Proton-catalyzed esterification

The first studies on esterifications were carried out by Berthelot233,234). Goldschmidt235-239 studied many proton-catalyzed esterifications in alcohol at relatively low temperatures (below 80 °C) without removal of water. He suggested a pseudo first-order mechanism ... 

Unlike carboxylic esters, open-chained S-alkyl thiocarboxylates cannot be obtained by direct proton-catalyzed esterification. Whereas thiocarboxylic acids react with alcohols to form esters as expected, thiol esters are formed on reaction of carboxylic acids with thiols, but the equilibrium is shifted towards the educts and the reaction is not useful for synthetic purposes. Therefore, activation is necessary and several methods have been developed to meet this requirement. 

Neutralization of Oils by Pre-Esterification. The free fatty acids present in the raw material are esterified with methanol. The proton-catalyzed esterification can be carried out homogeneously [204]. However, the result is that some of the acidic catalyst is... 

In this article we critically review most of the literature concerning non-catalyzed, proton-catalyzed and metal-catalyzed polyesterifications. Kinetic data relate both to model esterifications and polyeste-rificatiom. Using our own results we analyze the experimental studies, kinetic results and mechanisms which have been reported until now. In the case of Ti(OBu)f catalyzed reactions we show that most results were obtained under experimental conditions which modify the nature of the catalyst. In fact, the true nature of active sites in the case of metal catalysts remains largely unknown. 

Activation Parameters of Non-catalyzed and Protonic Acid-catalyzed Esterifications and Polyesterifications. 83... 

As noted below, such deprotonation/protonation chemistry occurs repeatedly without any loss of the hyperbranched graft as measured by ellipsometry (vide infra). This stability to acid and base mirrors the stability noted above for these hyperbranched grafts in continuous extractions and acid-catalyzed esterification chemistry and is thought to be a consequence of the light cross-linking that occurs during the grafting process. 

In addition to their acidic properties, carboxylic acids also can act as weak bases when the carbonyl oxygen accepts a proton from a strong acid, such as H2S04, HC104, or HSbF6 in S02 (Equation 18-4). Such protonation is an important step in acid-catalyzed esterification, as discussed in Section 15-4D ... 

TMSCl, MeOH, 2,2-dimethoxypropane, rt, 95-99% yield. As with the above case, aromatic acids are not esterified by this method which generates HCl in situ In general, it is more difficult to prepare aromatic esters by acid catalyzed esterification than aliphatic esters because aromatic acids are not as easily protonated. BCI3 in MeOH has been used to prepare methyl esters and this combination of reagents also produces HCl. ... 

This pattern of increased reactivity resulting from carbonyl group protonation has been seen before in nucleophilic additions to aldehydes and ketones (Section 17.6) and in the mechanism of the acid-catalyzed esterification of carboxylic acids (Section 19.14). Many biological reactions involve nucleophilic acyl substitution and are catalyzed by enzymes that act by donating a proton to the carbonyl oxygen, the leaving group, or both. 

SAMPLE SOLUTION (a) The reaction given is the acid-catalyzed esterification of methanoi by benzoic anhydride. The first step is the activation of the anhydride toward nucieophiiic addition by protonation. 

The study of gas-phase reactions has obviously been of much assistance in understanding solution-phase reactions. Sometimes, however, this relationship breaks down because the reactants give a different reaction in the gas phase than in the solution phase. Caserio and Kim present such an example in Chapter 5, a thorough study of gas-phase reactions (ion cyclotron resonance) of alcohol nucleophiles with protonated carboxylates, carbonates, and phosphates. The goal of this work is to understand solvent and counterion effects on acid-catalyzed esterification. The gas-phase reaction pathways, however, turn out to be different from those in solution phase. 

These results and discussion in favor of the p-toluene sulfonic acid (pTSA) as an effective catalyst for polyesterification of dimer acid and butanediol strongly supports the proton-catalyzed nature of polyesterification. Usually, the proton-catalyzed mechanism for esterification is extrapolated to proton-catalyzed polyesterification [104]. The polyesterification of dimer acid and butanediol involves protonation of the dicarboxylic acid by the reaction of protonated species with the hydroxy group of glycol to yield the polyester. The proton catalyzing the protonation of carboxylic acid is provided by the carboxyl group of the monomer, i.e., dimer acid, and by pTSA in absence and presence of added catalyst, respectively. 

A mechanism frequently proposed for the Bronsted acid-catalyzed esterification reactions is based on fatty acid carbonyl activation by proton generated by the catalyst, followed by alcohol attacks in the carbonyhc carbon, generating a protonate intermediate who after water elimination results in the ester formation (Figure 5). 

Esterification. Add-catalyzed esterification of carboxylic acid with alcohol produces water as a coproduct. However, a greener method of esterification of carboxylic acid is via the addition of olefin, which does not produce any co-products and therefore, displays 100% atom efficiency. Proton-exchanged montmorillonite (H+-mont) can be used as a catalyst for addition reactions of carboxylic acids to alkenes (Fig. 7) (38)a). For example, the reaction of benzoic acid with norbomene in the presence of the H+-mont catalyst gave 2-benzoyloxynorbomane. Even with less-reactive simple alkenes, such as cyclopentene and cyclohexene, the corresponding esters were obtained in excellent yields. 

The kinetics of Dowex 50 Wx8-catalyzed esterification was studied between acetic acid and benzyl alcohol (Ali Merchant, 2009). The swelling ratio of Dowex 50 Wx8 in different solvent was measured, and the results show that it decreases in the order of water, benzyl alcohol, acetic add, benzyl acetate. Water appears to be preferentially adsorbed by the catalyst from a binary solution of acetic acid and water, and hinders the approach of butanol to the protonated acid. Therefore, the water exerts an adverse effect on the esterification rate. 

Mechanism of Acid-Catalyzed Esterification Step 1. Protonation of the carboxy group... 

Esters undergo nucleophilic substitution reactions by means of addition-elimination pathways, albeit with reduced reactivity relative to halides and anhydrides. Thus, catalysis by acid or base becomes a frequent necessity. For example, esters are cleaved to carboxylic acids and alcohols in the presence of excess water and strong acid, and the reaction requires heating to proceed at a reasonable rate. The mechanism of this transformation is the reverse of acid-catalyzed esterification (Section 19-9). As in esterification, the acid serves two purposes It protonates the carbonyl oxygen to make the ester more reactive toward nucleophilic attack, and it protonates the alkoxy oxygen in the tetrahedral intermediate to make it a better leaving group. 

Acid-catalyzed ester hydrolysis can occur by more than one mechanism, depending on the structure of the ester. The usual pathway, however, is just the reverse of a Fischer esterification reaction (Section 21.3). The ester is first activated toward nucleophilic attack by protonation of the carboxyl oxygen atom, and nucleophilic addition of water then occurs. Transfer of a proton and elimination of alcohol yields the carboxylic acid (Figure 21.8). Because this hydrolysis reaction is the reverse of a Fischer esterification reaction, Figure 21.8 is the reverse of Figure 21.4. 

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. 

Other reactions in which cations other than protons are catalyti-cally effective are esterification and acetal formation, catalyzed by calcium salts,277 and the bromination of ethyl cyclopentanone-2-carboxylate, catalyzed by magnesium, calcium, cupric, and nickel, but not by sodium or potassium ions.278 One interpretative difficulty, of course, is the separation of catalysis from the less specific salt effects. The boundary line between salt effects (medium effects) and salt effects (catalysis) is not sharp either in concept or experimentally. 

The esterification of TPA is catalyzed by protons and in standard industrial operations neither an additional esterification catalyst nor a polycondensation catalyst is added to the esterification reactor. Some new antimony-free polycondensation catalysts [125-128] also affect the speed of esterification significantly and it could be advantageous to add them directly into the slurry preparation vessel. Co-monomers, which should be randomly incorporated into the polymer chains, are usually fed into the slurry preparation vessel. How and when additives, catalysts, colorants and co-monomers are added influences the overall reaction rate and therefore affects the product quality. 

In the case of the esterification of the diacid, the reaction is self-catalyzed as the terephthalic acid acts as its own acid catalyst. The reverse reaction, the formation of TPA and EG from BHET is catalytic with regard to the usual metal oxides used to make PET, but is enhanced by either the presence of hydroxyl groups or protons. In the case of transesterification of dimethyl terephthalate with ethylene glycol, the reaction is catalytic, with a metal oxide needed to bring the reaction rate to commercial potential. The catalysts used to produce BHET are the same as those needed to depolymerize both the polymer to BHET and BHET to its simpler esters. Typically, titanium, manganese and zinc oxides are used for catalysts. 

Consider the polyesterification of a diacid and a diol to illustrate the general form of the kinetics of a typical step polymerization. Simple esterification is a well-known acid-catalyzed reaction and polyesterification follows the same course [Otton and Ratton, 1988 Vancso-Szmercsanyi and Makay-Bodi, 1969]. The reaction involves protonation of the carboxylic acid,... 

Fig. 6. 22. Aa[2 mechanism of the acid-catalyzed hydrolysis of carboxylic esters (read from left to right) Aa[2 mechanism of the Fischer esterification of carboxylic acids (read from right to left). H means migration of a proton.

Nucleophilic acyl substitution also takes place in acid. Under acidic conditions, no strong nucleophile is present to attack the carbonyl group. The carbonyl group must become protonated, activating it toward nucleophilic acyl substitution. Attack by a weak nucleophile gives a tetrahedral intermediate. In most cases, the leaving group becomes protonated before it leaves, so it leaves as a neutral molecule. We now cover the Fischer esterification, a particularly useful example of an acid-catalyzed nucleophilic acyl substitution. 

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