請(qǐng)問由碳酸二甲酯合成甲基異氰酸酯的反應(yīng)方程式

老師：您好！ 我已經(jīng)多次請(qǐng)教由碳酸二甲酯合成甲基異氰酸酯的反應(yīng)方程式，可就是無人能回答，孟老師能告訴我嗎？

咨詢者：劉先生　咨詢時(shí)間：2010-04-08　狀態(tài)：已回復(fù)

甲基異氰酸酯有幾種生產(chǎn)方法.多半以甲胺酰氯為原料.但未見以碳酸二甲酯為原料的方法.要因地制宜搞創(chuàng)新,首先就要考慮原料成本和實(shí)施可能性.
介紹一篇美國(guó)專利提到的一些方法供你和你的同事參考.如要進(jìn)一步討論的話,請(qǐng)保持聯(lián)絡(luò).

United States Patent 4,255,350
Aliev , et al. March 10, 1981
This invention relates to processes for preparing isocyanates and more particularly to the process of preparing methyl isocyanate which is useful as starting material in the manufacture of e.g. N-methylcarbamino acid esters possessing a wide range of the activity against insects.

2. Description of the Prior Art

Isocyanates currently find extensive application in chemical industry. Development of processes for preparing isocyanates is in progress now, and is aimed at providing compositions and reaction conditions which offer higher yields.

Known in the art is a process for preparing methyl isocyanates (U.S. Pat. No. 3,644,461) by reacting phosgene with methyl amine or with chlorine hydrate of the same at 250.degree. C. giving rise to a formation of N-methyl carbamoyl chloride and decomposition thereof until methyl isocyanate is formed: ##STR1##

Gaseous methyl amine and phosgene are reacted at the first stage in a continuous fashion at a temperature range of 350.degree. to 400.degree. C. and atmospheric pressure. The reaction gases, which are represented by N-methyl carbamoyl chloride and HCl, are flown through a tower wherein carbon tetrachloride cooled to -10.degree. C. is sprayed to cool the reaction gases to 65.degree.-70.degree. C.

A solution of methyl carbamoyl chloride in carbon tetrachloride thus obtained is passed to a tower to strip unreacted phosgene. Next, hydrogen chloride is split off methyl carbamoyl chloride present in solution with carbon tetrachloride by methyl aniline at 45.degree. to 50.degree. C. to produce methyl isocyanate. Methyl isocyanate is prepared in two consecutive towers. The yield of methyl isocyanate is .apprxeq.90% of the theoretical. The methyl isocyanate obtained contains 98% of the basic substance and 2% carbon tetrachloride.

Though a rather high yield of methyl isocyanate is provided by this process, it still follows a multi-stage procedure, may be carried out with a complicated equipment, is based on costly and what is extremely important on highly toxic starting materials. Also, the hydrochloric acid produced in the process causes the equipment to corrode. Difficulties are also met with in regeneration of auxiliary substances involved in the process.

Thus, waste materials from the regeneration of dimethyl aniline and carbon tetrachloride need to be subjected to thermal decomposition with resulting environmental pollution.

Other prior art processes for preparing methyl isocyanate include such as by thermally decomposing N,N-diphenyl-N'-methylurea at 240.degree. to 249.degree. C. (Siefken, Ann. (1949) 562, 75) or phenyl-N-methyl carbamate at 240.degree. to 250.degree. C. (Japanese Patent Publication No. 1652-73).

Methyl isocyanate may also be prepared by reacting methyl amine with diphenyl carbonate at a temperature of 200.degree. C. but lower than the boiling point of diaryl carbonate (Japanese Patent Publication No. 20534-73) or by reacting N,N-dimethyl area with diaryl carbonate in a mole ratio of 1:1.5 to 1.4 and temperature of 200.degree.-300.degree. C. (Japanese Patent Publication No. 1652-73).

To practice these processes, however, there is need for scarce materials, which are produced in the processes involving phosgene and scarce amines.

U.S. Pat. No. 3,584,028 discloses a process for the manufacture of organic isocyanates having from C.sub.1 to C.sub.20 in the carbon group comprising reacting an alkali metal cyanate and/or alkaline earth metal cyanate with organic chlorides in an organic solvent in the presence of a catalyst, namely an alkali metal bromide or alkaline earth metal bromide, or alkali metal iodide, or alkaline earth metal iodide at elevated temperature.

To practice the process a reaction mixture conjointly contains 0.5 to 5 moles, preferably 0.8 to 1 mole of the cyanates, 0.01 to 10 moles, preferably 0.5 to 0.25 mole of the catalyst, 10 to 100 moles, preferably 25 to 50 moles of the organic solvent, per 1 mole of the organic chloride. The reaction temperature is 25.degree. to 300.degree. C., preferably 50.degree. to 150.degree. C., pressure may be from below 1 to 703 kg/cm.sup.2, and the time of contacting equals 50 min.

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