Carboxylate decompositions, secondary

Hydroperoxide formation is characteristic of alkenes possessing tertiary allylic hydrogen. Allylic rearrangement resulting in the formation of isomeric products is common. Secondary products (alcohols, carbonyl compounds, carboxylic acids) may arise from the decomposition of alkenyl hydroperoxide at higher temperature. 

Primary and secondary alkyl peroxyesters thermally decompose by a nonradical process, giving almost quantitative yields of carboxylic acids and carbonyl compounds. Art-Alkyl peroxyesters are much less sensitive to radical-induced decompositions than diacyl peroxides. Induced decomposition is only significant in peroxyesters containing nonhindered a-hydrogens or a, /(-unsaturation. 

The inclusion of heat stabilizers is essential to protect the system against thermal decomposition at elevated temperatures during processing. For this purpose, tin carboxylate esters or liquid calcium-zinc stabilizers are preferred. Thio-tin compounds are very effective as heat stabilizers but must be regarded with caution, bearing in mind that they can lead to unpleasant and unacceptable residual odours. Secondary stabilizers that can be used include epox-idized soya bean oil. 

The experiments of Schwenker and Beck also throw further light on the mechanism of the secondary reactions. In these experiments, cellulose was pyrolyzed in atmospheres of air, nitrogen, and helium, and essentially the same products were obtained, regardless of the conditions of pyrolysis. Therefore, it was concluded that the secondary decomposition reactions must, by-and-large, be non-oxidative in nature, in contrast with the primary reactions of cellulose on slow heating, which involve oxidation of the compound to provide carbonyl and carboxyl groups. 

Phosgene and tertiary carboxylic acid amides form very labile adducts (17 equation 6 not yet isolated or used for preparative purposes as such), which decompose with loss of CO2 very rapidly to give amide chlorides (see Section 2.7.2.2.1.i). Decomposition with evolution of CO2 is a common fate of primary adducts of carbonic acid chloride derivatives. Primary adducts from DMF and chloroformic acid esters (18), for example, decompose immediately to give alkoxymethyleneiminium chlorides, which react to give alkyl chlorides and DMF (equation 7). Adducts (19) from secondary and tertiary carboxamides... 

Decompositions of other metal carboxylates, such as acetates and other anions containing hydrocarbon radicals, give a more complicated range of volatile products and the solid product may also contain the metal carbide, identified as being formed by secondary reactions. It is useful to be able to predict, on thermodynamic grounds, the probable solid product which, in turn, may provide information about the gaseous products to be expected. 

The kinetics of the decomposition [17] of thorium tetraformate to ThOj can be described by the Prout-Tompkins equation with = 150 kJ mol" from 498 to 553 K. The autocatalytic process was ascribed to participation of the oxide in breakdown of the carboxyl groups at the reaction interface to yield ThOj, formaldehyde and carbon dioxide as the primary products of reaction. The volatile products could, however, react further on the surface of the active solid to yield a number of secondary products amongst which the following gases were identified Hj, CO, HjO, CHjOH, HCOOCHj, HCOOH and (CHj). Addition of nickel formate to the reactant not only accelerated decomposition but also influenced the composition of the gases evolved, yielding predominantly CO, COj and H2 (which are the main products of nickel formate decomposition). 

Reactions of carboxylates containing the more electropositive cations yield product carbonates, or sometimes the basic carbonates. Some of these salts, e.g., those of the alkali metals, melt before decomposition. The oxide products from decomposition of the lanthanide compounds may contain carbon deposited as a result of carbon monoxide disproportionation. Kinetic measurements must include due consideration of the possible retention of carbon dioxide by the product (as COj ) and the secondary reactions involved in carbon deposition. 

Because oxidative decarboxylation of carboxylic acids by lead tetraacetate depends on the reaction conditions, the co-reagents, and the structures of the acids, a variety of products such as acetate esters, alkanes, alkenes, and alkyl hahdes can be obtained. Mixed lead(IV) carboxylates are involved as intermediates as a result of their thermal or photolytic decomposition decarboxylation occurs and alkyl radicals are formed. Oxidation of alkyl radicals by lead(IV) species gives carbocations a variety of products is then obtained from the intermediate alkyl radicals and the carbocations. Decarboxylation of primary and secondary acids usually affords acetate esters as the main products (Scheme 13.41) [63]. 

At temperatures greater than a 100°C, thermal degradation of carboxylic acids produces methane and carbon dioxide (Surdam et ai, 1984). As the carboxylic acid anions are consumed due to increasing temperature, the carbonate system becomes internally buffered, and thus the pH may decrease due to increased in the system, leading to carbonate dissolution and the enhancement of secondary porosity (Surdam et ai, 1984). Factors influencing the thermal destruction rate of organic acids include coupled sulphate reduction and hydrocarbon oxidation, and the mineralogy of host sediments (Bell, 1991) the presence of hematite causes rapid rates of acetic acid decomposition. 

It should also be noted that decarboxylation of / -oxo acids is subject to specific catalysis by primary amines as well as to general catalysis. For example, the very smooth decarboxylation of 2,2-dimethylacetoacetic acid in water is uninfluenced by addition of a secondary or tertiary amine but its rate is increased by a factor of 10 on addition of aniline. The explanation lies in the fact that primary amines can react to form / -imino acids, whose imino-nitro-gen atom, being considerably more strongly basic than the ketonic oxygen atom, causes almost complete transfer of the proton from the carboxyl group, and it is this transfer that initiates the decomposition. A further example is the violent decomposition to acetone and carbon dioxide that occurs when a small amount of aniline is added to acetonedicarboxylic acid. 

Reaction (a) represents the ketone decomposition, reaction (b) the acid decomposition. E and Ei can be used to represent a variety of different groups. The exact relation of E to the carboxyl group with which it is combined is of secondary importance, as long as it does not change in the course of the reaction. The distribution of the charges, or the valence of the atoms follows from the principles already given. In reaction (6), the acid decomposition, there is apparently no oxidation or reduction involved. This reaction involves the use of concentrated alcoholic alkali, a reagent which is supposed to cause or assist oxida-... 

Hydroperoxides are the first products of oxidation (Carlsson et al. 1987a), which thermally and mechanically decompose and, over time, produce radicals. These continue the oxidation process, as was shown in Scheme 7. Further products of thermal, mechanical, or photolitic decomposition of hydroperoxides are ketones, carboxylic acids, and esters. The main products of thermal and photolitical decomposition of the secondary hydroperoxides formed in UHMWPE are ketones (Scheme 8). 

Mild Hydrolysis. Acetates of primary and secondary alcohols such as cyclopropyl acetate [205] and methyl or ethyl carboxylates (such as the labile cyclopentadiene ester [206]) can be selectively hydrolyzed under mild conditions using PLE, avoiding decomposition reactions which would occur during a chemical hydrolysis under acid or base catalysis (Scheme 2.22). For example, this strategy has been used for the final deprotection of the carboxyl moiety of prostaglandin Ei avoiding the destruction of the delicate molecule [207, 208],... 

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