Dimerization of carboxylic acids

An example of self-associating hydrogen bond is a dimer of carboxylic acids while hydrogen bonding between a carboxylic acid and a nonselfassociating functional group, such as ether or ester, is an example of the latter type. 

Consider a cyclic dimer of carboxylic acid involving two H-bond bridges, as depicted on Fig. 14. The two parts of the dimer are labeled r = a, b. According to Sections VI.B and VI.C, for such dimers, there are two degenerate high frequency modes and also two degenerate low frequency H-bond vibrations. 

Fig. 9 H/D isotope effects on the hydrogen bond geometries H/D isotope effects, A5 = 8Hd - SHh. for the encapsulated benzoic acid (BA) derivatives (d12 mesitylene, open circles) and non-encapsulated (CDF3/ CDF2C1 filled circles) dimers of carboxylic acids as a function of the bridging proton chemical shift 8Hh...

D Experimental Results I Cyclic Dimers of Carboxylic Acids and Amides... 

A second type of study that corroborates the basic assumption is any measurement which specifically shows that the H bonded species are cyclic. Infrared data can offer such evidence, provided the only important species over a wide concentration range are monomers and dimers. For such a system the peak intensities of absorption by the monomer (Im) and dimer (Id) are simply related the ratio IdIIm is a constant. Such behavior has been observed and cited in favor of cyclic dimers of S-valerolactam and of -caprolactam by Tsuboi (2050), and of y-bu-tyrolactam by Klemperer et al. (1117). However, no such deduction can be made for other amides (e.g., see 1117), phenols (e.g., see 411), or alcohols (e.g., see 1375 and 1150). There is much evidence in favor of cyclic dimers of carboxylic acids in the gas phase (1081), and some referring to solutions (1652 and 445). On the other hand, the IR data for liquid formic acid are more complicated. Ghapman (373) concludes that the liquid contains a mixture of cyclic dimers and chain polymers. Dielectric data have been interpreted to show that formic acid is not entirely dimeric (1046). By use of thermo-electric osmometry Davies and Thomas have measured AH of dimerization of several amides and have concluded that trichloroacetamide and A -methyltrichloroacet-amide form cyclic dimers, whereeis iV-methylaceteunide, iV-methylform-... 

The alkyl groups of two identical carboxylic acids can be coupled to symmetrical dimers in the presence of a fair number of functional groups (equation 1). Since free radicals are the reactive intermediates, polar substituents need not be protected. This saves the steps for protection and deprotection that are necessary in such cases when electrophilic or nucleophilic C—C bond-forming reactions are involved. Furthermore, carboxylic acids are available in a wide variety from natural or petrochemical sources, or can be readily prepared from a large variety of precursors. Compared to chemicd methods for the construction of symmetrical compounds, such as nucleophilic substitution or addition, decomposition of azo compounds or of diacyl peroxides, these advantages make the Kolbe electrolysis the method of choice for the synthesis of symmetrical target molecules. No other chemical method is available that allows the decarboxylative dimerization of carboxylic acids. 

Orthoboric acid is a trihydroxy-compound, B(OH)3, and in the crystalline state each molecule is hydrogen-bonded to neighbouring molecules by six 0—H-0 bonds. These are in pairs to three adjacent molecules, an arrangement similar to that in the dimers of carboxylic acids ... 

Replacement of H in H3PO4 by a radical R gives substituted phosphoric acids 0P(0H)20R and OP(OH)(OR)2 and finally the phosphoric ester, OP(OR)3. The structure of dibenzylphosphoric acid, HP02(0CH2C6Hs)2, was described in Chapter 8 the molecules are linked in pairs by O-H-0 bonds as in dimers of carboxylic acids. Examples of mono- and di-substituted ions are the phenyl-phosphate ion, (h), studied in the K salt, and the diethylphosphate ion, (i), in the Ba salt, and of an ester, triphenylphosphate,... 

A very few highly diluted gases have, however, been the objects of coherent electron scattering experiments. In more dense gases, absorption of electrons rapidly becomes predominant and hinders measuring scattering intensities. Diffraction of electrons thus allowed determining with precision the structure of H-bonded cyclic dimers of carboxylic acids (26-28) that constitute model systems of H-bonds we have already seen in Ch. 2 and shall see later in this book. They nevertheless remain scarcely used methods. 

Figure 7.1 IR spectra of cyclic H-bonded dimers of carboxylic acids (CD3COOD)2 and (CD3C00H)2 (bottom) and of corresponding not H-bonded free monomers (upper spectra). Spectra are offset for clarity. The structure of these dimers is shown in Figure 4.4.

Figure 7.2 bands of cyclic H-bonded dimers of carboxylic acids mixed dimers CD3COOH-CD3COOD (solid line) and homodimers (CD3COOH)2 and (CD3COOD)2, same as in Figure 7.1 (dotted lines). The numbers of mixed dimers is approximately twice that of each homodimer, so that i j(O-H - ) and vJ.O-D---) bands have comparable intensities. The structure of aU these H-bonded cyclic dimers is that represented in Figure 4.4. 

INFRARED SPECTRA OF CYCLIC DIMERS OF CARBOXYLIC ACIDS THE MECHANICS OF H-BONDS AND RELATED PROBLEMS, Yves Marechal... 

Cyclic dimers of carboxylic acids represent important model systems forming two coupled intermolecular hydrogen bonds [Fig. 15.1 (d), inset of Fig. 15.4]. The linear vibrational spectra of carboxylic acid dimers have been studied in detail, both in the gas and the liquid phase, and a substantial theoretical effort has been undertaken to understand the line shape of their 0-H and/or O-D stretching bands. In contrast, there have been only a few experiments on the nonlinear vibrational... 

In summary, the following experimental conditions should be used for a successful dimerization of carboxylic acids. An undivided beaker type cell (Fig. 2) is used, equipped with a smooth platinum anode and a platinum, steel or nickel cathode at a close distance a current density of 0.25 A cm or higher should be provided by a regulated power supply a slightly acidic or neutral electrolyte, preferably methanol as solvent and a cooling device to maintain temperatures between 10 and 45 °C should be employed. With this simple procedure and equipment, yields of coupling product as high as 90% can be obtained, provided the intermediate radical is not easily further oxidized. 

The kinetic measurements are consistent with these relationships and the rate coefficients fei, fe2, and k3 are given in Table 14. In calculating rate coefficients, the dimerization of carboxylic acids through hydrogen bonding was taken into account. The ratio fei/(fei + fe2) increases with increasing temperature for example, for stearic acid it is 0.20 at 125° and 0.24 at 145° C. 

Dimerization of carboxylic acids is also used for the formation of nonmesomorphic molecular assemblies in solid states such as one-dimensional polymeric structures [131, 132], two- or three-dimensional networks [133], and dendritic materials [134]. Dimer formation due to quadruple H-bonding has been designed to obtain highly stable molecular association [135]. The quadruply H-bonded polymer associates from bifunctional compounds show polymer-like behavior in the solution state due to large association constants [135]. 

To determine whether this association is due to carboxylic acids or whether it also involves the aluminum salt of carboxylic acid, the acid-functionalized polymer was evaluated using X-ray photoelectron spectroscopy (XPS). Figure 2 shows that there is no metal aluminum. Thus the association is most likely due to dimerization of carboxylic acids, as is commonly seen in smaller organic molecules. The XPS spectrum also shows the incorporation of oxygen functionality into the copolymer by the acylation reaction. The reaction scheme is as follows ... 

Gibbs Free Energy for the Dimerization of Carboxylic Acids... 

Doan, V., Koppe, R., and Kasai, R. H. (1997). Dimerization of Carboxylic Acids and Salts An IR Study in Rerfluoropolyether Media. Journal of the American Chemical Society, 119(41), 9810-9815. 

The first experimental quantity is the dimerization constant and the corresponding standard free energy of dimerization of carboxylic acids in various solvents (Schrier et ah, 1964 Schneider et al., 1965 Moon et al., 1965 Kunimitsu et al, 1968). Consider the dimerization constants for association of... 

Fig. 4.2 Two possible configurations of dimers of carboxylic acid (a) when the two alkyl groups (R) are near each other, and (b) when they are far apart.

The simplest organic supramolecular aggregates are the dimers of carboxylic acids, 35, and the cyclic tetramers of alcohols, 36. Of the organic compounds... 

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