Carboxylate decompositions, controlling

The chemical properties of the residual solid products from metal carboxylate decomposition are largely controlled by the reactant cation present and may include one or more of the following metal, carbide, oxide or oxides, carbonate. 

Solution Deposition of Thin Films. Chemical methods of preparation may also be used for the fabrication of ceramic thin films (qv). MetaHo-organic precursors, notably metal alkoxides (see Alkoxides, metal) and metal carboxylates, are most frequently used for film preparation by sol-gel or metallo-organic decomposition (MOD) solution deposition processes (see Sol-GEL technology). These methods involve dissolution of the precursors in a mutual solvent control of solution characteristics such as viscosity and concentration, film deposition by spin-casting or dip-coating, and heat treatment to remove volatile organic species and induce crystaHhation of the as-deposited amorphous film into the desired stmcture. 

Panke et al. (2003) also demonstrated enhanced reaction control, with respect to the temperature-sensitive synthesis of 2-methyl-4-nitro-5-propyl-2H-pyrazole-3-carboxylic acid 219, a key intermediate in the synthesis of the lifestyle drug Sildenafil (220) (Scheme 64). When performing the nitration of 2-methyl-5-propyl-2H-pyrazole-3-carboxylic acid 219 under adiabatic conditions, with a dilution of 6.01kg 1), Dale et al. (2000) observed a temperature rise of 42 °C (from 50 to 92 °C) upon addition of the nitrating solution. As Scheme 63 illustrates, this proved problematic as at 100 °C decomposition of the product 219 was observed and in order to reduce thermal decomposition of pyrazole 219, and increase process safety, the authors investigated addition of the nitrating solution in three aliquots, which resulted in a reduced reaction temperature of 71 °C and an increase in chemoselectivity unfortunately, the reaction time was increased from 8 to 10 h. 

Fixed oils and fats are naturally occurring products, usually of plant origin, that are used extensively in pharmaceutical formulation. They are very susceptible to oxidative decomposition (a process called rancidity) and special precautions must be taken to control their stability and prevent their decomposition. Compounds of this type exist as complex mixtures of structurally similar oils, the composition of which can vary from year to year depending on factors such as climate, time of harvest, etc. Chemically, fixed oils and fats are esters of the alcohol glycerol (propane-1,2,3-triol) with three molecules of long-chain carboxylic acids, called fatty acids, which may all be the same or may differ depending on the oil (Figure 8.15). 

Broadbent et al. [69] showed that ar-time curves for the decomposition of copper(II) oxalate (503 to 533 K) were sigmoidal and that data for the vacuum reaction fitted the Avrami-Erofeev equation with values of = 2.9 initially and later n = 3.5 ( , = 136 kJ mol ). Electron transfer was identified as the step controlling the reaction. There was no evidence from X-ray diffraction studies for the intervention of the Cu salt the orthorhombic structure was present until disappearance of the reactant and product copper metal was detected. However, many metal carboxylates, chilled after dehydration, yield anhydrous salts that are amorphous to X-rays or poorly crystalline, see, for example [70]. 

Steps which have been proposed as rate controlling in the decompositions of one or more carboxylates include the following. 

From the above comparisons it is evident that both structure and composition of the anion may influence the mechanism of decomposition of nickel carboxylates. The crystal structure of the reactant can probably be discounted as a rate controlling parameter because dehydration usually yields amorphous materials. Depending on temperature, carbon deposited on the surface of a germ metallic nucleus may effectively prevent or inhibit growth, it may be accommodated in the structure to yield carbide, or be deposited elsewhere (by carbide decomposition). These mechanistic interpretations are based on the relative reactivities of the nickel salt and of nickel carbide, for which the temperature of decomposition is known, 570 K [150]. 

For the decompositions of the metal carboxylates, alternative mechanisms involving breaking of each of the different principal linkages in the R-CO-O-M group have been proposed as rate controlling (reference [40] p.210), in addition to mechanisms based on charge transfer, interface strain and catalysis by an active product. 

Attempts at identifying the influence of structme on stability have generally been inconclusive. For example, some alkali metal permanganates with comparable stmctures show similarities of decomposition behaviour [29], while, in contrast, the decompositions of several cobalt(lll) ammine azides show little evidence of structural influences [76], Significant differences in behaviour were found for the various crystal forms of the LiK tartrate hydrates [87] and, also, for the dehydrations of the isomorphic alums [20,43], However, some reactants, for example those prepared by the dehydration of hydrated metal carboxylates [5], may be amorphous to X-rays, thus preventing recognition of any control of stability by crystal structure. 

A new decarboxylative route to free radicals, which has proved particularly successful in preparative work, embodies the thermal (or photochemical) decomposition reaction of 1-hy-droxypyridine-2(l/f)-thione esters 23 with tributyltin hydride, /er/-butanethiol, or a similar hydrogen donor.These esters can be easily prepared from acyl halides and the sodium salt of l-hydroxypyridine-2(l//)-thione, or from the carboxylic acid, dicyclohexylcarbodiimide and l-hydroxypyridine-2(l/f)-thione. The intermediate radicals were readily reduced to the corresponding hydrocarbons 24 in efficient chain reactions with organotin hydrides or thiols as reaction partners, and the proportion of rearranged to unrearranged products could be controlled by the choice of hydrogen donor, its concentration and the temperature. This system was sufficiently quantitative and well behaved for use in kinetic studies, and the rate constants of the (S-scission reactions of the listed cyclopropylmethyl species were determined. 

The interaction of a alk-l-ene and phosphorus pentachloride to form a complex of the general composition RCH=CH2 2PCl5, now recognized as having the phosphonium salt structure 73, has been known for some time. Very many examples are now known of the decomposition of such complexes with SO2 (or in some cases with P40io ) under controlled conditions when the products are (2-chloroalkyl)phosphonic dichlorides (74) or derivatives thereof. The acids from but-l-ene and pent-l-ene hex-l-ene and hept-1-ene and oct-l-ene and dec-l-ene ° have all been reported. The stability of the initial adducts appears to very considerably, and dehydrochlorination may occur readily if the reaction is carried out with insufficient control. Vinyl and isopropenyl esters of carboxylic... 

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