Carbamates reactions

Reaction 1 is highly exothermic. The heat of reaction at 25°C and 101.3 kPa (1 atm) is ia the range of 159 kj/mol (38 kcal/mol) of soHd carbamate (9). The excess heat must be removed from the reaction. The rate and the equilibrium of reaction 1 depend gready upon pressure and temperature, because large volume changes take place. This reaction may only occur at a pressure that is below the pressure of ammonium carbamate at which dissociation begias or, conversely, the operating pressure of the reactor must be maintained above the vapor pressure of ammonium carbamate. Reaction 2 is endothermic by ca 31.4 kJ / mol (7.5 kcal/mol) of urea formed. It takes place mainly ia the Hquid phase the rate ia the soHd phase is much slower with minor variations ia volume. 

This approach is compHcated by the fact that the isocyanate is produced via the thermolytic cleavage of the methyl carbamate. Reactions with the unconverted carbamate caimot be prevented. Much effort has been focused on improving the selectivity of the latter step. 

Trialkanolamines, lacking an amine hydrogen, do not undergo die carbamate reaction. 

Ethyl carbamate, reaction with hydrox-ylamine to form hydroxyurea, 40, 60... 

Figure 25.8 N,N -carbonyldiimidazole (CDI) may be used to activate the terminal hydroxyl of mPEG to an imidazole carbamate. Reaction of this intermediate with an amine-containing compound results in the formation of a stable carbamate linkage.

When sulfamate esters 114 are used as substrates, six-membered-ring formation is favored, and results in the selective formation of 1,2,3-oxathiazinane-2,2-dioxide heterocycles 115.251 Nevertheless, five-membered cyclic sulfamidates could be obtained when no alternative cyclization was possible. 1,3-Amino alcohols and related /2-amino acids are thus readily accessible from the same simple alcohols 113 by converting them into sulfamates 114 (Equation (90)). Furthermore, in comparison to the carbamate reaction (Scheme 9), the sulfamate substrates have... 

HC03, C03, NH2C00. For given temperature and total molalities of NHt and C02 there remain 10 equations to determine all liquid concentrations, the total pressure and the composition of the vapor. Two purely chemical equilibria The carbamate reaction (reaction 8) and... 

The equilibrium constant for the carbamate reaction (eq.VIII) was simplified by assuming a o = an< re placing all other activities by molalities. Numbers for Kg(T) at 20, 40 and 60 oc were determined from experimental results. (Van Krevelen et al. only report discrete numbers or diagrams for some constants. For inter- and extrapolation these numbers were replaced by equations, wherein the dimensions of m and T are moles/ kg H2O and Kelvin, respectively.) ... 

Fig. 8.7. a) Mechanism of the HCT-catalyzed, (and presumably also of the enzyme-catalyzed) hydrolysis of carbonic acid esters, b) Alternative mechanisms of the HO -catalyzed hydrolysis ofN-carbamates. Reaction b is restricted to monosubstituted carbamates, whereas Reaction a is also possible for A.iV-disubstituted carbamates. 

Peptide Conformations. I. Nuclear Magnetic Resonance Study of the Carbamate Reaction of Amino Acids and Peptides, R. U. Lemieux and M. A. Barton, Can. J. Chem., 49 (1971) 767-776. 

Simulation studies based on different kinetic descriptions of the monoethanola-mine carbamate reaction (see Section 9.53.4) using aqueous solutions of MEA and MDEA lead to deviations in the calculated column height of up to 15 % (Fig. 9.7). This demonstrates the importance of a proper kinetics measurement and description. 

The chemical name of this reagent is l,3-bis(hydroxymethyl)-4,5-dihydroxy-imidazolidinone-2 but it is usually called DMDHEU or the glyoxal reactant because it is prepared from glyoxal, urea, and formaldehyde. Other methylolamide agents that have been used for producing wrinkle resistance in cotton include the aforementioned urea formaldehyde, dimethylolurea, dimethylolethyleneurea, and formaldehyde adducts of melamines (triazines), acetylenediurea, propyleneurea, uron, triazones, and alkyl carbamates. Reactions between methylolamides and cellulose occur in the presence of acid (or Lewis acid) catalysts and are very fast at elevated temperatures—sufficiently so that they are adaptable to the requirements of rapid, commercial processing of cotton fabrics. 

Cellulose reacts with isocyanates in anhydrous pyridine or with urea and substituted ureas at relatively high temperature to yield carbamates. The optimum carbamation reaction of microcrystalline cellulose with urea in a dry solid mixture has been studied [51]. In addition, a preferentially C6-modified cellulose carbamate derivative has been obtained [52]. Heating of cellulose with thiourea at 180°C yielded cellulose thiocarbamate [53]. Heat treatment of cellulose isocyanate products has been utilized for the production of urethanes [54]. When ceUuIose was treated with phenylisocyanate at 100 C in DMF in the presence of dibutyltin dilaurate and triethylenediamine, celiuiose bisphenylcarbamate was formed [55]. Treatment of cellulose with urea at temperatures at or above the latter s melting point (where urea decomposes into isocyanic acid and ammonia) has been employed for the production of cellulose carbamates fibers [56]. The advantages and disadvantages of using urea as an intermediate for production of fiber have been discussed [57]. 

Recent work has shown that the carbamate reaction shows no legioselectivity in the rather demanding case of 1,2,3,6-tetrahydropyiidines 1,2-dihydropyiidines react at the 5,6 double bond, but with no selectivity in the N,0 orientation. The regioselectivities of the Sharpless oxyaminations have been rationalize, and the react on has recently been studied from the point of view of the inoiganic chemist These procedures and ther cis hydroxyaminations below produce stereochemistry complementaiy to that provided by ring Of ming of the epoxidized alkene. ... 

The reaction between DMC and primary or secondary amines yields carbamates [reaction (17)]. Suitable catalysts are needed to achieve good reaction rates... 

The reverse of the carbamation reaction, which occurs in lungs or gills (where 02 concentration is high, is equally important. Here, the high O2 concentration favors oxygenation and hence the oxy form of the molecule. When this switch occurs, stabilization of the carbamated N-termini is decreased, and CO2 is expelled and expired. 

Carbamates Reaction of organic azides with Me P, treatment of the ensuing iminophosphoranes with chloroformic esters, and hydrolytic cleavage of the aminophos-phonium salts complete the conversion. 

The degree of stripping efficiency is adjusted to ensure that as much low-pressure (LP) steam is produced by the carbamate reaction in the pool condenser as is needed in the downstream sections of the plant. 

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