Esters solvation effects

Dejaegere, A., Liang, X. and Karplus, M. Phosphate ester hydrolysis calculation of gas-phase reaction paths and solvation effects, J.Chem.Soc., Faraday Trans., 90 (1994), 1763-1770... 

Solvation effects on the conformation of esters of three /i-snbstituted 1-phenyletha-nols with 2-flnoro-2-phenyl acetic acid (FCDA) were studied both experimentally (in five solvents ranging from CDCb to DMSO) and quantum mechanically. Semi-empiri-cal (AMI of MJS Dewar and PM3 of JJP Stewart) and ab initio (RHF/3-21 G) calculations were undertaken. Energy maps for the conformers of the esters as a function of the dihedral angles alpha (F-C-alpha acid-C=0) and beta (CO-O-C-alcohol-H) were obtained. Solvent effect calculations, through the self-consistent reaction field on the most stable conformers, were also carried out (Hamman et al., 1996). 

Another very important property of plasticizers for PVC resins is the ability to act as a dispersing medium for polyvinyl chloride plastisols. The performance of the particular plasticizer in a plastisol application depends to a great extent on a property known as its viscosity stability. The stability of the plastisol to viscosity build up is the manifestation of the solvating effects of the plasticizer, and thus, those materials with higher solvating efficiency tend to display an increase in plastisol viscosity on standing. Figure 3 is the plot of the viscosity stability of PVC o-phthalate ester plastisol compositions and, in essence, compares the alcohol-derived plasticizers with that of the olefin-derived plasticizer. The data show that... 

The most common deviation is the exceptionally high reactivity of nucleophiles, such as hydroperoxide, hypochlorite and hydroxamate ions, with atoms bearing lone-pair electrons next to the nucleophilic centre. This phenomenon, known as the alpha-effect287, is found for aminolysis reactions of esters also285, and is commonly observed for attack at electrophilic centres where reactivity depends fairly strongly on the basicity of the nucleophile. Negative deviations may be evidence of steric hindrance, or in a few cases, in particular that of hydroxide ion, may reflect special solvation effects on the pKa or the nucleophilicity (or both) of the nucleophile. 

G. Alagona, C. Ghio, J. Igual, and J. Tomasi, /. Mol. Struct. (THEOCHEM), 204, 253 (1990). An Appraisal of Solvation Effects on Chemical Function Groups The Amidic and the Esteric Linkages. 

This theoretical examination of carbonyl addition reactions serves to emphasize the enormous role that solvation effects play. As indicated in Figure 3.22 and other studies, gas phase addition of hydroxide ion to esters is calculated to be exothermic and to encounter only a very small barrier at TSla and TS2a. The major contribution to the activation barrier (18.5kcal/mol) that is observed in solution is the energy of desolvation of the hydroxide ion. We return to a discussion of solvation effects on carbonyl additions is Section 3.8. 

P. George, R.J. Witonsky, M. Hachtman, C. Wu, W. Dorwart, L. Richman, W. Richman, E Shurayh, and B. Lentz, Squiggle-HaO An Enquiry into the Importance of Solvation Effects in Phosphate Ester and Anhydride Reactions. Biochim. Biophys. Acta, 223,1-15, 1970. 

It should always be borne in mind that solvent effects can modify the energy of both the reactants and the transition state. It is the difference in the two solvation effects that is the basis for changes in activation energies and reaction rates. Thus, although it is common to see solvent effects discussed solely in terms of reactant solvation or transition state solvation, this is usually an oversimplification. One case that illustrates this point is the hydrolysis of esters by hydroxide ion. 

The effect of a carboxy group is illustrated by the reactivity of 2-bromopyridine-3- and 6-carboxylic acids (resonance and inductive activation, respectively) (cf. 166) to aqueous acid under conditions which do not give hydroxy-debromination of 2-bromopyridine and also by the hydroxy-dechlorination of 3-chloropyridine-4-car-boxylic acid. The intervention of intermolecular bifunctional autocatalysis by the carboxy group (cf. 237) is quite possible. In the amino-dechlorination (80°, 4 hr, petroleum ether) of 5-carbethoxy-4-chloropyrimidine there is opportunity for built-in solvation (167) in addition to electronic activation. This effect of the carboxylate ion, ester, and acid and its variation with charge on the nucleophile are discussed in Sections I,D,2,a, I,D,2,b, and II,B, 1. A 5-amidino group activates 2-methylsulfonylpyridine toward methanolic am-... 

Therefore, the polar group influences the reactivity of ester in reactions with peroxyl radicals (see later). Due to the polar groups, the effect of multidipole interaction was observed in reactions of polyesters with R02 , 02, and ROOH (see Section 9.3.4). Ester as a polar media solvates the polar TS and influences the reactivity of polar reagents. 

Steric effects on both the amide and the acyloxyl side chain are similar. Tert-butyl and adamantyl groups on the amide side chain in 29v, 29x, 29c, and 29e (Table 2 entries 53 and 54, 63 and 65) result in lower stretch frequencies that, on average, are only 40 cm-1 higher than their precurser hydroxamic esters. Streck and coworkers have suggested that such changes in dialkyl ketones can be ascribed to destabilisation of resonance form II through steric hindrance to solvation which, in the case of tert-butyl counteracts the inductive stabilisation.127... 

The behavior of metal ions in reversed micelles may be more interesting, since the reversed micelle provides less solvated metal ions in its core (Sunamoto and Hamada, 1978). Through kinetic studies on the hydrolysis of the p-nitrophenyl ester of norleucine in reversed micelles of Aerosol OT and CC14 which solubilize aqueous cupric nitrate, Sunamoto et al. (1978) observed the formation of naked copper(II) ion this easily formed a complex with the substrate ester (formation constant kc = 108—109). The complexed substrate was rapidly hydrolyzed by free water molecules acting as effective nucleophiles. 

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