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The PETN detection universe

29 December 2009 - Counter terrorism

klik hier voor de afbeelding op ware grootteAs the son of a high ranking Nigerian banking official suspected terrorist Abdulmutallab of course carried a diplomatic passport while boarding Northwest Airlines Flight 253, at Schiphol Airport all regular personnel was replaced by inexperienced temporary staff on account of Christmas, even the bomb dog was having a day off and the so-called 500,000 TIDE list is so severely polluted and outdated that no one is supposed to look at it. Questions for this blog however are strictly chemistry related. Would 80 grams of PETN have done the job and what does it take to detect the explosive before it gets on board?.

With regards to the first question missile and explosives expert Herman Schöyer in an interview to NRC thinks not unless Abdulmutallab "placed the PETN in a box against a wall" first. Detonating a quantity of loose powder will definitely not work. This blog realises it is never smart to give terrorists free advice (like Hermann just did) but as PETN dissolves very well in acetone how about impregnating your clothing with the stuff and then stuff it the box!. By the way, the news reports continue to mention a syringe with which the explosive was somehow activated. From a chemistry point of view this does not make sense as there is no apparent two-component mechanism at work.

With regard to the second question - how to detect PETN at the airport - the general complaint according to the news media is the lack of sophisticated and very expensive backscatter X-ray equipment. Problem is that this equipment would detect powder in a condom hidden on a body but not the hypothetical PETN impregnated clothing. Backscatter X-ray primarily scans objects, it does not detect chemicals. On the other hand the radiation analyzed is sensitive to the low atomic number elements (C,N,O) and it therefore makes organic materials stand out compared to metals. If backscatter equipment exists ultra sensitive to nitrogen is another matter. In one report at least (DOI) with a lot of more free advice on how not to smuggle illicit goods, a backscatter image identifies a parcel of plastic explosive in a batch of chocolate parcels.

An alternative to backscatter X-ray and sometimes found in US airports is the puffer machine: a puff of air flushes out any airborne chemical on a passenger and the chemicals are then tested inside a mass spectrometer. Now this machine can actually tell if someone has been in contact with PETN which only leaves the job of excluding any medical personnel because very inconveniently PETN is also a cardiac drug.

Third option overlooked thus far by the media is the good old fashioned bomb sniffing dog. Here is some free advice for Schiphol Airport Security: bombdog4sale. Dog Miya has degrees in TNT, RDX and PETN and does not mind working irregular shifts.

See also: the chemists working on the other side of the fence

Update (30-12): explosives expert Jimmie C. Oxley in the NYT(link) is also clueless on the syringe: according to Oxley PETN itself does not detonate very easily the reason why in the 2001 shoe bomb plot the detonator for the PETN was TATP. However as TATP is again a solid this does still not explain the syringe.

Update (13-01-2010): Thanks to reader M. for interesting comments. The primary interest of this blog is the way the media have been reporting on this incident with respect to the technology involved. Do the body scanners that Schiphol Airport plans to have installed really detect explosive powders or just powders? With respect to the mystery syringe the relevant question is: was the suspected terrorist actually capable with chemicals?.
Also thanks to reader D. Yes, Dr Oxley was not exactly clueless, she did have a theory but told the NYT reporter but I'm not going to tell you - no free advice there.

CO + NN + HCl = oxamide

23 December 2009 - Dinitrogen cleavage

Splitting dinitrogen is not easy. Although nature is very efficient in converting nitrogen to ammonia in nitrogen fixation humans can only do it in a crude high temperature high pressure method called the Haber process. Finding a catalyst that will do the job with moderate reaction conditions must be one of the holy grails in chemistry. And profitable: ammonia itself is the raw material for the production of fertilizers critical to agriculture. The first to try successfully was Richard R. Schrock in 2003 (DOI) with a molybdenum system although with a lousy turnover number of 8. The latest system described by Knobloch, Lobkovsky and Chirik (DOI) is not even catalytic but manages to combine nitrogen and carbon monoxide to form oxamide which is a fertilizer in itself. Thus hafnocene 1 is reacted with nitrogen and sodium amalgam to dinitrogen complex 2. It takes 6 electrons to reduce dinitrogen and 2 are supplied by each hafnium center. The nitrogen center can be considered a N24- ligand. The two remaining electrons are donated by CO in the next step to intermediate 3 and treatment with hydrochloric acid liberates oxamide.

The ambient temperature claim in the article summary however is somewhat optimistic: sure the nitrogen adduct product is formed at 25 °C but addition of CO takes place at - 196°C. The process stops at higher temperatures because then hafnium misbehaves and inserts itself in one of the t-butyl C-H bonds rather then allowing CO insertion.

Update 02-01-2010: switched to Cp ligands in image. Many thanks to vigilant reader S.
dinitrogencleavage knobloch 2009

Metal-free carbon dioxide reduction

14 December 2009 - FLP chemistry

Three months ago Stephan et al. demonstrated carbon dioxide sequestration by FLP chemistry (frustrated Lewis pairs) (previous episode here) and this month Ashley et al. have taken it a step further and report carbon dioxide reduction (DOI), chemistry relevant to the methanol economy . Combine TMP, tris(pentafluorophenyl)boron and hydrogen to form the (TMPH)(HB(C6F5)3) salt (1), splitting the H2 molecule. Next add CO2 to a solution of 1 in toluene at 100°C and the formatoborate (HCOOB(C6F5)3)(TMPH) 2 forms quantitatively. Above 100 °C the reactions are no longer reversible. Attack by free BAr3 forms intermediate 3 which can be reduced by another equivalent of (TMPH)(HB(C6F5)3) salt to 4. This acetal is unstable and a TMP proton induces cleavage to intermediate 5 which is reduced again by (TMPH)(HB(C6F5)3) salt to the CH3OBAr3TMPH complex 6. Unable to dissociate into its constituent parts even at 160°C, a proton attacks one of the aromatic rings splitting of C6F5H and forming CH3OBAr2 + TMP. Finally distillation of the reaction product yields methanol in a 17-25% yield.

A catalytic system is in the making.


08 December 2009 - Fullerenes

Circulenes are closed rings consisting of benzene rings. As of this month the smallest possible ring is no longer 5 but 4 thanks to research by a big multinational team headed by Benjamin King (University of Nevada, Reno) (Bhola et al. 2009 DOI). The compound on the right is called a quadrannulene in reference to corannulene. It was synthesized from the tetraalkyne on the left by cyclotrimerization with bistrimethylsilylacetylene followed by oxidation with the ferrocenecompound Cp2FePF6 in an irrelevant yield of 2%. The molecule is a geodesic polyarene and bowl shaped with the 4 double bonds radiating from the cyclobutane ring heavily pyramidalized although the compound itself is very stable (moderately air stable, stable up to 170 °C). Interestingly the carbon framework of this molecule can be found in the hypothetical octahedral 4,6-C24-fullerene.

Wikibooks: Isomerism

08 December 2009 - Part I

A new feature of Wikipedia is called Wiki Books that you can create from within wikipedia itself. You collect a number of wikipedia articles that you want to turn into a single pdf file (the book), place them in a userpage in a folder /Books/ and hit the - load book - link. It is really that easy. The app behind it breaks up the book into chapters and takes care of an index, page numbers and takes care of layout issues. At the end of the book there is a list of article sources (specifying the edits) and contributors. The Image licences and general licence are also taken care of. As an example of a wikibook, this blog has helped to create the definitive work on isomerism:

Open here: Wikibooks/Isomerism.pdf (8.5 MB)

With 120 pages and 8.5 MB it contains everything you need to know of trans isomers, enantiomers and tacticity. Several hundreds of editors have contributed and are listed in the back. Important note: the wikibooks folder does not allow search engine crawlers: no search engine pollution!.

Wikibooks is a great way to curate wikipedia. Especially in the chemistry desk the number of volunteers is dwindling and at some point the quality of information can no longer be guaranteed. By offering wikibooks from a separate entity there is at least a backup. Next up: The Wikipedia Handbook of Organic Reactions.

The double dehydro-Diels-Alder Reaction

04 December 2009 - Organic synthesis

One organic reaction that over the course of a century has been tried but again and again failed to deliver is the double dehydro-Diels-Alder reaction of 1,5-dien-3-ynes. A new attempt by Fallon et al. (DOI) explains exactly why this reaction performs so miserably. The reaction of enyne 1,2-dicyclohexenyl acetylene and dienophile N-Methylmaleimide (5 equivalents, close cousin of N-ethylmaleimide) was improved from bad yields 11-22% to a yield of 61% by making use of microwave chemistry. The ene reaction product was isolated in only a 4% yield but the remainder - a tangle of polar degradation products - is believed to originate from that same ene product. Interestingly only one stereoisomer was found for the product with both dienophile units added in an counter intuitive appearant syn fashion. Every trick known to chemistry has been exhausted in getting the reaction right and even the addition of Lewis acid AlMeCl2 only helped the reactant to escape in a 2+2 cycloaddition.