Toxicity transesterification

Biodiesel is a fuel derived from renewable natural resources such as soybean and rapeseed and consists of alkyl esters derived from transesterification of triglycerides with methanol. In spite of all the advantages of biodiesel, such as low emissiotts, biodegradability, non-toxicity, and lubricity, the major hurdle in penetration of biodiesel is its high cost because of the expensive food grade refined vegetable oil feedstock. 

The transesterification of sucrose has been performed with a fatty acid ester of a volatile alcohol in the presence of an alkaline catalyst in a dipolar, aprotic solvent.142 The reaction of sucrose (293 mmoles) with methyl dodecanoate (293 mmoles) in A/,N-dimethylformamide in the presence of sodium methoxide in a pressure bomb for 8 h at 130° gave, after solvent extraction and crystallization, sucrose mono(dodecanoate) (m.p. 72-80° [a]D+52°) in 50% yield.142 Commercialization of these sucrose esters has so far been limited, in part because of the use of expensive solvents, and, in part, because solvent remaining in the product makes it unsuitable for use as a food emulsifier. In view of this situation, methods have been developed in which the use of toxic and expensive solvents has been avoided. 

There is an additional factor contributing to the toxicity of cocaine, namely its interaction with ethanol [122] [123], Many cocaine (ab)users simultaneously ingest ethanol, probably to experience potentiation of effects and attenuation of headaches. It is now known that ethanol interferes in two ways with the metabolism of cocaine, first by inhibiting its hydrolysis and second by allowing transesterification to form benzoylecgonine ethyl ester (7.61, Fig. 7.8) commonly known as cocaethylene. These metabolic effects are illustrated by studies in the rat (Table 7.3), with ethanol inhibiting the formation of... 

Many acetate esters (such as those of isoamyl, benzyl, citroneUyl, and geranyl alcohols) are components of natural flavors. They can be obtained by Upase-cata-lyzed esteriflcation in organic solvents, but the major problem with enzymatic acetylations is deactivation of lipases by acetic acid [8, 9]. Most of the Upase-catalyzed syntheses of esters have been carried out by transesterification to avoid free acid toxicity and water formation. Claon and Akoh [10] found that immobilized lipases from Candida antarctica promote highly effective direct esterification of geraniol and citronellol with acetic acid. 

To avoid using thallium, sodium 2-(dimethylamino)ethoxide may be used, although yields may be lower. The catalyst, thallous 2-(dimethylamino)ethoxide, is prepared by mixing thallous ethoxide (0.75 g, 3.0 mmol) with DMF (42 mL), DMAE (21 mL), and benzene (8mL), and distilling at 50°C under water aspirator vacuum until 10 mL distillate is obtained. This operation removes EtOH, which wUl yield stable ethyl esters if it is allowed to remain. Use an appropriate aliquot of this soln to catalyze the transesterification. To avoid using toxic thalhum, prepare the sodium salt in a similar fashion from sodium ethoxide (204 mg, 3.0 mmol).W... 

A novel metabolite of methylphenidate, ethylphenidate, has recently been detected in human blood and liver samples obtained from two suicide victims who had taken a large dose of methylphenidate and ethanol (49). The concentrations of ethylphenidate were small relative to those of methylphenidate and ritalinic acid. Nevertheless, given the frequent occurrence of methylphenidate and ethanol co-ingestion, the detection of ethylphenidate warrants further studies on the extent of its formation, as well as any associated toxicity associated with normal doses. A transesterification reaction, analogous to that known to produce cocaethylene after cocaine and ethanol co-ingestion (50), has been described with methylphenidate (51). 

Organotin compounds are used in many synthetic procedures as stoichiometric reagents (hydrostannolysis, hydrostannation, allylation, Stille coupling, Sn/Li transmetallation, transesterification, etc), and this presents the problem of removing the organotin residues from the required product. This is particularly important in the preparation of pharmaceuticals as these organotin compounds are to some degree toxic. 

Cocaine Metabolism. Because cocaine (3)has two ester functions, both can be hydrolyzed in vivo to generate metabolites. Hydrolysis of the methyl ester leads to benzo-ylecgonine (12), and hydrolysis of the benzoyl ester leads to ecgonine methyl ester (13).Tro-pan-3j3-ol-2j8-carboxylic acid is known as ecgonine (14). In cocaine users who also consume significant amounts of ethanol, a transesterification product (cocaethylene, 15) is also detected. Cocaethylene is also a potent psychostimulant, with about four times higher potency as a local anesthetic than that of cocaine itself (69), and can enhance the cardio-toxicity associated with cocaine use. 

An alternative route is the transesterification reaction between diphenyl carbonate and bisphenol-A in the presence of a strong base catalyst with the liberation of phenol under reduced pressure. However, this method also requires the use of phosgene in the preparation of precursors, so there is no way that use of the toxic compound can be avoided (Clagett and Shafer, 1989). The use of copolymers such as the poly(ester carbonates) and blends with other thermoplastics has extended the range of properties... 

Crown ethers also enhance the activity of enzymes in organic solvents.100 The enzyme is lyophilized in the presence of the crown ether before use. a-Chymotrypsin treated in this way with 18-crown-6 was 640 times as active in transesterification of amino acid esters as the untreated enzyme. However, the rate was still 50 times lower than in water. The use of surfactants to improve activity in organic solvents seems preferable because of a greater rate increase. They also avoid the toxicity and expense of the crown ethers. The cross-linked enzyme crystals also offer a way to obtain reasonable rates in organic solvents. 

In summary, considering the environmental issues as well as the process optimization that requires the substitution of toxic catalysts by more friendly solid catalysts, we have shown new catalysts based on modified silica that can efficiently catalyze transesterification. This reaction was carried out by an ecofriendly technology as the production of effluents harmful to the environment is avoided. 

Solid base catalysts have gradually gained importance in the catalytic field due to their well-known advantages of non-corrosive and easier product separation[l]. However, in contrast to the extensive application of solid acid catalysts, the utilization of solid bases was limited for their rapid catalytic deactivation [2]. The deactivation problem was also foimd in the continuous synthesis of dimethyl carbonate (DMC). DMC has attracted more and more attention in recent years because of its low toxic and nicely biodegradable property [3,4]. Transesterification between methanol and propylene carbonate (PC) or ethylene carbonate (EC) is an attractive route for the synthesis of DMC. Both acid and base catalysts catalyze the reaction, and base catalyst was reported to be more effective [5]. Among bases, CaO showed unique catalytic activity for the transesterification reaction with high yield and selectivity [6]. Unfortunately, when CaO based catalyst was employed in the continuous synthesis of DMC, its activity gradually decayed with time-on-stream due to... 

The alkali process for biodiesel production can achieve high purity and yield of biodiesel in a short time. However, vegetable oils high in free fatty acids result in the production of soap and the loss of catalyst in the alkali process. To overcome this, the free tatty acids should be removed before the transesterification reaction. Because a homogeneous acid catalyst like sulfuric acid can not be recovered and is toxic, a heterogeneous acid catalyst can be used for the esterification of free fatty acids. Solid catalysts can be easily recovered after the reaction and reused [10-12]. 

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