Carbamates, basicity

Localized pre-boiler scale and corrosion debris deposits. Combination of New phosphate, iron, copper, and silica deposition Old re-deposited debris Transport of Fe, Cu, Ni, Zn, Cr oxides to HP boiler section, leading to deposition, fouling, and possible tube failures Transport of minerals and debris including malachite, ammonium carbamate, basic ferric ammonium carbonate Precipitation in FW line of phosphates, iron, and silicates... 

Carbamates are substituted esters of carbamic acid (NH2COOH) with aliphatic or aromatic substituents on the oxygen and nitrogen atoms. Carbamate insecticides have an aryl or oxime N-methylcarbamate structure, and their mode of action is based on the inhibition of the enzyme acethylcholine esterase (1). However, this inhibition is reversible, and recovery from sublethal doses occurs rapidly. Some carbamate fungicides have a dithio, bisdithio, or benzimidazole carbamate basic structure, and dithiocarbamate fungicides inhibit the enzyme aldehyde deshydro-genase (2). The herbicides have an /V-alkylthiocarbamate or A-phenylcarbamate structure and interfere with photosynthetic activity or affect meristematic activity or lipid metabolism (3). Representative structures of carbamate pesticides are shown in Fig. 1. 

Bourbon Distillation. The basic distiUation system for the production of bourbon and other straight whiskeys consists of a beer stiU and a beer heater, thumper, or doubler (Fig. 4). The whiskey stiU consists of between 14 and 21 stripping trays. The upper portion of the stiU is fitted with either a bubble cap section or a section packed with copper rings to enhance the removal of unwanted flavors and ethyl carbamate precursors. The reduction of carbamate precursors requites strict adherence to a cleaning protocol with a 5% caustic solution as often as twice a week. 

Sulfonamides (R2NSO2R ) are prepared from an amine and sulfonyl chloride in the presence of pyridine or aqueous base. The sulfonamide is one of the most stable nitrogen protective groups. Arylsulfonamides are stable to alkaline hydrolysis, and to catalytic reduction they are cleaved by Na/NH3, Na/butanol, sodium naphthalenide, or sodium anthracenide, and by refluxing in acid (48% HBr/cat. phenol). Sulfonamides of less basic amines such as pyrroles and indoles are much easier to cleave than are those of the more basic alkyl amines. In fact, sulfonamides of the less basic amines (pyrroles, indoles, and imidazoles) can be cleaved by basic hydrolysis, which is almost impossible for the alkyl amines. Because of the inherent differences between the aromatic — NH group and simple aliphatic amines, the protection of these compounds (pyrroles, indoles, and imidazoles) will be described in a separate section. One appealing proj>erty of sulfonamides is that the derivatives are more crystalline than amides or carbamates. 

Either the iodo carbamate (96) or the iodo amine salt (94) from above can be converted to the aziridine by refluxing about 2.5 g of the respective product in 100 ml of ethanol which contains 10 ml of water and 10 g of potassium hydroxide for ca. 2 hr. The aziridine is then isolated by pouring the basic reaction mixture into 250 ml water and extracting with 200 ml ether. The ether extract is washed several times with water and dried (MgS04). Evaporation of the ether on a steam bath yields 2j5,3j5-iminocholestane (95, 25-95 %) as a clear oil which solidifies on standing mp 103-105°. This aziridine is not easily crystallized. 

Carbonates, like esters, can be cleaved by basic hydrolysis, but generally are much less susceptible to hydrolysis because of the resonance effect of the second oxygen. In general, carbonates are cleaved by taking advantage of the properties of the second alkyl substituent (e.g., zinc reduction of the 2,2,2-trichloroethyl carbonate). The reagents used to introduce the carbonate onto alcohols react readily with amines as well. As expected, basic hydrolysis of the resulting carbamate is considerably more difficult than basic hydrolysis of a carbonate. 

Carbamates are formed from an amine with a wide variety of reagents, the chlo-roformate being the most common amides are formed from the acid chloride. n-Alkyl carbamates are cleaved by acid-catalyzed hydrolysis A-alkylamides are cleaved by acidic or basic hydrolysis at reflux and by ammonolysis, conditions that cleave peptide bonds. 

Gaseous CO2 is extensively used to carbonate soft drinks and this use alone accounts for 20% of production. Other quasi-chemical applications are its use as a gas purge, as an inert protective gas for welding, and for the neutralization of caustic and alkaline waste waters. Small amounts are also used in the manufacture of sodium salicylate, basic lead carbonate ( white lead ), and various carbonates such as M2CO3 and M HC03 (M = Na, K, NH4, etc.). One of the most important uses of CO2 is to manufacture urea via ammonium carbamate ... 

Hydroxy-l-alkenyl diisopropylcarbamates 2 (X = OCb), in this respect, occupy a medium position since they are stable in strongly acidic and basic protic solvents. For deblocking vinyl carbamates, the presence of catalytic amounts of mercuric or palladium(II) salts is required. Due to this stability, several reactions of homoallylic alcohols, proceeding with high diastereo-selectivity, e g., epoxidation, are applicable in order to introduce further hetero-substituents. 

The A-(l-alkoxyalkyl)amides and -carbamates are stable compounds, thus allowing reactions under neutral or basic conditions to be carried out elsewhere in the molecule before generation of the A-acyliminium ion with an acidic catalyst. 

Ammonium carbamate NH2Fe2(OH)4(CO3)2-H20 Hydrated basic ferric The transport of pre-boiler corrosion debris to the boiler section will includes these minerals. Also the oxides of Fe, Cu, Ni, Zn, and Cr as... 

These reactions can be opened up to all substances that can yield aniline derivatives in acid or basic medium. Carbamic acid derivatives, numerous variations of which are used as plant treatment agents, provide a striking application. As do urea herbicides and a variety of drug substances e. g. benzodiazepines or phenylbutazone derivatives. 

Oxime carbamates have high polarity and solubility in water and are relatively chemically and thermally unstable. They are relatively stable in weakly acidic to neutral media (pH 4-6) but unstable in strongly acidic and basic media. Rapid hydrolysis occurs in strongly basic aqueous solutions (pH > 9) to form the parent oxime/alcohol and methylamine, which is enhanced at elevated temperature. Additionally, oxime carbamates are, generally, stable in most organic solvents and readily soluble in acetone, methanol, acetonitrile, and ethyl acetate, with the exception of aliphatic hydrocarbons. Furthermore, most oxime carbamates contain an active -alkyl (methyl) moiety that can be easily oxidized to form the corresponding sulfoxide or sulfone metabolites. 

Oxime carbamates are generally stable in aqueous solutions at pH 4-6. Their chemical degradation (hydrolysis) in water depends strongly on pH. Strongly basic conditions... 

Alternatively, two phosgene equivalents were studied, methyl chloroformate and p-nitrophenyl chloroformate. When methyl chloroformate was used for the end game, N-carbamate 54 was obtained smoothly but subsequent cyclization to ben-zoxazinone 1 was sluggish. Furthermore, removal of the unreacted intermediate methyl carbamate 54 from Efavirenz was not trivial, thus we did not pursue this method. On the other hand, reaction of 53 and p-nitrophenyl chloroformate initially provided the corresponding p-nitrophenyl carbamate 55 under mild basic conditions (KHC03). Carbamate 55 was smoothly cyclized to 1 upon increasing... 

Muscle relaxant activity is found in the aminoxy-methylnaphthalene structure of nafomine (27). The synthesis proceeds from l-chloromethyl-2-methylnaph-thalene (26), which is reacted with N-carbethoxy-hydroxylamine and base. In this way, the basic nitrogen is protected as the carbamate. Loss of the carbethoxy group either during reaction or on workup affords nafomine (27).10... 

Absorbents based on a variety of amines are by far the most common. Amines that have been used include monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), diisopropanolamine (DIPA), methyl diethanolamine (MDEA), and digly-colamine (DGA). C02 is typically absorbed at 80-140°F temperature and up to 1000 psig pressure by chemically reacting with the basic amine functional group in the absorbent, for example, primary amine, MEA, reacts with C02 by the following reaction forming a carbamate salt ... 

Carbamate nerve agents are readily destroyed by high pH (i.e., basic solutions). Use an aqueous caustic solution (minimum of 10% by weight sodium hydroxide or sodium carbonate) or use undiluted household bleach. Basic peroxides also rapidly detoxify carbamate nerve agents. 

The secondary aryl amine 24 is far less basic than primary or secondary alkyl amines, and does not form the carbamic acid to any detectable extent in the presence of scC02 [31]. Therefore, 24 is extracted readily from the catalyst-containing IL phase, which can be recycled without noticeable loss of activity and selectivity [13]. In fact, it transpires that the active species is more stable towards oxygen in the IL than in organic solvents. Furthermore, the choice of anion of the IL largely controls the performance of the active cationic species, allowing even the use of an otherwise inactive iridium chloride precursor [ Ir(cod)Cl 2] to form in-situ catalysts... 

This one-step procedure is a convenient and general method for the preparation of carbamates. It is substantially simpler, quicker, and safer than the multistep methods hitherto used for the preparation of carbamates of tertiary alcohols. This procedure is applicable to the preparation of carbamates of primary, secondary, and tertiary alcohols and mercaptans, polyhydric alcohols, acetylenic alcohols, phenols, and oximes. It has also been extended to the preparation of carbamyl derivatives (i.e., ureas) of inert (non-basic) amines.10... 

The stability of Molsidomine 84 and SIN-1 85 in aqueous systems under acidic, basic, thermic and photolytic conditions was investigated by Asahi et al. [92] (Scheme 6.17). In alkaline solutions the carbamate group is hydrolysed and decarboxylated to 85. This product immediately opens to SIN-1 A 105 that, in turn, is transformed into SIN-1C 106. The authors postulated that elimination of HNO underlies this last transformation. 

In order to demonstrate the use of laser flash photolysis in elucidation of the MDI based polyurethane photolysis mechanism, three polyurethanes, two aryl biscarbamate models, an aryl monocarbamate model, and an aromatic amine were selected. Two of the polyurethanes are based on MDI while the third is based on TDI (mixture of 2,4 and 2,6 isomers in 80/20 ratio). The MDI based polyurethanes all have the same basic carbamate repeat unit. The MDI elastomer (MDI-PUE) is soluble in tetrahydrofuran (THF). The simple polyurethane (MDI-PU) based on MDI and 1,4-butanediol is used in the tert-butoxy abstraction reactions since it does not contain a polyether backbone. (See page 47 for structures of polymers and models.)... 

In the case of carbonylation (Z)-(2-en-4-ynyl)amines, a peculiar promoting effect by CO2 has been found. In fact, CO2 buffers the basicity of the substrate, with formation of a carbamate anion, thus freeing the HI neces-... 

Figures 3 and 4 illustrate typical rates of hydrolysis for carbamate derivatives of cellulose and chitin. The rates of release at a pH value 11.3 were considerably higher in both systems than at pH values of 3.1 and 7.0. After seven days in the basic medium the cellulose derivative had delivered 27.3 percent of the available aniline. In the acidic medium and neutral medium 15.6 and 10.6 percent were delivered. After seven days the chitin derivative delivered 27.7, 10.0, and 9.5 percent of the available p-methylaniline in the basic, acidic, and neutral media, respectively.

Symmetrical and unsymmetrical carbonates have also been obtained from the reaction of chloroformates with alcohols under soliddiquid conditions [55], and the reaction of carbamoyl fluorides with alcohols produces alkyl carbamates [58]. r-Butyloxycarbonylation of alcohols and phenols is effected by trans-esterification of di-r-butyl carbonate under basic phase-transfer catalysed conditions [59]. Yields tend to be high for the reaction with the phenols (>85%), but only moderate with the alcohols (30-81%). 

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