Chemical Safety Note: Explosion Hazard during the Distillation of

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Organic Process Research & Development

Chemical Safety Note: Explosion Hazard during the Distillation of Propargyl Thiocyanate

Klaus Banert,* Frank Richter, and Manfred Hagedorn Organic Chemistry, Technische Universität Chemnitz, Strasse der Nationen 62, 09111 Chemnitz, Germany

Corresponding Author *E-mail: [email protected]

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ABSTRACT

This study is based on the efforts to understand the cause of a laboratory accident at a university in Germany. An explosion occurred during the distillation of propargyl thiocyanate (1), a well-known compound. It is assumed, that an unwanted [3,3]-sigmatropic rearrangment under thermal conditions led to the accumulation of highly reactive isothiocyanatopropa-1,2diene (2), which actually was the explosive substance. To assess the hazard potential of the established purification procedure, pure 1 was analyzed by DTA and 2 by isothermal reaction calorimetry. The results support our hypothesis of unintended formation of dangerous 2, and safety advice is given to prevent future incidents.

KEYWORDS

exothermic reaction, explosion, laboratory accident, propadienyl isothiocyanate, prop-2-ynyl thiocyanate



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INTRODUCTION

This study is motivated by an accident that happened in our laboratories during a practical course for students of chemistry in 2010. A fierce deflagration occurred while performing a standard vacuum distillation of simple propargyl thiocyanate (1) synthesized on multigram scale according to the corresponding, literature-known protocol (Scheme 1).1 Protocols from such textbooks, especially the one mentioned1 and similar procedures2 to prepare and purify 1, found their way into student practical courses, which illustrates the importance of hazard assessment. The synthesis of 1 from propargyl bromide1,2,3a–e or benzenesulfonate3f and potassium thiocyanate was described several times, and quite different values, such as 65 °C/15 Torr,1,2 23 °C/0.1 mbar (0.075 Torr),3a 68 °C/9 Torr,3b 83 °C/3.5 Torr,3c and 35– 36 °C/1 Torr,3f were reported as boiling point of the product. The vigorous deflagration was most likely caused by performing the vacuum distillation of 1 at too high temperature and/or by pressure variation at the end of the distillation, which resulted in a sudden pressure increase igniting the bottoms fraction. Parts of the apparatus were destroyed, and several fragments were explosively distributed over the workplace like shrapnel. Fortunately, the student performing the experiment was not seriously harmed during the incident, but got a shock and soot covered face. Safety goggles as part of laboratory regular personal protection equipment prevented eye injuries. We presume that during distillation [3,3]-sigmatropic rearrangement of 1 occurred and the succeeding product isothiocyanatopropa-1,2-diene (2) was most probably accumulated in the bottoms fraction. This compound is known to spontaneously undergo highly exothermic polymerisation; when a gram of pure 2 was first synthesized by flash vacuum pyrolysis of 1, isolated at low temperature, and then warmed to room temperature to determine the yield, the neat compound suddenly underwent a very vigorous reaction without any recognizable reason just upon standing.4 Etheral solutions of cumulene 2 were later found to ensure safe handling, but not 4 ACS Paragon Plus Environment

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without minimal self-heating, as sign of slow polymerization. On treatment with nucleophiles (NuH), solutions of 2 lead to thiazoles of type 3, and several similar heterocyclic products can be prepared with high yields by the reactions of 2 in one-pot procedures.4–6 A sudden increase in pressure on the cumulene-enriched bottoms fraction during the distillation of 1 seems sufficient to trigger an explosion. We report here on the evalutaion of the safety hazard potential of propargyl thiocyanate (1), which involves differential thermal analysis of 1 and measurment of the heat of reaction resulting from polymerization of the isomerization product 2 with the help of reaction calorimetry.

Scheme 1. Synthesis and sigmatropic rearrangment of propargyl thiocyanate (1), and secondary reactions of allenyl isothiocyanate (2)

RESULTS AND DISCUSSION

The differential thermal analysis (DTA) of 1 (heating rate 10 K/min) showed an exothermic conversion at an onset temperature Tonset = 117.15 °C. The temperature with maximum heat flow was found to be Tmax = 165.95 °C. After conversion of 1 to 2 via sigmatropic rearrangement (Scheme 1), the highly unsaturated compound polymerized completely. Both 5 ACS Paragon Plus Environment


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processes occurred concertedly and could not be resolved. The overall reaction enthalpy was determined to be ΔH = –236.5 kJ/mol.

Figure 1. Differential thermal analysis (DTA) of propargyl thiocyanate (1)

The energetic share of this value coming from polymerization of 2 was determined by isothermal reaction calorimetry. A sufficient amount of isothiocyanatopropa-1,2-diene (2) was synthesized by flash vacuum pyrolysis and dissolved in anhydrous toluene.2 This solution only tends to slow polymerization, and can be handled without explosion risk. Thus, the resulting self-heating was recorded and analyzed to give the heat of polymerization with ΔH = –218.5 kJ/mol. The reaction enthalpy resulting from the thermal transformation of the precursor 1 is higher than that of the allene polymerization itself, which indicates that the rearrangement 1 → 2 is an exothermic step. Although the corresponding methods, DTA and calorimetry, are hard to compare in a direct manner, a slightly exothermic [3,3]-sigmatropic rearrangement to form isothiocyanatopropa-1,2-diene (2) seems plausible. Under the conditions of flash vacuum pyrolysis at 400 °C and 0.75 Torr (or higher temperatures), the isomerisation of 1 to 2 is not completely irreversible. In this case, an equilibrium with 1 : 2 = 2 : 98 is established.4 Thus, we assume that the [3,3]-sigmatropic rearrangement 1 → 2 is slightly exothermic; however, this reaction does not seem to hold enough chemical energy to explain the described explosion by thermal runaway. The incident was rather caused by isothiocyanatopropa-1,2-diene (2) in the bottoms fraction of the distillation of propargyl 6 ACS Paragon Plus Environment

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thiocyanate (1), which was formed at elevated temperatures and insufficient vacuum. Our experience with compound 2 shows that no thermal or mechanical energy input is needed to trigger an explosion-like polymerization of 2. The mere self-heating of the neat substance is enough to cause a fatal ignition. When 1 is synthesized in practical courses, students have sometimes problems with efficient vacuum and thus perform the distillation of this product at higher temperatures. Furthermore, they can get the wrong impression from published1–3 boiling points that the distillation with bath temperatures of 100 °C or even higher temperatures is safe and without potential hazard. We strongly recommend to add safety advice to the corresponding literatureknown protocols of textbooks.1,2 The vacuum distillation of 1 should be conducted with lower bath temperatures (80 °C or even lower) and possibly with an additional protective shield. Moreover, at the end of the distillation or if the procedure has to be interrupted, the apparatus should cool down before slow air inlet is initiated. Since [3,3]-sigmatropic rearrangement can also occur in the case of substituted propargyl thiocyanates4,5a,b and unwanted formation of the corresponding allenyl isothiocyanates can result in an exothermic polymerization process, vacuum distillation of such thiocyanates7 should likewise be performed at lower temperatures and with appropriate precautions. Especially in practical courses, scaling down the batch will additionally decrease the risk. It is remarkable that in the first attempt of preparing 1, published in 1873, the distillation of this product was conducted at too high temperature, which led to complete decomposition of the compound.3e Most probably, the amount of 1 was low and no vigorous reaction was observed.8 On the other hand, distillation of allyl thiocyanate at normal pressure resulted in isomerisation yielding allyl isothiocyanate.9 These early experiments achieved the very first [3,3]-sigmatropic rearrangement already in 1875.9,10 Even with boiling allyl isothiocyanate (b.p. 150–152 °C), to the best of our knowledge, no highly exothermic polymerization has been reported. 7 ACS Paragon Plus Environment


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CONCLUSION

This article describes a deflagration during the vacuum distillation of propargyl thiocyanate. In all probability, this incident originated from very exothermic polymerization of isothiocyanatopropadiene, which was formed by [3,3]-sigmatropic rearrangement of the thiocyanate. Therefore, propargyl thiocyanate should be distilled with efficient vacuum and relative low bath temperatures (≤ 80 °C). Furthermore, appropriate precautions, such as an additional protective shield, are also useful in view of the intrinsic dangerous properties of alkynes, especially small molecules with a terminal C≡C bond.11 EXPERIMENTAL SECTION

Equipment

Isothermal reaction calorimetry was performed using a Mettler-Toledo Reaction Calorimeter RC1e. Differential thermal analysis was performed using a PerkinElmer TGA 7 Thermogravimetric analyzer.

Procedures Propargyl thiocyanate (1)1–3 (CAS 24309-48-6) and isothiocyanatopropa-1,2-diene (2)4 (CAS 137768-73-1) were synthesized according to literature known procedures. By DTA of propargyl thiocyanate (1) (1.578 mg heated from 30 to 250 °C with a heating rate of 10 K/min in 20 mL/min nitrogen flow), a reaction enthalpy value of ΔH = –236.5 kJ/mol was determined for the conversion of 1 to allene 2 accompanied by direct polymerization.

Isothermal reaction calorimetry of isothiocyanatopropa-1,2-diene (2) was performed on a 15.4 m% (percent by weight) concentrated solution of 48.0 g (0.494 mol) of allene 2 in anhydrous toluene (303 mL). The concentration of this solution was also confirmed by 1H NMR 8 ACS Paragon Plus Environment

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spectroscopy. After calibrating the system with pure toluene (determination of calibration factor UA, formula 1),12 the freshly prepared solution of 2 in toluene was introduced into the reaction calorimeter vessel before spontaneous self-heating could occur. The reactor temperature TR was kept constant at 30 °C. The heat flow Qflow was recorded until no significant change of the reactor jacket temperature TJ was detected anymore and the reaction presumably stopped. Qflow = UA (TR–TJ)

(1)

The allene content of the toluene solution was determined after completion of the calorimetry by 1H NMR spectroscopy to be 5.2 m%, which equals 66% conversion. The heat of polymerization was calculated to be ΔH = –218.5 kJ/mol.

AUTHOR INFORMATION

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

We are thankful to our cooperation partners of Technical and Physical Chemistry Departments of the Chemnitz University of Technology. We thank especially our colleagues Dipl.-Chem. René Schmidt for the support in conducting the reaction calorimetry and Dr. Cornell Wüstner for the help in performing the DTA.

REFERENCES (1) Brandsma, L. Synthesis of Acetylenes, Allenes and Cumulenes, Methods and Techniques; Elsevier: Amsterdam, 2004; p 394. 9 ACS Paragon Plus Environment


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(2) Brandsma, L.; Verkruijsse, H. D. Synthesis of Acetylenes, Allenes and Cumulenes, a Laboratory Manual; Elsevier: Amsterdam, 1981; p 227. (3) (a) Møllendal, H.; Konovalov, A.; Guillemin, J.-C. J. Phys. Chem. A 2010, 114, 2300– 2305. (b) Midtgaard, T.; Gundersen, G.; Nielsen, C. J. J. Mol. Struct. 1988, 176, 159–179. (c) Yura, Y. Chem. Pharm. Bull. 1962, 10, 1094–1098. (d) Austin, P. W. (Imperial Chemical Industries PLC) Substituted alkynic thiocyanates, biocidal compositions containing them and uses of such compositions. Eur. Pat. Appl. EP 244962, 1987 [Chem. Abstr. 1988, 108, 89483d]. (e) Henry, L. Ber. Dtsch. Chem. Ges. 1873, 6, 728–730. (f) Vizgert, R. V.; Sendega, R. V. J. Org. Chem., USSR (Engl. Transl.) 1969, 5, 475–478. (4) (a) Banert, K.; Hückstädt, H.; Vrobel, K. Angew. Chem. 1992, 104, 72–74; Angew. Chem. Int. Ed. Engl. 1992, 31, 90–92. (b) Banert, K.; Hückstädt, H.; Groth, S.; Lehmann, J.; Schlott, J.; Vrobel, K. Synthesis 2002, 1423–1433. (5) (a) Banert, K.; Jawabrah Al-Hourani, B.; Groth S.; Vrobel, K. Synthesis 2005, 2920– 2926. (b) Jawabrah Al-Hourani, B.; Banert, K.; Gomaa, N.; Vrobel, K.; Tetrahedron 2008, 64, 5590–5597. (c) Jawabrah Al-Hourani, B.; Banert, K.; Rüffer, T.; Walfort, B.; Lang, H. Heterocycles 2008, 75, 2667–2679. (d) Jawabrah Al-Hourani, B.; Richter, F.; Vrobel, K.; Banert, K.; Korb, M.; Rüffer, T.; Walfort, B.; Lang, H. Eur. J. Org. Chem. 2014, 2899–2906. (e) Banert, K.; Bochmann, S.; Hagedorn, M.; Richter, F. Tetrahedron Lett. 2013, 54, 6185– 6188. (6) Reviews: (a) Banert, K. Liebigs Ann./Recueil 1997, 2005–2018. (b) Banert, K. Targets in Heterocyclic Systems 2000, 3, 1–32. (7) Meijer, J.; Vermeer, P.; Bos, H. J. T.; Brandsma, L. Recl. Trav. Chim. Pays-Bas 1974, 93, 26–29. (8) Unfortunately, the author in ref. (3e) did not give any detail of the distillation, such as amount of the starting compound, temperature, boiling point, or pressure.


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(9) (a) Billeter, O. Ber. Dtsch. Chem. Ges. 1875, 8, 462–466. (b) Gerlich, G. Justus Liebigs Ann. Chem. 1875, 178, 80–91. (c) Emerson, D. W. J. Chem. Educ. 1971, 48, 81–83. (10) (a) Hansen, H.-J. Chimia 1999, 53, 163–173. (b) Hansen, H.-J. Chimia 2000, 54, 105– 119. (11) Jäger, V.; Viehe, H. G. In: Müller, E., Ed.; Houben–Weyl, Methoden der organischen Chemie, Vol. 5/2a; Thieme: Stuttgart, 1977, pp 4–7. (12) Zogg, A.; Stoessel, F.; Fischer, U.; Hungerbühler, K. Thermochim. Acta 2004, 419, 1– 17.


Chemical Safety Note: Explosion Hazard during the Distillation of

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