SUN Hu-Yun LI Ming-Jing LIU Yong-Jun

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A DFT Study on the Reaction Mechanism Involved in the Synthesis of Sodium Azide vi1a Hydrazine Hydrate Method①

SUN Hua-YunaLI Ming-JingaLIU Yong-Junb②

a(256414)b(250100)

Sodium azide is a widely used inorganic compound. Besides the commonly used method of “Wislicenus process” which uses ammonia, nitrous and sodium as materials, the hydrazine hydrate route is also employed for the preparation of sodium azide particularly in laboratory. However, because many species are involved in the reaction system, the reaction details for the hydrazine hydrate route are still unclear. A comprehensive understanding of the reaction mechanism may provide meaningful help for optimizing the production process. In this work, the reaction mechanism for the synthesis of sodium azide by hydrazine hydrate route has been studied using density function theory (DFT) method. On the basis of our calculations, the reaction details, including the energetics of ten elementary steps, the structures of intermediates and transition states as well as the influence of inorganic acids and alcohols, were illuminated at the atomistic level. Both the two steps, the generation of key intermediate (NH2-NH-NO) and thetransformation of NH2-NH-NO, are suggested to be the possible rate-limiting step, corresponding to the energy barriers of 20.3 and 22.7 kcal/mol, respectively. In the early reaction steps to generate NH2-NH-NO, the main role of sulphuric acid is to donate proton, which can be replaced by nitric acid or hydrochloric acid. From the energy point of view, isopropanol has similar reactivity as methanol and ethanol.

sodium azide, hydrazine hydrate, DFT method, isopropanol, sulphuric acid;

1 INTRODUCTION

Sodium azide is acolorless inorganic com- pound with the formula NaN3. It has long been applied in relative small amounts by industry to produce heavy metal azides for use in explosive detonators, to introduce the azide functional group by displacement of halides in organic synthesis[1, 2], and to produce cephalosporin[3]and sartan drugs[4]for treating hypertensive patients. Small quantities of this high toxic substance are also used as radical scavenger in laboratory preservatives, in the produc- tion of biocides, in anti-corrosion solutions and to inflate airline safety chutes, and in the formulation of getters in electric discharge tubes[5-9]. Over the past two decades, production of sodium azide was surged to meet the new demand for automobile air- bag inflator propellant[10]. Upon impact, an initiator squib is fired by an electromechanical trigger that causes the sodium azide to thermally decompose,yielding the nitrogen gas that inflates the air bag momentarily[11]. The United States National Highway Traffic Administration (NHTSA) mandated that new passenger vehicles sold in the United States after 1996 should have both driver-side and passen- ger-side air bags. Therefore, sodium azide demand exceeds 5 million kg in the U.S. by 1995 and has quickly grown as more new vehicles fitted with driver-side and passenger-side airbags[10].

The common synthesis method is the “Wisli- cenus process”[10, 12], which uses ammonia, nitrous and sodium as materials, and provides the basis of industrial route. In the first step, ammonia is conver- ted to sodium amide:

2Na + 2NH3→ 2NaNH2+ H2(1)

The sodium amide is subsequently combined with nitrous oxide:

2NaNH2+ N2O → NaN3+ NaOH + NH3(2)

The final product is a white crystalline solid. The capacity exceeds 9 × 106kg each in the U.S[10].

Another production process is the so called “hydrazine hydrate method”[13]. Nitrite ester is firstly generated by the reaction of sodium nitrite with alcohol (usually the ethanol is used), then, nitrite ester is converted to sodium azide using hydrazine hydrate:

2NaNO2+ 2ROH + H2SO4→

RON=O + Na2SO4+ 2H2O (3)

RON=O + NaOH + NH2-NH2?H2O →

NaN3+ ROH + 3H2O (4)

In reaction (4), sodium azide is prepared by feeding nitr ite ester gas into the alcohol solution of hydrazine hydrate and sodium hydroxide. This method has highyield and is easy to operate. However, it has two fatal flaws, limiting its indus- trial application. On one hand, a large amount of alcohol is used as reactant and solvent, which pro- duces a great quantity of waste liquid. It unavoidably contains small amounts of raw materials and product, and is unfriendly to the environment. On the other hand, the reaction occurs in the gas-liquid interface, and thereby the reactionefficiency is low and the period is relatively longer. In 2014, Yangimproved this production process through a water treatment technique[13], which employs water as the reaction medium. Although the improved method can greatly decrease the consumption of alcohol and reaction time, the reaction should be carried out under high pressure, and the production process is hard to control. As we all know, nitrite ester has high toxicity and low boiling point (290 ℃ at 1 atm), which is an undesirable intermediate in practice. Therefore, our laboratory aims to improve the hydrazine hydrate method, probably by changing the inorganic acid, alcohol or solvent. Based on this, we want to know more details regarding the reaction mechanism of the hydrazine hydrate method. For example, what is the detailed reaction mechanism about the synthesis of sodium azide? Which elementary step is rate-determining? Can we use other alcohol to replace ethanol?

By investigating literatures, we found that there is still no systematic study about the reaction mecha- nism of hydrazine hydrate method. Therefore, in this work, the reaction mechanism of the hydrazine hydrate route was studied using density function theory (DFT) method[14-16]. This theoretical method has been successfully applied in studying reaction mechanisms[17-22]. On the basis of our calculations, the whole reaction process, the energetics of the ten elementary steps, the structures of intermediates and transition states as well as the influences of inor- ganic acids and organic reagent, were illuminated at atomistic level.

2 COMPUTATIONAL METHODS

The reaction mechanism was modeled using Gaussian 09 software[23]. GaussView 5.0 software[24]was used for the visualization of molecular struc- tures and vibrational motions. All of the calculations were performed by employing DFT method with B3LYP functional and 6-311++G(d, p) basis set for all atoms, unless otherwise mentioned[25, 26]. All transition states were fully optimized and harmonic vibrational frequencies were calculated in order to confirm the transition state only has one single imaginary frequency. The intrinsic reaction coor- dinate (IRC) pathways were computed to verify that the two desired minima points are connected by the transition state. Solvent effects were treated through the polarizable continuum model (PCM) in water medium at 25 ℃[26, 27]. The structures obtained at the gas phase were further optimized to include the solvent effect. In order to obtain more reliable ener- gies, single point energy calculations were done at the PCM//B3LYP/6-311++G(2d, 2p) level.

3 RESULTS AND DISCUSSION

Based on our calculations, the overall reaction contains ten elementary steps, as shown in Fig. 1. For a clear description, the reaction cycle can be divided into two distinct chemical parts: the first- half reaction (step 1 to step 4) corresponding to the synthesis of alkyl nitrite, and the second-half reac- tion (step 5 to step 10) to the generation of sodium azide. In the first-half reaction, the key intermediate alkyl nitrite is generated, in which the sulfuric acid, sodium nitrite and alcohol are involved in the reaction. In the second-half reaction, the generated alkyl nitrite reacts with hydrazine hydrate, genera- ting the final product sodium azide via a series of intermediates and transition states. To explore how the inorganic acid and alcohol will influence the reaction barrier, in step 2, the commonly used sul- furic acid was replaced by nitric acid or hydrochloric acid, and in step 3 to step 5 isopropanol was repla- ced by methanol or ethanol.

Fig. 1. Proposed mechanism for the synthesis of sodium azide based on our calculations

3. 1 The first-half reaction

The first-half reaction (step 1 to 4) corresponds to the synthesis of alkyl nitrite. The optimized intermediates and transition states are displayed in Fig. 2. In the first step from R to IM1, NO2-is converted to HNO2by abstracting a proton from H2SO4. This process is calculated to be very easy with an energy barrier of only 0.6 kcal/mol (Fig. 3). In the second step, HNO2abstracts another proton from H2SO4generating the high reactive species (NO+) with an energy barrier of 5.8 kcal/mol. In the third step, NO+is attacked by the hydroxyl oxygen of isopropanol. Meanwhile, another NO2-abstracts a proton from the hydroxyl of isopropanol, yielding the isopropyl nitrite. The forth step corresponds to the transformation ofisopropanol nitrite to itsisomer. During this process, the N=O group rotates around the N-O bond for facilitating the following reaction with hydrazine hydrate. As shown in Fig. 2, the dihedral angle of a-b-c-d changes from –179.89°in IM3 to 0.63°in IM481.70°in TS4. Although theform of isopropanol nitrite (IM3) was calculated to be slightly more stable than IM4 by 1.3 kcal/mol, theisomer has a relative little steric hindrance for the subsequent attack by hydrazine hydrate. The energy barrier of step 4 is only 14.4 kcal/mol, which means thetransformation can proceed at room temperature.

During the first-half reaction, inorganic acid is involved in steps 1 and 2, which are calculated to be very easy. Based on the above calculation results, the main role of inorganic acid is to provide protons to facilitate the formation of NO+. Thus, H2SO4can be replaced by HNO3or HCl. Table 1 lists the corres- ponding energy values when HNO3or HCl is used in the second step. One can see that the use of HNO3and HCl does not significantly increase the energy barrier of step 2 (The optimized structures are not given here).

Fig. 2. Optimized stationary points during the reaction process. Distances are given in angstrom and dihedral angles in degree

Fig. 3. Energy profile of the reaction process. Energies are given in kcal/mol

Table 1. Relative Energies (in kcal/mol) of the Optimized Species in Step 2

3. 2 The second-half reaction

The second half-reaction from step 5 to step 10 corresponds to the generation of sodium azide. In step 5, as shown in Fig. 1, the nitrogen of hydrazine hydrate combines with the nitrogen of isopropanol nitrite, and simultaneously the oxygen atom of isopropanol nitrite abstracts a proton from hydrazine hydrate, generating IM5. This step corresponds to an energy barrier of 20.3 kcal/mol. As shown in Fig. 2, the distance of N-N bond shortens from 3.69 ? in IM4 to 1.31 ? in IM5 via 1.85 ? in TS5, indicative of the formation of another key intermediate NO-NH-NH2(IM5). Accompanied by the forma- tion of IM5, isopropanol is regenerated. So far, the function of isopropanol as reactant has been finished.

The sixth step corresponds to a conformational change of IM5, in which the N=O double bond of IM5 rotates around the N-N bond to generate IM6. IM5 and IM6 are two isomers. Due to the con- jugated effect in IM5, the rotation around N-N bond is not very easy, corresponding to an energy barrier of 22.7 kcal/mol. From the energy profile in Fig. 3, one can see that this conformational change is the rate-determining step of the overall reaction process. During this step, the N-N bond length changes from 1.31 ? in IM5 to 1.54 ? in TS6, and returns to 1.31 ? in IM6. This means the N-N bond has the double bond characteristic in both IM5 and IM6. In other words, the transformation of IM5 to IM6 should break the-bond of N-N, thereby corresponding to a high energy barrier.

The seventh step corresponds to an intramolecular proton transfer of IM6 mediated by a water molecule. In TS7, the distance of N-H bond is 1.33 ? and the distance between O of N=O group and H of water is 1.39 ?, which reveals that this step proceeds in a concerted manner with an energy barrier of 13.8 kcal/mol. In the eighth step, the hydroxyl anion captures a proton from the amino group of IM7, generating IM8 with an energy barrier of 5.1 kcal/mol. In TS8, the hydroxyl anion is stabilized by forming three hydrogen bonds in which three water molecules have distances of 1.80, 1.89 and 1.91 ?, respectively. The ninth step corresponds to the departure of the hydroxyl anion from IM8, genera- ting hydrazoic acid. This step corresponds to an energy barrier of 10.2 kcal/mol. The last step is a typical acid-base reaction, in which the hydrazoic acid reacts with the sodium hydroxide to yield the final product sodium azide. Since it is a very fast process, we did explore it.

As has been previously reported[13], in the second- half reaction, methyl nitrite and ethyl nitrite can also be used for the synthesis of sodium azide. To explore the different reactivity of methyl nitrite, ethyl nitrite and isopropanol nitrite, methanol and ethanol were also used in the calculations of step 3 to step 5. The corresponding energy values are listed in Table 2 (The optimized structures are not given here). Table 2 shows that, when isopropanol was replaced by methanol and ethanol, the energy values are basi- cally unchanged, which means that, from the energy barrier point of view, all these three alcohols can be used in the synthesis of sodium azide. Considering isopropanol nitrite has high boiling point (39 ℃ at 1 atm) and lower toxicity than those of nitrite ester, isopropanol may be an alternative for the synthesis of sodium azide.

Table 2. Relative Energies (in kcal/mol) of Optimized Species for the Reaction Process from Steps 3 to 5

4 CONCLUSION

In the present work, the detailed reaction mechanism involving the synthesis of sodium azide via hydrazine hydrate method has been investigated using a density functional theory (DFT) approach. Details of each elementary step and the energetics of the whole reaction process have been calculated. The calculated results suggest that the whole reaction process contains ten elementary steps. Both the two steps, the generation of key intermediate (NH2-NH-NO) and thetransformation of NH2-NH-NO, are suggested to be possible rate- determining, corresponding to energy barriers of 20.3 and 22.7 kcal/mol, respectively. In the early reaction steps to generate NH2-NH-NO, the main role of sulphuric acid is to provide proton, which can be replaced by nitric acid or hydrochloric acid. From the energy point of view, isopropanol has similar reactivity as methanol and ethanol. Our results would provide useful information for the synthesis of sodium azide.

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24 August 2017;

2 May 2018

① This project was supported by the National Natural Science Foundation of China (21773138)

. Liu Yong-Jun, born in 1964, professor, majoring in physical chemistry. E-mail: yongjunliu_1@sdu.edu.cn

10.14102/j.cnki.0254-5861.2011-1813