Transesterification acid-catalysed

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). 

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. 

Acid-catalysed ester formation and hydrolysis are the exact reverse of one another the only way we can control the reaction is by altering concentrations of reagents to drive the reaction the way we want it to go. The same principles can be used to convert to convert an ester of one alcohol into an ester of another, a process known as transesterification. It is possible, for example, to force this equilibrium to the right by distilling methanol (which has a lower boiling point than the other components of the reaction) out of the mixture,... 

The mechanism for this transesterification simply consists of adding one alcohol (here BuOH) and eliminating the other (here MeOH), both processes being acid-catalysed. Notice how easy it is now to confirm that the reaction is catalytic in H+. Notice also that protonation always occurs on the carbonyl oxygen atom. 

Acetates are generally cleaved under mildly basic conditions but they can be cleaved by acid-catalysed solvolysis (transesterification) as well however, in the... 

Transesterification of the /erf-butyl ester 29.1 to the corresponding methyl ester 29 2 [Scheme 6.29] is not straightforward. Typical acid-catalysed cleavage of the tm-butyl ester is precluded by the acid-sensitivity of the fiiran. whereas basic methanolysis suffers from the electrophilicity of the unsaturated lactone. However, brief immersion of a thin layer of 29.1 in a round bottom flask in an oil bath pre-heated to 210 °C, followed by treatment of the crude product with (trimethylsi)y])diazomethane afforded the methyl ester 29.2 in near-quantitative yield.76 The elimination did not proceed in refluxing decalin (190 °C). 

Sensitive carboxylic acids can be prepared from their esters by acid-catalysed transesterification e.g., acrylic acid can be obtained from its methyl ester by means of formic acid containing sulfuric acid.75... 

Rozenbaha, I., Odham, G., Jamberg, U., Alsberg, T., and Klavins, M., Characterization of humic substances by acid catalysed transesterification. Anal. Chim. Acta, 452, 105-114, 2002. 

The first step is a typical Claisen ester condensation and the second is an acid-catalysed thermodynamically controlled transesterification (the lactone and ethyl ester exchange alcohol partners) to give the more stable six-membered lactone, followed by decarboxylation. Now the Fischer indole synthesis works well and work-up with dry HCl in methanol gave the alkyl... 

Kulkami, M. G., Gopinath, R., Meher, L. C., Dalai, A. K. (2006). Solid acid catalysed biodiesel production by simultaneous esterification and transesterification. Green Chemistry, 8, 1056-1062. 

In acid-catalysed processes, the addition ofp-toluene sulfonic acid was found to cause a reduction in torque in the mixer, darkening, a loss of melt viscosity and enhanced crystalhsabiHty of the PC in the presence of 10-20% PCL. Shuster et al. [131] suggested that the enhanced crystalHnity meant that there was no transesterification and p-toluene sulfonic acid promoted carbonate-ncarbonate and not carbonate-l-ester interchange. Previously Smith et al. [132] had reported that transesterification produced block copolymer and loss of crystallisation and crystalhsabiHty. 

The degree of blockiness in the structure of partially hydrolysed, poly(vinyl acetate) depends on the method of hydrolysis. Saponification with alkali gives vinyl acetate-alcohol copolymers with a highly blocked structure, transesterification with methanolic methoxides gives intermediate blockiness, and acid-catalysed equilibrium hydrolysis gives near-random copolymers. These structures respond differently in iodination analyses. ... 

This is an example of an intramolecular transesterification reaction catalysed by an acid... 

As with acid-catalysed transesterification procedures, an additional solvent, such as toluene or... 

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