Ammonium isocyanate

A leading research idea in the laboratory was the hypothesis that atoms within a molecule are in constant motion, thus explaining the mechanisms of tautomerism. For example, the transformation of ammonium isocyanate to urea can be explained by motion of a hydrogen atom 35... 

The history of organic synthesis is generally traced back to Wohler s synthesis of the natural product urea from ammonium isocyanate in 1828. This laid to rest the vis vitalis (vital force) theory, which maintained that a substance produced by a living organism could not be produced synthetically. The discovery had monumental significance, because it showed that, in principle, aU organic compounds are amenable to synthesis in the laboratory. 

Ammonium isocyanate O = C = N— NH, is converted into urea by heat. 

Those containing alcoholic radicals may be obtained, as urea is obtained from ammonium isocyanate, from the oyanate of the corresponding compound ammonium or by the action of NHsj or of the compound ammonias, upon the cyanic ethers. 

The year 1828 was a great one for chemistry. In Germany, Friedrich Wohler demolished the division between natural and synthetic chemicals when he converted ammonium isocyanate into urea. In Holland, Conrad Van Houten accomplished something that would have consequences that were almost as far-reaching. What did he do ... 

Wohler s synthesis A synthesis of urea performed by Friedrich Wohler in 1828. He discovered that urea (CO(NH2)2) was formed when a solution of ammonium isocyanate (NH4NCO) was evaporated. At the time it was believed that organic substances such as urea could be made only by living organisms, and its production from an inorganic... 

With the probable exception of Wohler s isomerization of ammonium isocyanate to urea, most of the major advances in organic chemistry have been preceded and presaged by a number of diverse and perhaps minor advances. In some cases the early work was recognized, extended and built upon. In other cases, early examples of particular phenomena were recognized only after a general statement of principles had been offered. Phase transfer catalysis (ptc) is a major advance which is preceded by earlier examples of related phenomena but most if not all of these examples were recognized as examples of phase transfer catalysis only after the principles had coalesced in several minds across the world. 

Wohler s synthesis The synthesis of urea from the evaporation of a solution of ammonium isocyanate. First carried out by Friedrich Wohler in 1828, the synthesis was significant at the time as organic substances were thought to be produced only by living organisms. 

FIGURE 1.1 Conversion of ammonium isocyanate to urea. Note. I hope you are familiar with the use of a capital Greek delta, A, to indicate heat. New symbols and acronyms will be explained the first time they are used, but they are also collected together in Appendix A. 

A simpler nonphosgene process for the manufacture of isocyanates consists of the reaction of amines with carbon dioxide in the presence of an aprotic organic solvent and a nitrogeneous base. The corresponding ammonium carbamate is treated with a dehydrating agent. This concept has been apphed to the synthesis of aromatic and aUphatic isocyanates. The process rehes on the facile formation of amine—carbon dioxide salts using acid haUdes such as phosphoryl chloride [10025-87-3] and thionyl chloride [7719-09-7] (30). 

The nitrogen of aHphatic and aromatic amines is alkylated rapidly by alkyl sulfates yielding the usual mixtures. Most tertiary amines and nitrogen heterocycles are converted to quaternary ammonium salts, unless the nitrogen is of very low basicity, eg, ia tn phenylamine. The position of dimethyl sulfate-produced methylation of several heterocycles with more than one heteroatom has been examined (22). Acyl cyanamides can be methylated (23). Metal cyanates are converted to methyl isocyanate or ethyl isocyanate ia high yields by heating the mixtures (24,25). 

Cyanuric acid can also be prepared from HNCO (100). Isocyanic acid [75-13-8] can be synthesized directiy by oxidation of HCN over a silver catalyst (101) or by reaction of H2, CO, and NO (60—75% yield) over palladium or iridium catalysts at 280—450°C (102). Ammonium cyanate and urea are by-products of the latter reaction. 

The isocyanurate reaction occurs when three equivalents of isocyanate react to form a six-membered ring, as shown in the fifth item of Fig. 1. Isocyanurate linkages are usually more stable than urethane linkages. Model compound studies show no degradation of the trimer of phenyl isocyanate below 270°C [10,11]. Catalysts are usually needed to form the isocyanurate bond. Alkali metals of carboxylic acids, such as potassium acetate, various quaternary ammonium salts, and even potassium or sodium hydroxide, are most commonly used as catalysts for the isocyanurate reaction. However, many others will work as well [12]. 

The hydroxyl derivative of X-CN is cyanic acid HO-CN it cannot be prepared pure due to rapid decomposition but it is probably present to the extent of about 3% when its tautomer, isocyanic acid (HNCO) is prepared from sodium cyanate and HCI. HNCO rapidly trimerizes to cyanuric acid (Fig. 8.25) from which it can be regenerated by pyrolysis. It is a fairly strong acid (Ka 1.2 x 10 at 0°) freezing at —86.8° and boiling at 23.5°C. Thermolysis of urea is an alternative route to HNCO and (HNCO)3 the reverse reaction, involving the isomerization of ammonium cyanate, is the clas.sic synthesis of urea by F. Wohler (1828) ... 

A new ionic polymeric polycarbamate was synthesized after steps of polyurethane chemistry using 3-iso-cyanatemethyl-3,5,5-trimethylcyclohexyl isocyanate, 2,5-dimethyl-2,5-dihydroperoxyhexane, 1,6-butanediol, 2,4-tolylenediisocyanate, and N,N -bis(j3-Hydroxy-ethyOpiperazine [27]. Modification of the nitrogen of the piperazine ring into quaternary ammonium salt by treatment with methyliodide gave the MPI high electroconductivity. 

Trimerization to isocyanurates (Scheme 4.14) is commonly used as a method for modifying the physical properties of both raw materials and polymeric products. For example, trimerization of aliphatic isocyanates is used to increase monomer functionality and reduce volatility (Section 4.2.2). This is especially important in raw materials for coatings applications where higher functionality is needed for crosslinking and decreased volatility is essential to reduce VOCs. Another application is rigid isocyanurate foams for insulation and structural support (Section 4.1.1) where trimerization is utilized to increase thermal stability and reduce combustibility and smoke formation. Effective trimer catalysts include potassium salts of carboxylic acids and quaternary ammonium salts for aliphatic isocyanates and Mannich bases for aromatic isocyanates. 

Silylation of amino acids such as r-leucine 180 with TCS 14 gives rise to the O-silylated ammonium salt 181, which reacts selectively with triphosgene and triethylamine to afford the isocyanate 182. Subsequent reaction of 182 with primary amines such as free r-leucine 180 or secondary amines such as N-BOC-pi-perazine 184 affords the ureas 183 and 185 in 49% or 77% overall yield, respectively [10] (Scheme 4.7). 

The following carbamoylimidazole, which was prepared from 14C-labeled methyl-ammonium chloride and CDI, was not isolated. It was instead fragmented by heating in vacuo into the isocyanate, which was then reacted with an alcohol to give the labeled carbamate [220]... 

N-Methylethylamine has been prepared by heating ethyl-amine with methyl iodide in alcohol at 100° 3 by the hydrolysis of N-methyl-N-ethylarenesulfonamides,4-5 -nitroso-N-methyl-N-ethylaniline,6 or methylethylbenzhydrylidene ammonium iodide 7 by catalytic hydrogenation of ethyl isocyanate or ethyl isocyanide 8 and by the reduction of ethyl isocyanate by lithium aluminum hydride,9 of N-methylacetisoaldoxime by sodium amalgam and acetic acid,10 or of a nitromethane/ethylmagnesium bromide adduct by zinc and hydrochloric acid.11... 

V-(Ethoxycarbonylmethyl)carbodiimides, obtained from the reaction of iminophos-phoranes with aryl isocyanates, undergo cyclization when treated with an excess of tetra-n-butylammonium fluoride at room temperature to yield l,3-oxazlin-5-ones [75] (Scheme 3.9). The ammonium fluoride probably enhances the electrophilic character... 

Cyclohexylurea has been prepared by the reaction of cyclo-hexyl isocyanate with gaseous ammonia or ammonium hydroxide, by thermal decomposition of cyclohexyl allophanamide, by treating cyclohexylamine hydrochloride with an aqueous solution of potassium cyanate," by heating nitrosomethylurea with cyclohexylamine, and by heating an ethanolic solution of cyclohexylamine and 3,5-dimethyl-l-carbamylpyrazole. 2,6-DimethyIphenyIthiourea has been synthesized by allowing 2,6-dimethylaniline hydrochloride to react with ammonium thiocyanate. ... 

Alkyl lV,lV-dinitramines (154) have been prepared from the reaction of the tetraalkylam-monium salts (155) of primary nitramines with nitryl fluoride in acetonitrile at subambient temperature. The same reaction with the primary nitramine or its alkali metal salts yields the corresponding nitrate ester. Treatment of the ammonium, potassium, or lithium salts of primary nitramines (156) with a solution of nitronium tetrafluoroborate in acetonitrile at subambient temperature yield alkyl iV,iV-dinitramines. ° The same reactions in ether or ester solvents enables the free nitramine to be used. The nitrolysis of A-alkylnitramides (157) and N,N-diacylamines with nitronium tetrafluoroborate in acetonitrile, and the nitration of aliphatic isocyanates with nitronium tetrafluoroborate and nitric acid in acetonitrile, also yield alkyl A,A-dinitramines (154). 

Nitric acid has a 65 25 fertilizer explosive end use ratio. The production of ammonium nitrate makes up nearly all of these two uses. The other 10% is made into miscellaneous compounds adipic acid, nitroglycerin, nitrocellulose, ammonium picrate, trinitrotoluene, nitrobenzene, silver nitrate, and various isocyanates. 

---