Esters azeotropes with

It is explosive, and distillation, even under reduced pressure as described, may be dangerous [1], A Hungarian patent describes a safe procedure for in-situ generation of the ester, azeotropic dehydration and subsequent metal-catalysed reaction with 1,3-dienes to give alkyl cyclopropanecarboxylates [2],... 

In the synthesis of esters of sugars, high yields were obtained in many cases, as reported in Table 12.5. This can be attributed to the high temperatures used in these experiments (usually experiments are conducted at 40-80 °C), since the removal of water in open vials can shift the reaction toward the product [27]. The same purpose was successfully achieved by using azeotropic distillation [25]. An increase in ester yield with decreasing temperature was observed, and this behavior was related to the decreased product solubility. 

Table 8.3 lists the properties of the azeotropes that 2-ethylhexanol and lauric acid form with water, both heterogeneous. It is remarkable that the solubility of water in acid and alcohol is large, while the reverse solubility, organic components in water, is extremely low, particularly for the acid. Pressure has little effect No information about water/fatty ester azeotrope is available, but this should not affect the analysis. 

Diazomethane methylation is a good way of making methyl esters from carboxylic acids on a small scale because yields are excellent and the only by-product is nitrogen. However, there is a drawback diazomethane has a boiling point of-24°C, and it is a toxic and highly explosive gas. It therefore has to be used in solution, usually in ether the solution must be dilute, because concentrated solutions of diazomethane are also explosive. It is usually produced by reaction of N-methyl-N-nitrosourea or N-methyl-N-nitrosotoluenesulfonamide with base, and distilled out of that reaction mixture as an azeotrope with ether, straight into a solution of the carboxylic acid. 

Hydrolytic enzymes such as lipases and proteases catalyze readily reversible reactions and will often promote reverse hydrolysis at reduced water activities. Water can be removed with desiccants, as an azeotrope with a solvent or through application of a vacuum. Lipases have proven particularly useful in this regard, allowing the formation of esters from alcohols and either free carboxylic acids or esters (see Figure 31.12). 

FIG. 13-98 Flow sheet for making esters which form a heterogeneous minimum-boiling azeotrope with water. 

Similar to the conventional method for the preparation of organic esters by esterification reactions, the alkoxides (orthoesters) of mainly metalloids as well as organometallic moieties (e.g., RXB, R,Ge, RxSn, or PhHg) can be synthesized (6, 34) by the following types of reactions. These reactions can be pushed to the right by removal of water, which is formed, azeotropically with an organic solvent (e.g., benzene/toluene). 

Methanol forms azeotropes with many substances, particularly esters, and often cannot be recovered from spent solvents or from reaction mixtures by simple distillation. Pervaporation provides a simple way to break these azeotropes. Used alone or in combination with distillation, such units provide an economical and reliable route to recover or remove methanol (Fig. 26). 

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