Ester exchange, mechanism

The mechanism is like that for ester exchange but hydroxide is used up in deprotonating the carboxylic acid produced so a whole equivalent of NaOH is needed. In acidic solution, protonation of the carbonyl oxygen atom makes the ester more electrophilic and attack by the weak nucleophile (water) is made faster but the acid catalyst is regenerated. In both these reactions nucleophilic attack is the rate-determining step. 

Amide group scission (termed as cA-elimination mechanism) and intramolecular rearrangement of two amide groups, leading to cyclic compounds [20] are the main pyrolysis reactions in acyclic polyamides. The former reaction is outlined in Scheme 12.1b and the ester exchange drawn in Scheme 12.2 is analogous to the latter one. 

This strongly confirms the ion exchange mechanism which initially had been suggested. The model compounds with free carboxylic groups show the strongest effect, which is consistent with the theory. The other compounds show weaker, but temperature-dependent effect. This means that even other functional groups, such as chemically bound carboxylic acid (open or ring-shaped esters) or chemical precursors for such carboxylic acid... 

The hydro complex has been suggested as an intermediate in the ester exchange reaction of soybean oil methyl ester catalyzed by PdClg-(PPh3)2SnCl2-2H20. The mechanism involves coordination of the ester... 

Fig. 22. Suggested mechanism for the ester exchange reaction catalyzed by palladium complexes.

Furthermore, a study of vinyl ester exchange, which proceeds by a mechanism analogous to the acetoxypalladation mechanism for olefin oxidation [see Section III, B, 1, Eqs. (174) and (175)], indicates that the dimer is the most catalytically active species, with the trimer next, and the monomer, (NaaPd(OAc)4), unreactive. In the study of Moiseev et ah, a maximum rate is attained at the point at which the concentration of Na2Pd2(OAc)g reaches a maximum. Thus the dimer is the reactive species rather than Na2Pd(OAc)4. However, the decrease in rate with increasing [NaOAc] is greater than can be explained on the basis of conversion of dimer to unreactive monomer [Eq. (5), Section II, A, 2]. 

It can be speculated that the ester exchange reaction has already occurred but complete randomization of the monomers has not yet occurred. In that case, a blocky structure would be expected. However, as will be demonstrated later, the observed DSC traces are not consistent with the occurrence of a blocky structure. Thus, the conclusion is reached that the present data suggests that transesterification is not a dominant mechanism in determining the behavior of the present blends. 

A number of chelates, structurally similar to bis[iV-(carboxymethyl methyl ester)salicylaldimine]copper(II), undergo repeated ester exchange. When this compound, which has a trans square-planar structure (LV), is refluxed in n-butyl alcohol for about 10 minutes, transesterification of the amino acid methyl ester occurs to give the butyl ester. When the compound is refluxed with n-butylamine, the corresponding butylamide is obtained (168, 202). Transesterification does not occur, however, with (1-carbo-methoxy)pentamethylenebis(salicylaldimine-l,5)copper(II), since the configuration of this compound is fixed in the cis form by the pentamethylene ring (LVI) (202). An important function of the metal ion is therefore to position the reactive centers in such a manner that the reaction is facilitated. The mechanism proposed for transesterification involves an attack by the... 

This is a difficult problem but examination of the proposed mechanism should show you that binding to the magnesium holds the side chain over the pyridine ring. Enzymatic reactions often use binding to metals to hold substrates in position. Of course, in this example, the substrate is covalently bound to the reagent but simple ester exchange with MeO" in MeOH releases it. 

Ramjit, H. G., The influence of stereochemical structure on the kinetics and mechanism of ester-ester exchange reaction by mass spectrometry,. Macromol Sci. Chem. Ed., A19, 41, 1983. 

---