Esterifications involving other acid catalysts

Sulphuric acid has also been used in esterification but, because it is less easily eliminated at the end of the reaction than hydrochloric acid, the isolation of the esters usually requires their extraction into a non-polar solvent such as hexane. Also, with sulphuric acid there is sometimes a risk of dehydration reactions, charring and/or oxidative side reactions, which is why it is not so popular. 

Free fatty acids are dissolved in a 4 1 mixture of the alcohol with light petroleum (b.p. 40-60 Q containing 

2% sulphuric acid at 70 °C for an hour. Triglyceride fats can be transesterified in the same way but this takes four hours [31]. Similar transesterification procedures have been described for triglyceride fats using chloroform [32] or ether [33] as solvent the use of acid conditions rather than the usual base-catalysed transesterification conditions was to avoid isomerization. 

These esters may be made from the free acids or by transesterification of the methyl esters using 2% sulphuric acid in 2-chloroethanol at 60 °C for two hours. The mixture is poured into water and the esters are extracted into petroleum ether [34]. These esters may also be made using the commercially available 10% boron trichloride in 2-chloroethanol (see Section 2.2.5). 

3 Methyl esters with boron trifluoride/methanol 

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The esterification of acetic acid with ethanol using sulfonic ion-exchange resins as catalyst/selective sorbent was studied by Mazzotti et al. [164]. The authors developed a detailed mathematical model, which was able to predict correctly the system s behavior. They succeeded in obtaining 100% conversion of acetic acid in addition to a complete separation. Several other studies involving enzymatic reactions were also carried out and will be presented in more detail in the next section. 

In the DOD, phytosterols are present in both the free and esterified forms with fatty acids. Therefore, the first step in the extraction of phytosterols is conversion of phytosterol fatty esters into free phytosterols. This is achieved either by hydrolysis or trani-esterification. Hydrolysis could be carried out under strong basic conditions (saponification with further acidulation), under strong acidic conditions, or under chemical or enzyme (specific or nonspecific) catalyzation. Re-esterification of phytosterols occurs during methyl ester distillation as a result of the high temperatures involved therefore, a further trani-esterification step for free sterols is required. Esterification of phytosterols or trani-esterification of sterol fatty acid esters is the second step in this process. Methanol is the most commonly used alcohol, and it leads to methyl esters, which are characterized by a higher volatility, however, other Ci to C4 alcohols may also be used. Esterification and trans-esterification of fatty acids or phytosterols can be catalyzed by metal alcoholates, or hydroxide, by organic catalysts, or by enzymes (Table 7). 

Several explosions involving methanol and sodium hypochlorite were attributed to formation of methyl hypochlorite, especially in presence of acids or other esterification catalyst. 

The process involves reacting the degummed oil with an excess of methyl alcohol in the presence of an alkaline catalyst such as sodium or potassium methoxide, reaction products between sodium or potassium hydroxide and methyl alcohol. The reaction is carried out at approximately 150°F under pressure of 20 psi and continues until trans-esterification is complete. Glycerol, free fatty acids and unreacted methyl alcohol are separated from the methyl ester product. The methyl ester is purified by removal of residual methyl alcohol and any other low-boiling-point compounds before its use as biodiesel fuel. From 7.3 lb of soybean oil, 1 gallon of biodiesel fuel can be produced. See FIGURE 12-5. 

Fig. 23.1 Microbial routes from natural raw materials to and between natural flavour compounds (solid arrows). Natural raw materials are depicted within the ellipse. Raw material fractions are derived from their natural sources by conventional means, such as extraction and hydrolysis (dotted arrows). De novo indicates flavour compounds which arise from microbial cultures by de novo biosynthesis (e.g. on glucose or other carbon sources) and not by biotransformation of an externally added precursor. It should be noted that there are many more flavour compounds accessible by biocatalysis using free enzymes which are not described in this chapter, especially flavour esters by esterification of natural alcohols (e.g. aliphatic or terpene alcohols) with natural acids by free lipases. For the sake of completeness, the C6 aldehydes are also shown although only the formation of the corresponding alcohols involves microbial cells as catalysts. The list of flavour compounds shown is not intended to be all-embracing but focuses on the examples discussed in this chapter... 

Esters are produced by acid-catalysed reaction of carboxylic acids with alcohols, known as Fischer esterification. They are also obtained from acid chlorides, acid anhydrides and other esters. The preparation of esters from other esters in the presence of an acid or a base catalyst is called transesterification. All these conversions involve nucleophilic acyl suhstitu-tions (see Section 5.5.5). 

The synthesis of vinyl esters of the higher monocarboxylic acids is troublesome and costly. The preparation may involve transvinylation with vinyl acetate and the higher carboxylic acid with a costly mercuric salt as a catalyst. In this procedure, the equilibrium situation is unfavorable and yields of product usually are low. Alternatively, a similar catalyst may be used for the addition of acetylene to the carboxylic acid. This procedure requires pressure equipment not usually available in a synthesis laboratory. The allyl esters, on the other hand are readily prepared by conventional esterification procedures. Therefore, they are of potential interest as plasticizers which might copolymerize with resins such as vinyl acetate that normally give rise to hard resins. Unfortunately, allyl esters of more ordinary carboxylic acids homopolymerize sluggishly. Their ability to enter into copolymer systems also seems to be marginal. 

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