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Edelweiss Chemical Science Journal (ISSN 2641-7383)

Do Thermally Mild Chemical Reactions (for Avoiding Fire Accidents) Give Rise to Unexpected Products?

Takashiro Akitsu*, Yuika Onami and Natsuki Katsuumi

Avaliable from October, 2020

Abstract

One of the most probable and frequent reasons for a chemical experiment fire accident may be dangerous heating as well as the reagent itself (e.g. volatility and ignition) during a chemical reaction. Some reaction methods without heating (such as long-time stirring at room temperature) and using advanced tools (such as a microfluidic device), which are associated with unusual intermediates kinetically and may result in yielding unexpected products thermodynamically. This paper focused on two complicated examples of crystal structure analysis of unexpected and/but normal products.

Introduction

 

Generally, heating during chemical reactions are common procedure to enhance reaction rate, though it may be common reason for fire accidents in chemical laboratory [1]. Not only for avoiding fire but also for obtaining products efficiently, alternative experimental conditions or methods can be selected at room temperature. One may be long-time reaction can help slow rate not to exceed activation energy to become usual intermediate kinetically. Another may be using sophisticated tools such as a microfluidic device, in which moving solvents, encounter of the reactant can be caused in a short time. Additionally, limited special environment may result in formation of unusual products thermodynamically [2].


In recent years, we have studied on preparation of (chiral) Schiff base metal complexes (Scheme 1) in hot alcohol solutions for several hours [3]. As for copper(II) complexes having chelate Schiff base ligands, typical reaction scheme can be described as Scheme 1.

 

Scheme1:Typical reaction scheme of metal complexes having (chiral) Schiff base ligands

 

However, some unusual reactions yielded products of single crystals, of which structure analysis was complicated because there was also possibility of unexpected products. According to such working hypothesis, two strange examples after reactions of failed conditions found in our group are introduced below.

Results and Discussion

 

Case 1: “Excess” hydrate of copper(II) acetate, (Cu(COOCH3)2.H2O).0.5H2O

As recovery of the reactant (namely, no reaction occurred), copper(II) acetate hydrate as a metal source was found by X-ray crystallography. However, its cell constants and composition (containing 0.5 crystalline water) were different from the original and re-determination reports. Adsorption of crystalline solvents under different reaction conditions was expected firstly [4]. Due to too large thermal displacement parameters for 0.5 H2O, re-calculation after removing weak diffraction data in reciprocal space were carried out (We soon realized the mistake, so we didn't leave any wrong results.). Then it was solved as the following unit cell data: C2/c, a=13.192(9), b=8.595(6), c=13.867(10)Å, β=116.913(11)°, V=1402.0(18) Å, being identical to the original report (Figure 1).

 

Figure1:Re-determined crystal structure of Cu(COOCH3)2.H2O as recovery of the reactant, which was identical to the original report

 

Case 2: Not monoclinic but “triclinic” polymorphism of 2-hydroxy-1-naphthaldehyde (C11H8O2)

As recovery of the reactant, a single crystal of 2-hydroxy-1-naphthaldehyde (instead of salicylaldehyde, o-C6H5(OH)(CHO)= C11H8O2) was obtained and its crystal structure was determined at low temperature (173K). An analysis program automatically judged to be triclinic P-1 with Z=4 based on unit cell constants (a=5.626(2), b= 9.360(4), c=15.522(7)Å, α=90.027(5), β= 80.695(6), γ= 90.087(6)°, V=806.7(6) Å. It cannot be negligible deviation of α and γ from exact 90 degrees according to standard deviation reasonably. Two crystallographically independent molecules in asymmetric unit were perfectly identical, which suggest n-glide plane as a proof of monoclinic P21-/n being similar to the original report measured at room temperature [5]. Therefore, we judged again systematic absence (0k0) manually and confirmed crystallographic symmetry of unit cells, namely, Laue group 2/m (of course, monoclinic crystal system) based on both 21 screw axis and n-glide plane. In this way, it was not polymorphism (same compounds taking different crystal structures) by phase transition but wrong determination of crystal system (Figure 2).

 

Figure2:Re-determined crystal structure of 2-hydroxy-1-naphthaldehyde at 173K as recovery of the reactant, which was identical to the original report.

 

Conclusion

 

In the two examples, humans corrected the crystallographic mistakes by the programs, but it can also be interpreted that the reaction conditions produced “subtle” products. In other word, this area is suspicious. Indeed, novel methods of chemical reaction or special condition of crystallization may result in thermodynamically unexpected products or unusual isolation of kinetically favored products, novel asymmetric synthesis of optical isomer (or racemization), unstable tautomer, chiral crystallization, new phase (polymorphism) of crystals, inclusion of guest molecules and so on, respectively. However, careful experiments must be also required to discuss such “delicate” chemistry.

 

Acknowledgement

 

This work was supported by a Grant-in-Aid for Scientific Research (A) KAKENHI (20H00336).

 

References

  1. Mizuno K, Ueki K and Akitsu T. Case studies on predictability in university chemistry experiment accidents (2018) Int J Fire Sci Technol 37: 1-16.https://www.jstage.jst.go.jp/article/fst/37/1/37_1/_article/-char/en
  2. Yoon DH, Jamshaid A, Ito J, Nakahara A, Tanaka D, et al. Active microdroplet merging by hydrodynamic flow control using a pneumatic actuator-assisted pillar structure (2014) Lab Chip 14: 3050-3055. https://doi.org/10.1039/c4lc00378k
  3. Nakagame R, Tsaturyan A, Haraguchi T, Pimonova Y, Lastovina T, et al. Photochemical reaction of amino acid Schiff base derived Cu complexes with extended π-system and their titanium oxide composites (2019) Inorg Chim Acta 486: 221-231. https://doi.org/10.1016/j.ica.2018.10.034
  4. Van Niekerk JN and Schoening FRL. A new type of copper complex as found in the crystal structure of cupric acetate, Cu2(CH3COO)4.2H2O (1953) Acta Crystallogr 6: 227-232. https://doi.org/10.1107/S0365110X53000715
  5. Maniukiewicz W and Bukowska-Strzyzewska M. Structure of 2-hydroxy-1-naphthaldehyde (1992) Acta Cryst C48: 1324-1326. https://doi.org/10.1107/S0108270191013628

 

Corresponding author

Takashiro Akitsu, Department of Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan, E-mail: akitsu2@rs.tus.ac.jp

 

Citation

Akitsu T, Onami Y and Katsuumi N. Do thermally mild chemical reactions (for avoiding fire accidents) give rise to unexpected products? (2020) Edelweiss Chem Sci J 3: 15-16.

Keywords

Chemical reaction, Temperature, Schiff base metal complexes, Unexpected products, Crystal structure analysis.