Benzoic dimer

A unique example of observation of tunneling splitting is given by Oppenlander et al. [1989]. Upon replacing the host benzoic acid dimer by a thioindigo molecule of nearly the same size, the resulting bias accidentally turns out to be small, of order of A. The 4x4 Hamiltonian of the complex of two dimers and the guest molecule is... 

Substituted phenylacetic acids form Kolbe dimers when the phenyl substituents are hydrogen or are electron attracting (Table 2, Nos. 20-23) they yield methyl ethers (non-Kolbe products), when the substituents are electron donating (see also chap. 8). Benzoic acid does not decarboxylate to diphenyl. Here the aromatic nucleus is rather oxidized to a radical cation, that undergoes aromatic substitution with the solvent [145]. 

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... 

However, the hydrogen-bonded mesogens that are of most interest in the context of this article are those elaborated initially by Kato and Frechet in the early 1990s [24-33]. In this approach, a pyridine, which may or may not have liquid crystal properties, was hydrogen bonded with a 4-substituted benzoic acid to form a new species with its own, distinct mesomorphism. For example, complex 9 shows a SmA phase that persists to 238 °C (n = 2, m = 4), while its free component pyridine is nematic to 213 °C the component benzoic acid is also nematic (as the H-bonded dimer) to 147 °C (although note that the notional monomer would not be liquid crystalline). 

The side products of the reaction between benzoylnitromethane 279 and dipolarophiles (norbornene, styrene, and phenylacetylene) in the presence of l,4-diazabicyclo[2.2.2]octane (DABCO) were identified as furazan derivatives (Scheme 72). The evidence reported indicates that benzoylnitromethane gives the dibenzoylfuroxan as a key intermediate, which is the dimerization product of the nitrile oxide. The furoxan then undergoes addition to the dipolarophile, hydrolysis, and ring rearrangement to the final products (furazans and benzoic acid) <2006EJ03016>. 

Abstract This presentation is a brief review on the resnlts of our work on iodine interaction with thioamides, selenoamides and amides. The thioamides, benzothia-zole-2-thione (BZT) (1), 6-n-propyl-2-thiouracil (PTU) (2), 5-chloro-2-mercap-tobenzothiazole (CMBZT) (3), N-methyl-benzothiazole-2-thione (NMBZT) (4), benzimidazole-2-thione (BZIM) (5), thiazolidine-2-thione (TZD) (6), 2-mercapto-pyridine (PYSH) (7), 2-mercapto-nicotinic acid (MNA) (8), 2-mercapto-benzoic acid (MBA) (9) and 2-mercapto-pyrimidine (PMT) (10) react with producing three type of complexes of formulae [(HL)IJ(l2) (HL= thioamide and n= 0, 1), [(HL) [I3 ] and [(HL-L)]+[l3 ]. The interaction of seleno-amides, derived from, 6-n-propyl-2-thiouracil (RSelJ) (R= Me- (11), Et- (12), n-Pr- (13) and i-Pr- (14)) with I, have also been studied and produced the complexes [(RSeU)IJ of spoke structure. These complexes are stable in non-polar solvents, but they decompose in polar solvents, producing dimeric diselenide compounds or undertake deselenation. 

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). 

Kolbe oxidation of carboxylate ions to radicals with loss of carbon dioxide (p. 312). The latter process gives highest yields of dimeric product at a platinum anode and only monomeric products from oxidation of the radical centre at a carbon anode. Oxidation of butadiene in methanol containing benzoic acid, at a smooth platinum anode, gives 45 % of the but-3-ene-l,4-diol diester [45]. 

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. 

Figure 1 Doubleproton transfer in benzoic add dimers. When the 0-0 bond length becomes shorter, tunneling is enhanced.

The trimerization of cyclopentadiene (6) is catalyzed by a homogeneous bifunctional palladium-acid catalyst system.7 The reaction gives trimers 7 and 8 as a 1 1 mixture in 70% yield with bis(acetylacetonato)palladium(II) [Pd(acac)2] or with bis(benzylideneacetone)-palladium(O) as the palladium component of the catalyst. As the phosphorus component, phosphanes like trimethyl-, triethyl-, or triphenylphosphane, and triisopropylphosphite or tris(2-methylphcnyl)phosphite, are suitable. A third component, an organic acid with 3 < pK < 5, is necessary in at least equimolar amounts, in the reaction with cyclopentadiene (6), as catalytic amounts are insufficient. Acids that can be used are acetic acid, chloroacetic acid, benzoic acid, and 2,2-dimethylpropanoic acid. Stronger acids, e.g. trichloroacetic acid, result in the formation of poly(cyclopentadiene). The new catalyst system is able to almost completely suppress the competing Diels-Alder reaction, thus preventing the formation of dimeric cyclopentadiene, even at reaction temperatures between 100 and 130°C. 

These authors consider two postulates (a) If the benzoic acid is solubilized in the micellar core, then dimerization should occur, in which case HB /[HB]2 const, (b) If the solubilization occurs at the micellar surface, then surface saturation analogous to Langmuir adsorption should occur. Test each postulate with the data provided and decide which gives the better fit. 

Dimeric carboxylic acids exhibit a very characteristic pattern of peaks in the OH and OD stretching region. In some cases, e.g. benzoic... 

Ai/12(dimerization in benzene) in kcal/mole dimer, o-methoxy benzoic add 3 4 0 2 p-mefcho r. f1) n> ei-r acid ... 

Oxidative coupling of N-benzoylpyrroles was achieved with palladium acetate. From a reaction at 110°C in acetic acid, benzopyrrolizinones (35) and 2,2 -dimeric compounds (36) were obtained. Under the same conditions 1-aroylindoles gave 37 and 38 but no 2,2 -dimeric compounds.30 Treatment of dibenzoylpyrrolizinones (37) with potassium t-butoxide/f-butyl alcohol containing a small amount of water at 82°C afforded o-(2-indo yI)benzoic acids in good yield.31... 

The >, > -dimer of 6-methylguaiacol (3,3 -dimethoxy-4,4 -dihydroxy-5,5 -dimethylbiphenyl-l,l ) exhibits the lowest C.O.P., and we found that this compound is even oxidized when exposed to the atmosphere (decrease of 80 mv.). The benzoic acid derivative (V), on the other hand, had the highest C.O.P. (caused by the electron attracting —COOH group in the 0-position) indicating its stability towards oxidation. 

The kinetics and mechanism of the phosphorus-catalysed dimerization of acrylonitrile to give 1,4-dicyanobut-l-ene and 2,4-dicyanobut-l-ene have been studied.114 The reactions of aryhminodimagnesium (138) with //-substituted p-cyanobenzophenones, l-cyano-9-fluorenenone, o-, m-, and p-dicyanobcnzcnes, and o-, m-, and p-nitrobenzonitriles have been examined.115 The effect of pressure on the reaction of 3 -methyl- l-(4-tolyl)triazene (139) and benzoic acid in chloroform and acetonitrile has been studied.116 The effect of acids on the rate of urethane formation from alcohols and isocyanates in the presence of alkyltin carboxylates has been examined.117 A Hammett a value has been reported for the amidine group N=CHNMe2 and used for the prediction of the basicity of sites in bifunctional amidines.118... 

Etter has shown that cocrystallization of mixed dimers of differently substituted benzoic acids can easily be achieved. We present here calculations that predict the relative stabilities of several possible mixed benzoic acid dimers that are variously substituted. 

Crystals of stoichiometric 1 1 mixtures of compounds that can complex with each other have been shown to form preferentially to pure crystals of the individual components. In some cases these crystals may have potential non-linear optical properties. An interesting example is the 1 1 mixture of p-aminobenzoic acid and 3,5-dinitrobenzoic acid. (15) A view of the crystal structure is shown in figure 3. Examination of this figure leads one to the hypothesis that the preference for the mixed crystal may be due to a) a more stable H-bonding interaction between the different benzoic acids in the hetero-dimer than in the homo-dimer b) the ability of the mixed crystal (hetero- dimers) to H-bond between their amino and nitro groups. It is likely that both of these factors play a role in the stability of the crystal structure. Calculational modelling can aid in determining the importance of these factors. 

In order to determine whether molecular orbital methods could be used to predict and explain preferences for cocrystalization analogous to that discussed above, we present here AMI calculations on the dimerization energies of variously substituted benzoic acids. 

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

Components of Substituted Benzoic acid Dimers Stabili-... 

AMI calculations on the dimers of nitroanilines and substituted benzoic acids are of considerable value in predicting their crystal structures. In particular, the intermolecular forces that dictate the relative orientations of the individual molecules in the crystal chains can be understood. It is likely that this methodology will be useful for modelling the kinds of interactions that might occur in other crystals. 

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