Rate determining step carboxylic esters with

Although more studies have been devoted to the mechanism of the acylation of amines with carboxylic esters than with other reagents, the mechanistic details are not yet entirely clear.898 In its broad outlines, the mechanism appears to be essentially BAC2.899 Under the normal basic conditions, the reaction is general base-catalyzed,900 indicating that a proton is being transferred in the rate-determining step and that two molecules of amine are in-... 

Many of the esters which are hydrolyzed by the AalI mechanism in acid are also hydrolyzed with alkyl-oxygen fission under neutral condi-tions60,67 74 75 84 85 88 89. These reactions have the high enthalpies and entropies of activation characteristic of unimolecular reactions, and involve the ionization of (usually) tertiary alkyl esters, to the carbonium ion and a carboxylate anion in the rate-determining step, viz. 

It was proposed that propagation proceeds by reaction of carboxyl anion with epoxide to form an ester-alkoxide anion which can react in turn with anhydride to form ester and another carboxyl anion. The rate-determining step was thought to be the reaction of carboxyl anion with epoxide, which would lead to first-order kinetics with respect to epoxide concentration. 

If the reaction between the enol and the electrophile proceeds extremely fast, the enol tautomer of a carbonyl or carboxyl compound might be consumed completely. The generation of enol becomes the rate-determining step. This situation occurs with the enol titration of ace-toacetic ester, (Figure 12.4). In this process, bromine is added to an equilibrium mixture of the ketone form (B) and the enol form (iso-B) of an acetoacetic ester. Bromine functionalizes the enol form via the intermediacy of the carboxonium ion E to form the bromoacetic ester D. The trick of conducting the enol titration is to capture the enol portion of a known amount of acetoacetic ester by adding exactly the equivalent amount of bromine. From the values for... 

On the other hand, the substrate may undergo nucleophilic attack by base, either in the rate-determining step — with or without formation of an intermediate — or in a fast pre-equilibrium step which is followed by rate-determining breakdown of the intermediate. These three possibilities are included in the B2 mechanism according to Ingold s nomenclature [14]. Examples of one-step B2 reactions (SN2 mechanisms) are the alkaline hydrolyses of sulfonic esters [14] and 2,4,6,-tri-f-butylbenzoic esters [18]. Intermediates are formed by carbonyl addition of hydroxide ion in the alkaline hydrolyses of (unhindered) carboxylic esters and amides. Addition of OH is partially or completely rate-determining in ester hydrolysis [4, 15], but probably not in amide hydrolysis [15]. 

In the NMR experiments carried out by Wenthe and Cordes [187] with methyl orthobenzoate and methyl orthocarbonate in CD3OD—D20 solutions, the rate coefficients for the disappearance of orthoester and those for the formation of CH3OD and of carboxylic ester have been found identical within experimental error (Table 15). This indicates that there is no exchange of methoxy groups prior to hydrolysis. The same result has been obtained from product analysis studies of the carboxylic esters formed. Consequently, the rate-determining step must be carbonium ion formation or a previous step. The findings do not support an A2 mechanism, for the following reason. As the nucleophilic reactivities of water and methanol are similar, the A2 reaction with attack of water... 

The experimental evidence obtained has shown that the reactivity of the estos of the aromatic and aliphatic carboxylic acids under study in the reaction of alcoholysis with cellulose changes substantially with changes in the chemical constitution of the acyl radical. According to existing con<%ptions (52), the alcoholysis of esters in the presence of an alkaline catalyst at the rate-determining step involves the attack of the alkoxy ion on the carbon atom erf the carbonyl group in the case of... 

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. 

The cr value alone can, of course, be used to understand the mechanism of those reactions which do not come under the ambit of cj+ and u values. For instance, the base hydrolysis of a benzoic acid ester may take two different pathways (a) nucleophilic attack of hydroxide ion on the carbonyl carbon to result in a tetrahedral intermediate, in a rate determining step, followed by its collapse into carboxylic acid or (b) nucleophilic attack of hydroxide ion on the alkyl carbon of the ester function, leading to the formation of the carboxylate directly. Since the carbonyl carbon is closest to the ring, the effect of a substituent felt by it must be much larger than the effect felt by the alkyl carbon. Hence, the rate of hydrolysis will be expected to increase much more in the former instance than in the latter with the increase in the substituent s a value. This is indeed the case as evident from the a versus rate constant k given in Table 3. The large increase in the rate of hydrolysis with the increase in cr could be justified only if the tetrahedral pathway is involved. The correlation of cr with log k is linear as seen from the plot in Fig. 5. 

Carbonylation and decarbonylation reactions of alkyl complexes in catalytic cycles have been reviewed . A full account of the carbonylation and homologation of formic and other carboxylic acid esters catalysed by Ru/CO/I systems at 200 C and 150-200 atm CO/H2 has appeared. In a novel reaction, cyclobutanones are converted to disiloxycyclopentenes with hydrosilane and CO in the presence of cobalt carbonyl (reaction 4) . The oxidative addition of Mel to [Rh(CO)2l2] in aprotic solvents (MeOH, CHCI3, THF, MeOAc), the rate determining step in carbonylation of methyl acetate and methyl halides, is promoted by iodides, such as Bu jN+I", and bases (eg 1-methylimidazole) . A further kinetic study of rhodium catalysed methanol carbonylation has appeared . The carbonylation of methanol by catalysts prepared by deposition of Rh complexes on silica alumina or zeolites is comparable with the homogeneous analogue . 

The imidazole-catalysed hydrolysis of polar substituted 2,4-dinitrophenyl acetates (21 X = Cl, OMe) has been investigated at different temperatures. The observed rates correspond to the bimolecular nucleophilic addition of the imidazole at the carboxylic carbon atom followed by a very fast hydrolysis of the (V-acetylimidazole in water. The influence of polar substituents in the acid moiety of the ester molecule on the hydrolysis reaction can be described by an electrostatic dipole-dipole interaction in the same way as the neutral hydrolysis of polar substituted ethyl acetates. By the use of both quantum and classical dynamics, a study of the neutral hydrolysis of 4-methoxyphenyl dichloroacetate (22) in water concluded that the rate-determining step is a proton transfer concerted with formation of a C-O bond. ... 

Oxalic and malonic acids, as well as a-hydroxy acids, easily react with cerium(IV) salts (Sheldon and Kochi, 1968). Simple alkanoic acids are much more resistant to attack by cerium(IV) salts. However, silver(I) salts catalyze the thermal decarboxylation of alkanoic acids by ammonium hexanitratocerate(IV) (Nagori et al., 1981). Cerium(IV) carboxylates can be decomposed by either a thermal or a photochemical reaction (Sheldon and Kochi, 1968). Alkyl radicals are released by the decarboxylation reaction, which yields alkanes, alkenes, esters and carbon dioxide. The oxidation of substituted benzilic acids by cerium(IV) salts affords the corresponding benzilic acids in quantitative yield (scheme 19) (Hanna and Sarac, 1977). Trahanovsky and coworkers reported that phenylacetic acid is decarboxylated by reaction with ammonium hexanitratocerate(IV) in aqueous acetonitrile containing nitric acid (Trahanovsky et al., 1974). The reaction products are benzyl alcohol, benzaldehyde, benzyl nitrate and carbon dioxide. The reaction is also applicable to substituted phenylacetic acids. The decarboxylation is a one-electron process and radicals are formed as intermediates. The rate-determining step is the decomposition of the phenylacetic acid/cerium(IV) complex into a benzyl radical and carbon dioxide. 

A study of QDC oxidation of heterocyclic aldehydes and carboxylic acids in AcOH-H2O supports a rate-determining decomposition of the chromate ester. The rate of oxidation of dimedone (DM) in HCIO4 is of first order in QDC, of order less than unity in DM and H+ ions, and increases with increasing dielectric constant. The mechanism postulated involves the formation of a complex between protonated Cr(VI) species and DM, which decomposes to a DM free radical and Cr(V) in a slow step followed by fast steps to give the products. The rate of oxidation of L-glutamic acid (Glu) in HCIO4 is first order in QDC and Glu, and increases with [H+] and the ionic strength. 

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