Dimerization of benzoic acid

To calculate the dissociation constant of dimerized benzoic acid in benzene solution from isopiestic measurements. 

Solutions in benzene of weighed amounts of phenanthrene Ci4Hio (chosen because of its low volatility) and of benzoic acid were equilibrated through the vapour phase and the volumes of the two solutions w ere then measured. Results so obtained at 43.9 °C by Wall and Rouse (J. Amer. Chem. Soc. 1941, 63, 3002) are given in table 1. The following notation is used 

We shall use the following further notation Wy mass of benzene (at equilibrium) containing phenanthrene tffy mass of benzene (at equilibrium) containing benzoic acid M 2 molar mass of phenanthrene M3 molar mass of benzoic acid monomer a degree of dissociation of dimer. 

The solutions are all sufficiently dilute for differences in density from that of the pure solvent to be ignored. We accordingly calculate the mass of benzene in each solution b the formulae 

Then assuming that Raoult s law is obeyed and that only dimers and monomers are present, we have 

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Cocrystal systems are assembled through the association of individual molecules into fundamental building block units that are known as supramolecular synthons [13]. For example, one such synthon would be formed by hydrogen-bond interactions between a phenyl-carboxylic acid and a phenyl-amide, with the molecules being linked into a dimeric species through 0 H-N and O-H O hydrogen bonds [14], This mode of interaction can be illustrated using the synthon that would result from the dimerization of benzoic acid and benzamide ... 

Fig. 3.13 The role of the hydrogen bond in formation of dimers of benzoic acid molecules (a) and cyclohexane-carboxylic acid molecules (b)...

The dimer of benzoic acid is a non-polar entity whereas the benzaldehyde monomer will have a significant dipole. It is therefore very important to purify organic materials to a very high level if one wishes to obtain the true crystal morphology. Generally, solutions may contain between 10 and 10 solid particles per cubic centimetre and these foreign particles can act to lower the interfacial energy necessary for cluster formation ... 

H Tsugane, Y Yagi, H Inomata, S Saito. Dimerization of benzoic acid in saturated solutions of supercritical carbon dioxide. J Chem Eng Japan 25 351, 1992. 

Of great interest is the study of Gur yanova and Beskina74 on dielectrometric and cryoscopic titrations of benzoic acids in benzene with amines they concluded that benzoic acid reacts as a dimer (HX)2 yielding with primary and... 

About 20 years later, Nemst realized that it was necessary to take into account the different reactions of the solute in each phase, such as dimerization in the organic phase and dissociation in the aqueous phase. Thus the distribution of benzoic acid (HBz) between the organic (benzene) phase and water could be written... 

The phenylation of thiophene with benzoyl peroxide gave a considerable amount of 2,2 -dithienyl one suggested mechanism involved the formation of 2-thienyl radicals by oxidation, and their subsequent dimerization. More recent studies indicate that the 2,2 -dithienyl is formed through an initial addition of benzoyloxy radicals to the thiophene nucleus followed by dimerization of the resulting radical and loss of two molecules of benzoic acid (Scheme 15). 

The single crystal structure of benzoic acid has been reported [52]. In its crystal, the molecule is approximately planar, but distances of0.042 A and 0.068 A displace the carboxylate carbon and one of its oxygen atoms, respectively, from the mean plane. The structure is characterized by the existence of hydrogen-bonded molecular dimers in the unit cell. 

Table IV. Interaction Stabilization Energies of Benzoic Acid Dimers (kcal /mole)...

Hydrogen bonded dimers of 4-RF-substituted benzoic acids [119, 120, 122, 126], /rcyclohexane carboxylic acids [120], and hydrogen bonded complexes of benzoic acids with 4,4 -bipyridine and stilbazole [166, 243-246] can be regarded as supramolecular rod-like mesogens with two fluorinated chains. Figures 33 and 34 show comparisons of selected benzoic acid dimers and their bipyridine and stilbazole complexes. Comparison of the benzoic acid dimers 122 and 123 with -alkyl chains and F-terminated -alkyl chains, respectively, indicates that a single F-atom at the end of the alkyl chain reduces the mesophase stability... 

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...

A total of 0.50 g of benzoic acid was added to 200 mL of water at pH 5.2. 100 mL of benzene was then added to the aqueous phase and thoroughly mixed. How much benzoic acid remains in the oil phase of the emulsion as dimers The pKa of benzoic acid is 4.2. The apparent distribution ratio is 38.5. 

Therefore, the concentration of benzoic acid dimers is given by ... 

As an example, we consider H NMR measurements on a single crystal of benzoic acid [ 14], carried out to investigate tunnelling dynamics in hydrogen bonded carboxylic acid dimers. 

The crystal structure of benzoic acid is monoclinic and contains four molecules per unit cell in the form of two magnetically inequivalent dimers with equal values of the chemical shift tensors but with their principal axis systems oriented... 

In benzene the molecules of benzoic acid CellsCOOH form a dimer. The equilibrium can be represented by the equation... 

Coupling reactions are not observed in all cases. With highly bulky substrates, radical dimerization becomes so slow that saturation (case of enones) is found to be the main reaction. This is particularly the case with the brassino steroids (potent plant growth promoters), and the compound depicted below for which a stereoselective saturation process was sought. The best conditions for this process are dimethylformamide containing an excess of benzoic acid and a glassy carbon rod as the cathode. The saturated ketone was obtained [62] with a yield of up to 60%. Other proton donors, like trifluoroacetic acid, were used as well. 

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