Carboxylates, solvent-dependent

Primary alcohols are oxidized to either aldehydes or carboxylic acids, depending on the reagents chosen and the conditions used. One of the best methods for preparing an aldehyde from a primary alcohol on a small laboratory scale, as opposed to a large industrial scale, is to use pyridinium chloro-chromate (PCC, CsH NCrO Cl) in dichloromethane solvent. 

The effect is observed in alcohols, phenols, amines and carboxylic acids and, as in the case of infrared spectra it is temperature, concentration and solvent dependent (p. 383). 

The rate of decarboxylation of activated carboxylate anions [e.g. (10)], shows strong solvent dependence. It is not surprising, therefore, that these reactions have been used to probe the microsolvent effects of micelles and CDs (Fendler and Fendler, 1975). In particular, it was anticipated that complexation with a CD might result in catalysis by providing an environment for the reaction that is less polar than water. 

The unusual rate enhancement of nucleophiles in micelles is a function of two interdependent effects, the enhanced nucleophilicity of the bound anion and the concentration of the reactants. In bimolecular reactions, it is not always easy to estimate the true reactivity of the bound anion separately. Unimolecular reactions would be better probes of the environmental effect on the anionic reactivity than bimolecular reactions, since one need not take the proximity term into account. The decarboxylation of carboxylic acids would meet this requirement, for it is unimolecular, almost free from acid and base catalysis, and the rate constants are extremely solvent dependent (Straub and Bender, 1972). 

Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate [52] has been most widely investigated in aqueous systems, since this reaction is remarkably solvent dependent (Kemp and Paul, 1970 Kemp and Paul, 1975 Kemp et al.,... 

Finally, even if these criteria are satisfied, there remains the question of whether the product will adhere to form a film or just precipitate homogeneously in the solution. This is the most difficult criterion to answer a priori. The hydroxide and/or oxy groups present on many substrates in aqueous solutions are likely to be quite different in a nonaqueous solvent (depending on whether hydroxide groups are present or not). Another factor that could conceivably explain the general lack of film formation in many organic solvents is the lower Hamaker constant of water compared with many other liquids this means that the interaction between a particle in the solvent and a solid surface will be somewhat more in water than in most other liquids (see Chapter 1, van der Waals forces). From the author s own experience, although slow precipitation can be readily accomplished from nonaqueous solutions, film formation appears to be the exception rather than the rule. The few examples described in the literature are confined to carboxylic acid solvents (see later). 

Earlier attempted interpretations of the hydrogen bond with the help of resonance or delocalization forces, e.g. in the case of the dimeric carboxylic acids (Illa-d) or, in particular, of substances containing intramolecular hydrogen bonds such as o-nitrosophenol (IVa-d), were shown to be untenable by unambiguous experimental evidence. Thus, in the dimer carboxylic acids the proton is not in the centre of the 0. O distance and the C=0 and C—OH distances are not identical [7], and derivatives of ortho-hydroxyazobenzene and of ortho-nitrosophenol have been shown to exist as solvent-dependent tautomeric equilibria (II), in spite of the presence of the internal hydrogen bond in both tautomers [3, 4]. 

The same authors describe an interesting interaction between phosphites (e.g. 188) and acetylenedicarboxylates in the presence of alcohols, phenols or carboxylic acids. For example, with benzoic acid present, (188) gives the ylide (189) which is in equilibrium with the isomer (190). This equilibration is very solvent dependent, and after several hours in dichloromethane solution finally yields the phosphonate (189a Scheme 31) (8iPS(10)18i). 

The photolytic products of ethyl 2-(l-isoquinolinyl)-5-oxo-2,5-dihydroisoxazole-4-carboxylates (357) were both wavelength and solvent dependent (92AJC1811 94AJC1249). No reaction occurred in any solvent... 

Subsequently, it was shown that the reaction was catalyzed by base (48M(79)106). Dicyandiamide reacts with nitriles (the most valuable method), amidines, cyanamides, ammonia, cyanates, thiocyanates, carboxylic acids and anhydrides to yield 1,3,5-triazines (Table 13). This synthetic route has been reviewed thoroughly several times (59HC(13)1, p. 219,61MI22000, p. 650, 73ZC408). The base-catalyzed reaction of dicyandiamide with alkyl or aryl nitriles (Scheme 65) proceeds via the imino ether anion and the rate determining step is solvent dependent. In DMSO the formation of the imino ether is rate determining, but in 2-methoxyethanol the reaction between the anion and dicyandiamide controls the rate (66T157). 

Both monensin (24) and nigericin (25) complex Na+ and K+ strongly but not selectively. The crystal structures of the Na+, Tl+ and Ag+ complexes all show the metal ion to be in an O-rich cavity. The carboxylate group is not involved however.97 With the antibiotics (26), (27) and (28) the thermodynamic stabilities (Table 9) are greater for the divalent than for the monovalent metal ions.98 The conformations adopted in these complexes axe very solvent dependent, and the implication of these to the biological transportation of the cations has been discussed.99... 

The acyloxymercuration-demercuration of alkenes provides an alternative route to esters which is probably less prone to caibon skeleton rearrangements than the direct addition of carboxylic acids to alkenes (equation 282). This reaction has recently been reviewed.477 The reaction is most commonly run using mercury(II) acetate in acetic acid, though other mercury salts may be used and aprotic solvents can also be employed. Equilibria have been measured for the reaction of mercury(II) trifluoroacetate and alkenes in tetrahydrofuran, and were found to be solvent dependent.478... 

The carbonyl group. Acetone (in cyclohexane solution) exhibits two absorption bands one appears at 190nm (e 1860) and corresponds to the n - n transition, while the second is at 280 nm (e 13) and corresponds to the n - n transition. The absorption maxima of these bands are solvent-dependent. Ultraviolet spectra of saturated aldehydes, carboxylic acids, esters and lactones exhibit a similar absorption profile, and in general are of little diagnostic value. 

Lastly, the effect of hydrogen-bonding should be noted. Protons which are hydrogen-bonded are deshielded relative to the non-bonded situation and their chemical shifts can vary over a wide range. The effect is observed in alcohols, phenols, amines and carboxylic acids and, as in the case of infrared spectra it is temperature, concentration and solvent dependent (p. 382). 

Primary alcohols are oxidized either to aldehydes or to carboxylic addsl depending on the reagents chosen and on the conditions used. Probably tlM best method for preparing sn aldehyde from a primary alcohol on a laboratory scale (as opposed Co an industrial scaIo> is by use of pyrirhnii chlorochromale (PCC, C tNCrO CU in dichloromcthane solvent. 

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