Thermochemical properties of 4-(4,6-Dimethoxylpyrimidin-2-yl)-3-thio-allophonic acid ethyl ester

Ren Yinghui¹, Song Jirong^1,2 ,Xu Kangzhen¹, Ma Haixia¹, Huang Jie¹, Fu Dingwei¹ ,Yang Xuwu³
(¹ School of Chemical Engineering / Shaanxi Key Laboratory of Physico-Inorganic Chemistry, Northwest University, Xi'an Shaanxi 710069; ²Conservation technology department, the Palace Museum, Beijing 100009; ³ Department of Chemistry / Shaanxi Key Laboratory of Physico-Inorganic Chemistry, Northwest University, Xi'an Shaanxi 710069, China)

Abstract The title compound was synthesized by mixing 2-amino-4,6-dimethoxylpyrimidine, potassium thiocyanate and ethyl chloroformate in ethyl acetate. It was characterized by IR and element analysis. The specific heat capacity (C_p)was determined by a microcalorimetry apparatus Micro-DSC III, and C_p=0.331＋3.463×10^-3T (293.0≤T≤344.2K).The constant-volume combustion energy of the compound, (), was determined as (16766.90 ±9.81) J·g^-1 by a precise rotating-bomb calorimeter at 298.15K. Its standard molar enthalpy of combustion,, and standard molar enthalpy of formation, (), were calculated as (-4794.09 ± 2.81）kJ·mol^-1 and（-1438.63 ±3.11）kJ·mol^-1, respectively.
Keywords synthesis; specific heat capacity; microcalorimetry; constant-volume combustion energy; standard molar enthalpy of formation

Thioureas as a kind of highly active bactericide have been studied broadly for many years because they could prevent many crops from disease efficiently with little harm to crops and low toxicity to mammals ^[1]. Many research groups have made important contributions in this field ^[2-8].
    The specific heat capacity is one of the essential thermodynamic data for substance, and importent to the relevant engineering technologic design of energies and materials. In this paper, the specific heat capacity and the constant-volume combustion energy of the title compound, 4-(4,6-dimethoxylpyrimidin-2-yl)-3-thio-allophonic acid ethyl ester, were determined by a microcalorimetry apparatus----Micro-DSC III and a precise rotating-bomb calorimeter respectively. Its standard molar enthalpy of combustion and standard molar enthalpy of formation were calculated on the basis of the constant-volume combustion energy of the compound. The final results would provide theoretical basis for enlarging their application field.

2. RESULTS AND DISCUSSION
2.1 Specific Heat Capacity of the compound  
The enthalpies of solution () of crystalline KCl (S.R.) in deionized water was measured at 298.15K. The result =17.267±0.074 kJ·mol^-1 is in agreement with the recommended value reported in [10] (17.241±0.018 kJ·mol^-1), which shows that the calorimetric system was reliable.
    Continuous heating model in Micro-DSC  III apparatus was adopted in the experiment. It is convenient to measure the specific heat capacity of solid samples with this method. The specific heat capacity obtained by the experiments is a continuous variable temperature equation, and it is easy to deduce the specific heat capacity at a certain temperature according to the equation.
    The schematic diagram of C_p determination is shown in Fig.1.Calculated formula is as equation (1).
（1）
where C_p (J·g^-1·K^-1) is the heat specific capacity, A_s and A_b (W) are the real-time heat flows of the sample and the blank respectively, m_s (g) is the mass of sample, b (K·s^-1) is the heating rate. The software program of apparatus can deal with the data and figure out the results automatically. The determination results with 0.28877g sample and 0.1K·min^-1 heating rate was shown in Fig.2. The specific heat capacity of the title compound is obtained as C_p=0.331＋3.463×10^-3T (293.0≤T≤344.2).

Fig.1 Schematic diagram of continuous specific heat capacity

Fig. 2 Determination results of the specific heat capacity

2.2 Combustion energy of the compound
2.2.1 Experimental condition           
The constant-volume combustion energy of the compound was determined by a precise rotating-bomb calorimeter (RBC-typeⅡ). The analytical methods and main experimental procedures were described previously ^[11].
    The initial temperature was regulated to (25.0000 ± 0.0005) ℃, and the initial oxygen pressure was 2.5MPa. The correct value of the heat exchange was calculated according to Linio-Pyfengdelel-Wsava formula ^[12].
    The calorimeter was calibrated with benzoic acid of 99.999% purity.(Chengdu Chemical Reagent Company), which has an isothermal heat of combustion of -2643 J·g^-1 at 25℃. The calibrated experimental result was (17775.09±7.43) J·K^-1 (Table 1), and the precision was 4.18×10^-4. To determine the standard combustion energy of sulfur-containing compounds, the constant-volume combustion energy of thianthrene (mass fraction: 99％, recrystallized before using, Tokyo Kasei Kogyo Co. Ltd.) has been determined as being (-33507.76 ± 14.13) J·g^-1 (Table 2), which suits well with the reported value as being(-33468 ± 4) J·g^-1 . The precision and the accuracy were 4.22×10^-4 and 1.19×10^-3 , respectively.
    The analysis methods of the final products (gas, liquid and solid) were the same as these in ref.[11].The analytical results of the final products indicated that the combustion reactions were complete. The calibrated experimental results were summarized in Table 1.　

Table 1 Result for calibration of energy equivalent of the rotating-bomb calorimeter at 298.15K

2.2.2 Constant-volume combustion energy of the compound                                
The methods of determination and calculation of the constant-volume combustion energy for the compound are the same as for the calibration of the calorimeter with benzoic acid. The values are calculated by the following equation:
(2)
Where (the title compound, s) denotes the constant-volume combustion energy of the compound (in J·g^-1), W is the energy equivalent of the rotating-bomb calorimeter (in J·K^-1 ), a is the length of the actual Ni-Cr wire consumed (in cm), G is the combustion enthalpy of Ni-Cr wire for ignition (0.9 J·cm¹), 5.97 is the formation enthalpy and solution enthalpy of acid corresponding to 1mL of 0.1000mol·L^-1 solution of NaOH (in J·mL^-1), b is the volume (in mL) of consumed 0.1000 mol·L^-1 solution of NaOH and is the correct value of the temperature rise. m is the mass (in g) of the title compound. The experiment results of the combustion energy of the compound were given in Table 2.

Table 2 Experimental results for the combustion energies of the sample

2.2.3 Standard combustion enthalpy of the compound
The standard molar combustion enthalpy of the compound,, refers to the combustion enthalpy change of the following ideal combustion reaction at 298.15K and 100kPa.
        (3)
The standard molar combustion enthalpy of the compound is calculated by the following equations:
(4)
(5)
Where n_g is the total amount in mole of gases present as products or as reactants, R=8.314J·K^-1·mol^-1,T=298.15K.The result of the calculation of is (-4794.09 ± 2.81）KJ·mol^-1.
2.2.4 Standard molar enthalpy of formation for the compound
The standard molar enthalpy of formation of the compound,, is calculated by Hess's law according to the above thermochemical equation (3)

Where
    The result of the calculation of is（-1438.63±3.11）KJ·mol^-1.
    The standard molar enthalpy of formation of the compound is negative, which indicates that the compound's stability is better, and it is easy for preparation, reserve and application.

REFERENCES
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化合物4-(4,6-二甲氧基嘧啶-2-基)-3-硫代脲酸乙酯的热化学性质研究
任莹辉¹，宋纪蓉^1,2，徐抗震¹，马海霞¹，黄洁¹，傅丁薇¹，杨旭武³
（¹西北大学化工学院/陕西省物理无机化学重点实验室，陕西 西安 710069; ²故宫博物院文保科技部 北京 100009 ;³西北大学化学系/ 陕西省物理无机化学重点实验室，陕西 西安 710069)
摘要 用2-氨基-4,6-二甲氧基嘧啶，硫氰酸钾和氯甲酸乙酯在乙酸乙酯中反应合成了标题化合物，通过红外光谱和元素分析法对化合物进行表征。用Micro-DSC Ⅲ微量量热仪测定了化合物的比热容C_p=0.331＋3.463×10^-3T (293.0≤T≤344.2K)。在298.15K下，用精密转动弹热量计测定出化合物的恒容燃烧能为(16766.90±9.81) J·g^-1，计算出标准摩尔燃烧焓和标准摩尔生成焓分别为(-4794.09 ± 2.81）KJ·mol^-1和（-1438.63±3.11）KJ·mol^-1 。
关键词 合成；比热容；微量量热仪；恒容燃烧能；标准摩尔生成焓

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