Esterification acid-catalysed

The reaction mechanism for the heterogeneous and homogeneous acid-catalysed esterification were reported to be similar (17). However, there is a major difference concerning the snrface hydrophobicity. Reaction pockets are created inside a hydrophobic environment, where the fatty acid molecules can be absorbed and react further. Water molecules are unlikely to be absorbed on sites enclosed in hydrophobic areas. 

Solid Acid Catalysed Esterification of Amino Acids and Other Bio-Based Acids... 

Yuen et a/.31 first demonstrated the nature of the information that can be obtained regarding chemical mapping within a fixed-bed reactor, using the liquid phase esterification of methanol and acetic acid catalysed within a fixed bed of H+ -ion... 

Acid-catalysed reaction measured in the range pH 1-4. Units dm5 mol-1 s-1 The reference intermolecular reaction is the esterification of acetic acid by ethanethiol under the same conditions c Storm and Koshland, 1972b... 

In the absence of ultrasound, the results show a substantial amount (49 %) of the dimer bicyclohexyl from the one-electron pathway, together with cyclohexylmethyl-ether, cyclohexanol and other products from the two-electron pathway (approx. 30%). The methyl cyclohexanoate ester (17%) can be thought to arise from the acid catalysed chemical esterification of the starting material with the solvent methanol. (As a result of the high current densities needed, (parasitic) discharge of the solvent methanol produces a large quantity of protons around the anode as a competitive reaction [54].)... 

Most hterature references to pharmaceutical primary process monitoring are for batch processes, where a model of the process is built from calibration experiments [110, 111]. Many of these examples have led to greater understanding of the process monitored and can therefore be a precursor to design of a continuous process. For example, the acid-catalysed esterification of butan-l-ol by acetic acid was monitored through a factorial designed series of experiments in order to establish reaction kinetics, rate constants, end points, yields, equilibrium constants and the influence of initial water. Statistical analysis demonstrated that high temperatures and an excess of acetic acid were the optimal conditions [112]. 

The most important reactions of carboxylic acids are the conversions to various carboxylic acid derivatives, e.g. acid chlorides, acid anhydrides and esters. Esters are prepared by the reaction of carboxylic acids and alcohols. The reaction is acid catalysed and is known as Fischer esterification (see Section 5.5.5). Acid chlorides are obtained from carboxylic acids by the treatment of thionyl chloride (SOCI2) or oxalyl chloride [(COCl)2], and acid anhydrides are produced from two carboxylic acids. A summary of the conversion of carboxylic acid is presented here. All these conversions involve nucleophilic acyl substitutions (see Section 5.5.5). 

Transesterification Transesterification occurs when an ester is treated with another alcohol. This reaction can be acid catalysed or base catalysed. This is where the alcohol part of the ester can be replaced with a new alcohol component. The reaction mechanism is very similar to the Fischer esterification. 

The acid-catalysed hydrolysis of an ester is the reverse reaction of the Fischer esterification. Addition of excess water drives the equilibrium towards the acid and alcohol formation. The base-catalysed hydrolysis of esters is also known as saponification, and this does not involve the equilibrium process observed for the Fischer esterification. 

Fig. 15. Taft correlation with steric substituent constants (Es) in the vapour phase esterification of carboxylic acids with ethanol over Na-poisoned silica—alumina at 250°C ( ) [126] and in homogeneous acid-catalysed esterification at 40°C (°) [419], Acids 1, acetic, 2, propionic, 3, butyric, 4, isobutyric, 5, isovaleric, 6, pivalic, 7, 2-ethylbutyric.

It follows from all the above considerations that the acidic character of the surface is necessary for the esterification reaction. This view is supported by the parallel found by some workers [405,406] between the rate of esterification and that of other typical acid-catalysed reactions. A linear correlation was established between the rate of acetic acid—ethanol esterification and that of deisopropylation of isopropylbenzene on a series of silica—alumina, alumina—boria and alumina catalysts [406] a similar relation was found between the rate coefficient of the same esterification reaction and the cracking activity of a series of silica—alumina catalysts prepared in a different way [405]. 

The homogeneous acid-catalysed esterification has been thoroughly studied and a lot of information on its mechanism has been accumulated. The assumed close relation to the heterogeneous mechanism can, therefore, be made use of in mechanistic considerations about the esterification catalysed by ion exchangers. 

The solid acids are highly active in typical Bronsted acid catalysed reactions including esterifications. Details of these will be reported elsewhere. 

The enantioselective esterification of 2-arylpropionic acids catalysed by a lipase was discussed earlier.26 Steady-state kinetics of the Pseudomonas cepacia lipase-catalysed hydrolysis of five analogous chiral and achiral esters (R)- and (.S )-(235 R1 = Me, R2 = H), (R)- and (reaction mixtures of water-insoluble substrates.212 The Km values were all die same and the apparent kcat values reflected the binding abilities of the alcoholate ions for the fast-reacting enantiomers. All the substrates are believed to be... 

Because the rates of this reaction are very rapid, normal sampling techniques were not satisfactory and an infrared technique was used. This esterification reaction was shown to be about 100 times faster than the disproportionation reaction and inter-intra-molecular assistance was also found to be important. This assistance seems to be a common pattern in acid-catalysed processes of oligosiloxanols in inert solvents. In dioxane solvent the redistribution kinetics can be interpreted in terms of an unzipping mechanism. The ratedetermining step is terminal silanol cleavage by water forming dimethylsilanediol which rapidly reacts with other substrate silanols (Scheme 4). 

The acid-catalysed esterification reaction usually proceeds via an acyl-oxygen fission process. This involves the cleavage of the bond between the original carbonyl-carbon atom and an oxygen of an hydroxyl group in the intermediate (6) arising from nucleophilic attack by an alcohol molecule on the pro-tonated carboxylic acid group (5). 

The process of acid-catalysed esterification in the presence of benzene, or, better, of toluene, is greatly facilitated if the water produced in the reaction is removed by distillation in a Dean and Stark water separation unit. This allows the separation and removal of water from the azeotrope, the organic phase being returned continuously to the reaction flask. Examples are provided in Expt 5.145. These include examples of the synthesis of esters where either the alcohol... 

A recent procedure for the preparation of methyl esters involves refluxing the carboxylic acid with methanol and 2,2-dimethoxypropane in the presence of toluene-p-sulphonic acid as the catalyst (Expt 5.146). The water produced in the esterification process is effectively removed by acid-catalysed reaction with the ketal to give acetone and methanol. 

Fries rearrangement and phenol acetylation The Fries rearrangement is the acid catalysed transformation of aryl esters into hydroxyarylketones. Both this rearrangement and the two-step transformation (esterification, Fries rearrangement) in one-pot operation of phenols with carboxylic acid or anhydrides will be examined hereafter. Most studies in which acid zeolites were used as catalysts (Tables 3.6 and 3.7) deal with the synthesis of o- and p-hydroxyacetophenones (o- and p-HAP) either by the Fries rearrangement of phenyl acetate [Reaction (3.5)] ... 

Figure 3.11 Solvent-free, acid catalysed esterification [PTSA = p-toluenesulfonic acid ...

Microwave heating and catalysis have been successfully used in the solvent-free synthesis of cosmetic fatty esters (Villa et al., 2003). Two kinds of reaction were performed acid-catalysed esterification (Figure 3.11) and phase-transfer catalysed alkylations, both reactions affording near quantitative yields when microwave heating was used. It should be noted that diethyl ether and water were used in the purification of the product, and alternative purification/separation procedures would be required if this process was performed on an industrial scale, due to the flammability risk of diethyl ether. 

Fig. H. Mechanism for the acid-catalysed esterification of a carboxylic acid.

Imagine the reaction between a chiral, but racemic alcohol and a chiral, but racemic carboxylic acid, to give an ester in an ordinary acid-catalysed esterification (Chapter 12). 

---