Dicarboxylic acids, solvent effect

The compounds Rh6(CO)16 and Re2(CO)10 are also effective homogeneous catalysts for autoxidating cyclic alcohols to dicarboxylic acids. Solvent effect data for cyclohexanol are shown in Table IV. Again low yields are found in benzene solvent, and considerably higher conversions in cyclohexane. The yields of carboxylic acids obtained from both cyclic and acyclic alcohols are shown in Table V. It is apparent that the acid yields are small for acyclic alcohols. There is no difference in catalytic activity whether the compound Rh6(CO)16 or Re2(CO)10 is used and low yields are obtained from both primary and secondary alcohols. 

The benzoic acid derivative 457 is formed by the carbonylation of iodoben-zene in aqueous DMF (1 1) without using a phosphine ligand at room temperature and 1 atm[311]. As optimum conditions for the technical synthesis of the anthranilic acid derivative 458, it has been found that A-acetyl protection, which has a chelating effect, is important[312]. Phase-transfer catalysis is combined with the Pd-catalyzed carbonylation of halides[3l3]. Carbonylation of 1,1-dibromoalkenes in the presence of a phase-transfer catalyst gives the gem-inal dicarboxylic acid 459. Use of a polar solvent is important[314]. Interestingly, addition of trimethylsilyl chloride (2 equiv.) increased yield of the lactone 460 remarkabiy[3l5]. Formate esters as a CO source and NaOR are used for the carbonylation of aryl iodides under a nitrogen atmosphere without using CO[316]. Chlorobenzene coordinated by Cr(CO)j is carbonylated with ethyl formate[3l7]. 

The kinetic solvent-isotope effects on these reactions are made up of primary and secondary kinetic isotope effects as well as a medium effect, and for either scheme it is difficult to estimate the size of these individual contributions. This means that the value of the isotope effect does not provide evidence for a choice between the two schemes (Kresge, 1973). The effect of gradual changes in solvent from an aqueous medium to 80% (v/v) Me2SO—H20 on the rate coefficient for hydroxide ion catalysed proton removal from the monoanions of several dicarboxylic acids was interpreted in terms of Scheme 6 (Jensen et al., 1966) but an equally reasonable explanation is provided by Scheme 5. 

The equilibrium constant for the isotope-exchange equilibrium can be expressed (6) in terms of the solvent isotope effects on the acid-dissociation constants and of the monocarboxylic acid and dicarboxylic acid monoanion, respectively. It follows that a lower value for the fractionation factor of the hydrogen-bonded proton means that the solvent isotope effect on the acid-dissociation constant will be lower for the dicarboxylic acid monoanion than for the monocarboxylic acid. 

Perhaps the simplest two-site cooperative systems are small molecules having two binding sites for protons, such as dicarboxylic acids and diamines. Despite their molecular simplicity, most of these molecules do not conform with the modelistic assumptions made in this chapter. Therefore, their theoretical treatment is much more intricate. The main reasons for this are (1) there is, in general, a continuous range of macrostates (2) the direct and indirect correlations are both strong and intertwined, so that factorization of the correlation function is impossible. In addition, as with any real biochemical system, the solvent can have a major effect on the binding properties of these molecules. 

Clearly, if one takes a smaller value of D, one gets a higher value of IVj j, for a given distance Rufj. Kirkwood and Westheimer (1938), Westheimer and Kirkwood (1938), and Westiieimer and Shookhoff (1939) indeed argued that one should take a much smaller dielectric constant, since the intervening medium between the two protons more closely resembles a hydrocarbon liquid rather than water. In fact, for any dicarboxylic acid one can define an effective dielectric constant to fit the experimental value of W, by an equation of the form (4.8.13), with Dg being dependent on the proton-proton distance, the type and size of the acid and the solvent. 

Fluorescence is measured in dilute solution of model compounds for polymers of 2,6-naphthalene dicarboxylic acid and eight different glycols. The ratio of excimer to monomer emission depends on the glycol used. Studies as functions of temperature and solvent show that, in contrast with the analogous polyesters in which the naphthalene moiety is replaced with a benzene ring, there can be a substantial dynamic component to the excimer emission. Extrapolation to media of infinite viscosity shows that in the absence of rotational isomerism during the lifetime of the singlet excited state, there is an odd-even effect In the series in which the flexible spacers differ in the number of methylene units, but not in the series in which the flexible spacers differ in the number of oxyethylene units. 

Preparation and Solubility of Polyesters. A number of polyesters were prepared from several diols and dicarboxylic acid esters to determine the effect of structure on the solubility in typical solvents used in lacquers. The data in Table I show that solubility in the solvents decreased in the following order toluene > methyl ethyl ketone > butyl acetate. Polymers that were soluble in all three solvents are examples 9-14. 

Broadbent et al. have found that the rhenium blacks prepared by reducing rhenium heptoxide are highly effective catalysts for the hydrogenation of carboxylic acids to alcohols at 150-170°C for monocarboxylic acids (eq. 10.3) and at 200-250°C for dicarboxylic acids (eq. 10.4) under 13.5-27 MPa H2.10,11 Rhenium heptoxide can be reduced to the active blacks in appropriate solvent (ethanol, 1,4-dioxane, acetic acid, or water) at 120-220°C and 15-21 MPa H2 for 1-2 h, or more conveniently, in situ in... 

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