Carbamate esters, alkaline hydrolysis

When a carbonyl group is bonded to a substituent group that can potentially depart as a Lewis base, addition of a nucleophile to the carbonyl carbon leads to elimination and the regeneration of a carbon-oxygen double bond. Esters undergo hydrolysis with alkali hydroxides to form alkali metal salts of carboxylic acids and alcohols. Amides undergo hydrolysis with mineral acids to form carboxylic acids and amine salts. Carbamates undergo alkaline hydrolysis to form amines, carbon dioxide, and alcohols. 

Carboxyhc acid ester, carbamate, organophosphate, and urea hydrolysis are important acid/base-catalyzed reactions. Typically, pesticides that are susceptible to chemical hydrolysis are also susceptible to biological hydrolysis the products of chemical vs biological hydrolysis are generally identical (see eqs. 8, 11, 13, and 14). Consequentiy, the two types of reactions can only be distinguished based on sterile controls or kinetic studies. As a general rule, carboxyhc acid esters, carbamates, and organophosphates are more susceptible to alkaline hydrolysis (24), whereas sulfonylureas are more susceptible to acid hydrolysis (25). 

Bilikova and Kuthan [87] developed a gas chromatographic method for the determination of submicrogram concentrations of Carbofuran (2,3-dihydro-2, 2,-dimethylbenzofuran-7-yl-methyl carbamate) in soils. Soil samples are mixed with methanol-water (80 20) and water samples are extracted with chloroform. After separation of the chloroform, and weak alkaline hydrolysis, derivatization is performed with l-fluoro-2,4-dinitrobenzene. The ester produced is isolated from benzene and cleaned up with acetone. The acetone extract is used to determine Carbofuran by gas chromatography with a nitrogen-specific detector. 

I. Christenson, Alkaline Hydrolysis of Some Carbamic Acid Esters , Acta Chem. Scand. 1964, 18, 904-922. 

Reference to Chapter 3 (Scheme 3) shows that the alkaline hydrolysis of aryl carbamate esters has a large Hammett p coefficient (2.54) whereas that for the alkaline hydrolysis of aryl acetate esters is only small (0.67) (Scheme 2). The small p value for alkaline hydrolysis of the acetate case is consistent with little bond fission in the transition structure and the large p for the carbamate case (see Chapter 3 for mechanism) indicates substantial bond fission and a gross difference in mechanism between the two reactions. The simple Hammett relationship only correlates the alkaline hydrolysis rates of the carbamate when meta substituents are employed. Para substituents, which withdraw negative charge by resonance (4-NO2, 4-CN, etcf require values to obtain a good correlation. This is interpreted to mean that the transition structure has sufficient aryl oxide ion character for resonance to occur. In... 

Ester 23c has been used in a short synthesis of (5)-isoserine (17) (Scheme 2) [28]. Treatment of 23c with diphenylphosphoryl azide gives an intermediate isocyanate 24 via a Curtius rearrangement. Spontaneous cyclization results in the formation of cyclic carbamate 25 in 74% yield. Alkaline hydrolysis affords 17 in high yield without any racemization. 

The hydrolysis of the four carbamate insecticides investigated in alkaline medium was a function of the hydroxyl ion concentration in the solution and was first order with respect to these ions. The second order rate constants, 2, for the mono 2V-methyl substituted esters, Sevin and baygon, were much higher than those for the 2V,IV-dimethyl substituted ones. Casida et al. (4), Dittert (5), and Christenson (2) also found that... 

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