Transesterification stepwise

Cleavage of this group is achieved with tetramethylguanidinium syn-2-pyridinecarboxaldoxime. Tris(hexafluoro-2-propyl) phosphites are sufficiently reactive to undergo transesterification with alcohols in a stepwise fashion. ... 

FIGURE 5 Stepwise synthesis of a triblock copolymer (PCL-PLA-PCL) of PCL and polylactic acid using aluminum coordination catalysts to minimize randomization of the block structure by transesterification. (From Ref. 43.)... 

The homogeneous acid-catalyzed transesterification process does not enjoy the same popularity in commercial application as its counterpart, the base-catalyzed process, one of the main reasons being that it is about 4000 times slower, due to the different mechanism [10]. Thus, in the reaction sequence triglyceride is converted stepwise to diglyceride, monoglyceride and finally glycerol with formation of one molecule of methyl ester at each step (Scheme 10.1). 

Transesterification converts one ester into another by reaction with an alcohol in the presence of an acid catalyst. Draw a stepwise mechanism for the following transesterification. 

The vast majority of modern phosphate esters are synthetic , using materials derived from petrochemical sources. For example, alcohols are from a-olefins by the 0X0 process and iso-propylated or t-butylated phenols are produced from phenols by reaction with propylene or butylene. Reaction of alcohol or phenol with phosphoryl chloride yields the crude product, which is washed, distilled, dried and decolourised to yield the finished product. Low molecular weight trialkyl esters are water soluble and require non-aqueous techniques. For mixed alkylaryl ester production the reactant phenol and alcohol are added separately, the reaction being conducted in a stepwise fashion. Reaction temperatures are kept as low as possible to avoid transesterification reactions. 

Effect of hexane addition on the transesterification of soybean oil using an ethanol/soybean oil molar ration of 3 (stepwise ethanol addition 1/3 at Oh, 1/3 after 0.5 h, and 1/3 after 1 h), 7% mim immobilized lipase from A. niger 11T53A14, at 40 C... 

Another useful method is the transesterification of diphenyl carbonate with bisphenols under elimination of phenol. However, the major disadvantages are that the process requires high temperature and vacuum. The polycmidensation reaction is carried out stepwise, i.e., the temperature and vacuum are raised slowly, in order to avoid diphenyl carbonate loss by sublimation at the beginning of the reaction. However, an excess of diphenyl carbonate is necessary in order to obtain high molecular weight. 

Aspirin is an anti-infiammatory agent because it inhibits the conversion of arachidonic acid to prostagiandins by the transfer of its acetyi group (CH3CO-) to an OH group at the active site of an enzyme (Section 19.6). This reaction, caiied transesterification, resuits in the conversion of one ester to another by a nucieophiiic acyi substitution reaction. Draw a stepwise mechanism for the given transesterification. 

Monoglyceride Methanol FIGURE 12.2 Scheme for stepwise transesterification mechanism. 

Transesterification of triglycerides has a stepwise reversible reaction route in which diglycerides and monoglycerides are intermediates (Scheme 2) ... 

An excess alcohol makes the forward reaction pseudo-first order whereas reverse reaction second order in the case of com oil (Meher et al., 2006). When com oil is transesterified in a pressiuized batch reactor in the presence of sodium methoxide and methanol, higher conversion can be obtained. Kinetic constants of the stepwise reactions are increased in the direction of the progressing steps of the transesterification (Velazquez 2007). 

The oil obtained from microalgae usually has a higher viscosity than diesel oil, which cannot be directly applied to engines. Transesterification is needed to reduce the viscosity and increase the fluidity. Transesterification occurs stepwise with the first conversion of TAG to DAG and then to MAG and finally to glycerol. In this process, large, branched triglycerides are transformed into smaller, straight-chain molecules, which are similar in size to the molecules of the species present in diesel fuel (Sinha et al., 2008). 

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