Esterification, acid catalysis mechanism

Exactly the same considerations apply to the esterification of hindered acids (182) in the reverse direction. It will be noticed that this mechanism requires protonation on the less favoured (cf. p. 240) hydroxyl oxygen atom (185) to allow the formation of the acyl carbocationic intermediate (184). Apart from a number of R3C types, a very well known example is 2,4,6-trimethylbenzoic (mesitoic) acid (186), which will not esterify under ordinary acid-catalysis conditions—and nor will its esters (187) hydrolyse. Dissolving acid or ester in cone. H2S04 and pouring this solution into told alcohol or water, respectively, is. found to effect essentially quantitative esterification or hydrolysis as required the reaction proceeds via the acyl cation (188) ... 

The widely accepted mechanism of the acid-catalyzed esterification is designated as the Aac2 process (A = acid catalysis, AC = acyl transfer, 2 = bimolecular). As shown in Schemes la and lb the rate-deter-... 

Esters react by both acid and nucleophile initiated mechanisms. Hydrolysis of esters by acid catalysis is exactly the reverse of the mechanism for the acid-catalyzed esterification of a carboxylic acid. Base-catalyzed hydrolysis of esters is called saponification. Hydroxide attacks to form a tetrahedral intermediate. Loss of alkoxide ion then occurs. The alkoxide neutralizes the resulting carboxylic acid to form the salt. 

Show how acid catalysis is used to synthesize acid derivatives, as in the Fischer esterification and in transesterification. Propose mechanisms for these reactions. 

This reaction can be catalyzed by (Lewis or protonic) acids through more or less complex mechanisms. In the case of a catalysis by protonic acids, various mechanisms can be written depending upon the nature of X and Y. For example, the esterification of alcohols is known to be self-catalyzed by antagonist carboxylic acids as shown below ... 

Kemkes256 assumes that the overall order relative to the esterification of terephthalic acid by 1,2-ethanediol in oligo(l,2-ethanediyl terephthalate) is two no mechanism has however been suggested. Mares257 considers that during the esterification of terephthalic acid with 1,2-ethanediol, two parallel kinetic paths take place, one corresponding to a reaction catalyzed by non-dissociated add and the other to a non-catalyzed process. In fact, Mares257 is reserved about the existence of protonic catalysis. Some other orders were found for the system terephthalic atid/l,2-ethanediol 0 (overall)318 2 (add) andO (alcohol)203 1 (add) and 1 (alcohol)181 1 (add)194 . These contradictory results could be partly due to the low solubility of terephthalic acid in 1,2-ethanediol. 

Catalysis by Solid Acids. Two aspects are considered here. The first aspect is concerned with transesterification reactions catalyzed by solid acids. Unfortunately, little research dealing with this subject has been reported in the literature. The second aspect deals with esterification reactions of carboxylic acids (or FFAs). This second part addresses an important characteristic of inexpensive TG feedstocks, i.e., high FFA content. Ideally, an active solid catalyst should be able to carry out transesterification and esterification simultaneously, thus eliminating pretreatment steps. It is likely that heterogeneous catalysts that perform well in esterification should also be good candidates for transesterification since the mechanisms for both reactions are quite similar. 

The combination of carboxyl activation by a carbodiimide and catalysis by DMAP provides a useful method for in situ activation of carboxylic acids for reaction with alcohols.10 The reaction proceeds at room temperature. Carbodiimides are widely applied in the synthesis of polypeptides from amino acids. The proposed mechanism for this esterification reaction involves activation of the acid via isourea 28 followed by reaction with another acid molecule to form anhydride... 

For catalysis by bases, addition esterification is the first and the fastes reaction. It can be followed by slower polyetherification reaction, if epoxy groups are in excess, or by condensation esterification (even slower), if carboxyl groups are in excess. This mechanism was verified on a model system caproic acid — phenylglycidyl ether... 

In commercial practice today reactions of the second type frequently utilize an excess of glycol, increasing the rate of esterification. Later stages of such polycondensations, as in the preparation of polyethylene terephthalate, take place by a transesterification mechanism with liberation of glycol. The direct esterification reaction may be catalysed by a second molecule of the carboxylic acid (self-catalysis) or by an independent acidic catalyst (catalysed esterification). 

Mares et al. [52] recently concluded that hydrogen-ion catalysis during this esterification was not likely and that catalysis by undissociated acid occurred together with a reaction in which, kinetically at least, no form of catalysis was involved. On the other hand, Vansco-Szmercsanyi et al. [53] concluded that polyesterification of maleic, fumaric and succinic acids with ethylene glycol or 1,2-propylene glycol was catalysed by protons. Much work clearly remains to be done on defining the detailed mechanism of catalysis by the acidic species during esterifications. 

Ester formation is a standard organic reaction between an alcohol and a carboxylic acid, which is an equilibrium reaction that has been shown to occur under catalysis by either acid or base. In polyesterification involving an organic acid, the substrate is itself the catalyst. It has been noted (Pilati, 1989) that, among the many reaction mechanisms, Scheme 1.1 is the most likely for acid-catalysed esterification, with the second reaction being the rate-determining step. 

A catalysis through a double Lewis acid activation of the scissile phosphate (coordination of two oxygen atoms of the phosphate onto the copper instead of one oxygen atom for a single Lewis acid assistance) was proposed by Chin (334) to account for the higher reactivity of 8 compared to 6. The mechanism proposed by Bashkin et al. for 6 is totally different and is based on the fact that the optimal rate of phosphate trans-esterification is at a pH value close to the value of the metal-bound water molecule (332, 335). Because monoaqua complexes as in 6 do not form stable four-membered ring phosphate coordinates, they may just behave as acid/base coreactants like histidine residues in ribonuclease A (336) or like imidazole buffer (337). 

These catalysis were employed in the esterification of benzoic acid w ith 1-bromobuiane m toluene or acetonitrile. Interestingly, the reaction rate depends on both loading of the solid catalyst and solvent utilized. For example, in toluene the low TBD-loaded polystyrene exhibits higher reactivity. This can be explained by considering the reaction mechanism initially the benzoic acid 5 reacts with the supported base 4 forming a salt that consists of... 

One of the chemical treatments of triglycerides of different vegetable and algae oils and animal fats produces the biofuel known as biodiesel. This development was based on the knowledge of the catalytic processes as well as on the evolution of instrumental techniques, which permitted the elucidation of the esterification mechanisms and consequently, the study of transesterification. Homogeneous and heterogeneous processes were studied focusing on basic, acid, and enzymatic catalysis [13, 15, 16, 22-36], Usually,... 

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