Organic solvents reaction order

Recent experiments have shown that the concentration of aromatic compound needed to maintain zeroth-order kinetics (see below) was much greater than for nitrations with solutions of nitric acid in some inert organic solvents reactions which were first order in the concentration of the aromatic were obtained when [ArH] < c. 2 x io mol 1 . ... 

Mechanistic aspects of SO2 insertion reactions have been studied by several groups, espedally for CpFe(CO)2R systems. While the intimate details of the mechanism are still not explained totally, some aspects are well understood. On the basis of kinetic studies, Jacobson and Wojdcki proposed a bimolecular electrophilic process in dilute organic solvents, first order in SO2 and CpFe(CO)2R, to rationalize observed kinetics. The reaction is characterized by large negative zlS values (typically -50 to -60 e.u.), which have been attributed to developing charges in the transition state and to extensive solvation by S02 The proposed mechanism, shown below, involves electrophilic... 

Obviously, this reaction can also be performed in organic solvents in order to prevent hydrolysis of the active esters. 

Several copolymers were synthesized in an organic solvent in order to analyze the final product in the absence of any additive required for a water dispersion. AIBN was used as the radical initiator and the reactions were performed in nitrogen atmosphere. The spectroscopic characteristics of the copolymers were compared with those of an analogous PVAc homopolymer obtained with the same procedure. 

We previously mentioned the importance of determining the appropriate solvent for acyl transfer reactions (esterification and transesterification). Nevertheless, it is difficult to select a universal solvent for the esterification of (R,S) 2-arylpropionic acids. In fact, hydrophobic solvents such as cyclohexane [98,102], isooctane [97], or the mixtures isooctane/ChC or isooctane/toluene [100] are recommended for the highly hydrophobic substrates naproxen and ibuprofen (see Tables 6 and 7). On the contrary, moderately hydrophilic acids such as ketoprofen (Table 5 [92]) or flurbiprofen (Table 8 [111]) are better esterified in sUghtly hydrophilic solvents such as cffisopropyl ether, methylwobutyl ketone, or 1,4-dioxane. Iherefore, we can conclude that depending on the hydrophobicity of the substrate we must select the organic solvent in order to obtain the best catalytic performance. 

The extreme influence water can exert on the Diels-Alder reaction was rediscovered by Breslow in 1980, much by coincidence . Whale studying the effect of p-cyclodextrin on the rate of a Diels-Alder reaction in water, accidentally, the addition of the cyclodextrin was omitted, but still rate constants were observed that were one to two orders of magnitude larger than those obtained in organic solvents. The investigations that followed this remarkable observation showed that the acceleration of Diels-Alder reactions by water is a general phenomenon. Table 1.2 contains a selection from the multitude of Diels-Alder reactions in aqueous media that have been studied Note that the rate enhancements induced by water can amount up to a factor 12,800 compared to organic solvents (entry 1 in Table 1.2). 

In order to obtain more insight into the local environment for the catalysed reaction, we investigated the influence of substituents on the rate of this process in micellar solution and compared this influence to the correspondirg effect in different aqueous and organic solvents. Plots of the logarithms of the rate constants versus the Hammett -value show good linear dependences for all... 

Nitration at a rate independent of the concentration of the compound being nitrated had previously been observed in reactions in organic solvents ( 3.2.1). Such kinetics would be observed if the bulk reactivity of the aromatic towards the nitrating species exceeded that of water, and the measured rate would then be the rate of production of the nitrating species. The identification of the slow reaction with the formation of the nitronium ion followed from the fact that the initial rate under zeroth-order conditions was the same, to within experimental error, as the rate of 0-exchange in a similar solution. It was inferred that the exchange of oxygen occurred via heterolysis to the nitronium ion, and that it was the rate of this heterolysis which limited the rates of nitration of reactive aromatic compounds. 

Nitration in organic solvents is strongly catalysed by small concentrations of strong acids typically a concentration of io mol 1 of sulphuric acid doubles the rate of reaction. Reaction under zeroth-order conditions is accelerated without disturbing the kinetic form, even under the influence of very strong catalysis. The effect of sulphuric acid on the nitration of benzene in nitromethane is tabulated in table 3.3. The catalysis is linear in the concentration of sulphuric acid. 

A similar circumstance is detectable for nitrations in organic solvents, and has been established for sulpholan, nitromethane, 7-5 % aqueous sulpholan, and 15 % aqueous nitromethane. Nitrations in the two organic solvents are, in some instances, zeroth order in the concentration of the aromatic compound (table 3.2). In these circumstances comparisons with benzene can only be made by the competitive method. In the aqueous organic solvents the reactions are first order in the concentration of the aromatic ( 3.2.3) and comparisons could be made either competitively or by directly measuring the second-order rate constants. Data are given in table 3.6, and compared there with data for nitration in perchloric and sulphuric acids (see table 2.6). Nitration at the encounter rate has been demonstrated in carbon tetrachloride, but less fully explored. ... 

In contrast to the hydrolysis of prochiral esters performed in aqueous solutions, the enzymatic acylation of prochiral diols is usually carried out in an inert organic solvent such as hexane, ether, toluene, or ethyl acetate. In order to increase the reaction rate and the degree of conversion, activated esters such as vinyl carboxylates are often used as acylating agents. The vinyl alcohol formed as a result of transesterification tautomerizes to acetaldehyde, making the reaction practically irreversible. The presence of a bulky substituent in the 2-position helps the enzyme to discriminate between enantiotopic faces as a result the enzymatic acylation of prochiral 2-benzoxy-l,3-propanediol (34) proceeds with excellent selectivity (ee > 96%) (49). In the case of the 2-methyl substituted diol (33) the selectivity is only moderate (50). 

Whether AH for a projected reaction is based on bond-energy data, tabulated thermochemical data, or MO computations, there remain some fundamental problems which prevent reaching a final conclusion about a reaction s feasibility. In the first place, most reactions of interest occur in solution, and the enthalpy, entropy, and fiee energy associated with any reaction depend strongly on the solvent medium. There is only a limited amount of tabulated thermochemical data that are directly suitable for treatment of reactions in organic solvents. Thermodynamic data usually pertain to the pure compound. MO calculations usually refer to the isolated (gas phase) molecule. Estimates of solvation effects must be made in order to apply either experimental or computational data to reactions occurring in solution. 

Reactions between A -(l-chloroalkyl)pyridinium chlorides 33 and amino acids in organic solvents have a low synthetic value because of the low solubility of the amine partner. A special protocol has been designed and tested in order to circumvent this drawback. Soon after the preparation of the salt, an aqueous solution of the amino acid was introduced in the reaction medium and the two-phase system obtained was heated under reflux for several hours. However, this was not too successful because sulfur dioxide, evolved during the preparation of the salt, was converted into sulfite that acted as an 5-nucleophile. As a result, A -(l-sulfonatoalkyl)pyridinium betaines such as 53 were obtained (Section IV,B,3) (97BSB383). To avoid the formation of such betaines, the salts 33 were isolated and reacted with an aqueous solution of L-cysteine (80) to afford thiazolidine-4-carboxylic acids hydrochlorides 81 (60-80% yields). 

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