Decomposition carbamic acid

Problem 16.52 The following carboxylic acids are unstable and their decomposition products are shown in parentheses carbonic acid, (H0)2C=0 (CO, + H O) carbamic acid, H,NCOOH (CO + NH,) and chloro-carbonic acid, CICOOH (CO + HCl). Indicate how the stable compounds below are derived from one or more of these unstable acids. Name those for which a common name is not given. 

Ammonium carbamate is prepared from dry ice and liquid ammonia [14]. These conditions are very similar to the conditions under which we have observed the formation of amine salts. To some readers, ammonium carbamate may seem to be an exotic compound. In fact, it is manufactured industrially on a multiton scale, because on heating (usually at 100-185°C) ammonium carbamate is converted to urea and water [14-16]. Urea is important for both the agricultural and the plastics industries. The ammonium carbamate is not always isolated during urea preparation. Instead, the reactions are carried out under conditions where the carbamate is just an intermediate. Ammonium carbamate is only moderately stable and it gradually loses ammonia in air. Although the data are sparse, the rate of decomposition of carbamates in solution seems to decrease as the volatility of the parent amine decreases [17]. Free carbamic acids in solution do not decompose spontaneously to free amine and C02. Instead, the acid ionizes by reaction with water the proton is transferred from the hydronium ion to the amine and then decomposition occurs [17]. Acids catalyze the decomposition. 

Some properties of H2NC02H have been the subject of theoretical studies [17] indeed, calculations have predicted that the syn conformer is more stable than its anti counterpart [17a]. It has been also calculated that gaseous carbamic acid can decompose exothermically into ammonia and carbon dioxide (AH° = -26 kj mol"1) [17b]. Moreover, ah initio calculations have shown that the zwitterion H3N+C02" is less stable than monomeric acid H2NC02H and, therefore, the zwitterion form has been proposed as the most probable intermediate for the decomposition of carbamic acid to ammonia and C02 [17c],... 

Fig. 8. 3. Zwitterion mechanism of the decomposition of neutral carbonic acid (derivatives) in a heterocumulene and a hetereo-nucleophile—examples carbonic acid (B), carbonic acid monomethylester (E), carbonic acid mono-tert-butylester (H), carbamic acid (K), and urea (M).

Fig. 8.5. Proton-induced decomposition of W-substituted carbamic acid C into carbon dioxide and a primary ammonium hydrochloride—a reaction occuring in the course of acidic hydrolysis of organic isocyanates.

Fig. 8.6. Proton-induced decomposition of /(/-substituted carbamic acid B into carbon dioxide and a primary ammonium trifluoroacetate—a reaction occuring during the acidic deprotection of a Boc-pro-tected amine.

METHYL((METHYL((((5-METHYL-1,3-OXATHIOLAN-4-YLIDENE)AMINO)-OXY)CARBONYL)AMINO)THIO) CARBAMIC ACID, ETHYL ESTER mf C10H17N3O5S2 mw 323.42 SAFETY PROFILE A poison by ingestion. When heated to decomposition it emits toxic vapors of NOx and SOx. 

Frequently this is not of kinetic significance (see, however. Section 5) on account of the rapid decomposition of carbamic acid. Naturally stabiliza-... 

XXII) while (33) represents the reversible decarboxylation of (XXII). In the absence of added acid catalysts reaction (34) is the decomposition of (XXI) catalysed by the carbamic acids HX. The kinetic behaviour of the reaction at constant non-zero CO2 pressure in the absence of added acids is complex on account of the catalysis by the carbamic acid species (XXII) (HX). Even at the lowest achievable partial pressures of CO2 the finite rate of decomposition of (XXII) (reaction (33)) affects the rate of polymerization, that is the rate coefficient fe2 is involved in the kinetics [17]. It is reasonable to assume a stationary state for carbamic acid concentration... 

In APCI mass spectra of carbamates, fragment ions are observed, which are most likely due to thermal decomposition in the heated nebulizer interface and snbseqnent ionization of the thermal decomposition products [11, 14, 20-23]. For example, base peaks were observed at m/z 163 for oxamyl, due to the loss of methyl isocyanate, at m/z 168 for propoxur, dne to the loss of propylene, and at m/z 157 for aldicarb, due to the loss of HjS. The APCI mass spectra of aldicaib and two of its metabolites, aldicarb sulfoxide and aldicarb snlfone, showed significant fragmentation. Major fragments for aldicarb were dne to the loss of carbamic acid (to m/z 116) and due to charge retention at [CH3-S-C(CH3)2]. For aldicarb sulfoxide and aldicarb sulfone, the loss of carbamic acid resnlted in the base peaks of the spectra (at m/z 132 and 148, respectively). 

The relative rates of these reaction sequences were found to depend on the nature of the medium (homogeneous or heterogeneous), rate of addition of water, temperature, reactivity of the amine (formed by the decomposition of the carbamic acid) with the isocyanate, concentration, and other factors. For example, in the reaction with phenyl isocyanate, cold water and heterogeneous medium favored the formation of diaryl urea while with boiling water the main product was aniline. Dilution also favored aniline formation. 

A more detailed quantitative study of the water-o-tolyl isocyanate reaction by Shkapenko et al. 51) showed that at 80° in dioxane solution and in the presence of triethylamine or other catalysts the consumption of the isocyanate was complete within a short period when only approximately half of the theoretical amount of carbon dioxide was released. The evolution of carbon dioxide proceeded from this point on at a slow rate. It was also demonstrated that by heating the reaction mixture to 100°, 30-35% of theoretical CO2 was released, and that this portion of the CO2 was given off by the decomposition of the carbamic acid anhydride formed from the acid and a second molecule of isocyanate. Additional tests showed that 4-5% of the isocyanate formed o-tolyl ammonium-W-o-tolyl carbamate, 18.7% of the water added remained unreacted, and that a trace of the free o-tolyl amine was also present. In addition, the presence of di-o-tolyl urea was proven. 

So, also the selectivity based on the CO consumed was very high (100%). However, when the aniline was carbonylated in the presence of methanol at T>100 °C, relevant amounts of CO2, increasing upon increase of temperature, were obtained. The CO2 formation only under these conditions suggests that it is not formed by direct oxidation of CO, but likely by hydrolysis of DPU (reaetion 3) followed by decomposition of resulting carbamic acid (CO(NHPh)(OH)) in CO2 and aniline. 

From the hydrolysis of a polyether-based PU a diamine (or a polyamine) such as toluene diamine or diphenylmethane diamine, a polyol and carbon dioxide are formed. The resulting diamines are the precursors used for the synthesis of isocyanates [11,12,16,18]. The resulting polyol is the polyether polyol used for the initial synthesis of PU. Carbon dioxide results from the decomposition of the very unstable carbamic acid formed by the hydrolysis (20.2) ... 

In general, the N-substituted derivatives of carbamic acid are unstable compounds, especially under alkaline conditions, their decomposition results in the formation of an alcohol or phenol, ammonia, amines and carbon dioxide. These salts and esters of N-substituted carbamic acid are more stable, this enhanced stability is the basis for their use as biologically active pesticides. 

This last reaction, the hydrolysis of coordinated nitriles, has been extensively studied by Ford and co-workers acidic hydrolysis of [Rh(NH3)5NCO] has been used as an alternate route to the hexaammine (equation 113). The reaction was shown to go through a carbamic acid intermediate, which was isolated and characterized. The kinetic study of the hexaammine formation was complicated by the presence of these two consecutive reactions, and by two different reaction paths. The mechanism proposed for the hydrolysis of the coordinated cyanate is shown in Scheme 14 at low acid concentrations (0.005 to 0.025 M) the rate determining step is HjO attack of the protonated species leading to a rate expression which is first-order in [H ] rate = ky [RhlfH" ]. At higher acid concentrations (0.2 to 1.0 M), decomposition of the carbamic acid complex is rate determining, and... 

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