjugate addition to cyclohexenone using a Cu-alkyl reagent.","author":"Henry Rzepa","date":"November 4, 2012","format":false,"excerpt":"The text books say that cyclohexenone A will react with a Grignard reagent by delivery of an alkyl (anion) to the carbon of the carbonyl (1,2-addition) but if dimethyl lithium cuprate is used, a conjugate 1,4-addition proceeds, to give the product B shown below. The standard explanation is that the\u2026","rel":"","context":"In \"metal\"","block_context":{"text":"metal","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=metal"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/11\/4.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\/16619","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=16619"}],"version-history":[{"count":35,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/16619\/revisions"}],"predecessor-version":[{"id":17802,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/16619\/revisions\/17802"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=16619"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=16619"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=16619"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=16619"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}