Bromide, formation constants with

The importance of the solvent, in many cases an excess of the quatemizing reagent, in the formation of heterocyclic salts was recognized early. The function of dielectric constants and other more detailed influences on quatemization are dealt with in Section VI, but a consideration of the subject from a preparative standpoint is presented here. Methanol and ethanol are used frequently as solvents, and acetone,chloroform, acetonitrile, nitrobenzene, and dimethyl-formamide have been used successfully. The last two solvents were among those considered by Coleman and Fuoss in their search for a suitable solvent for kinetic experiments both solvents gave rise to side reactions when used for the reaction of pyridine with i-butyl bromide. Their observation with nitrobenzene is unexpected, and no other workers have reported difficulties. However, tetramethylene sulfone, 2,4-dimethylsulfolane, ethylene and propylene carbonates, and salicylaldehyde were satisfactory, giving relatively rapid reactions and clean products. Ethylene dichloride, used quite frequently for Friedel-Crafts reactions, would be expected to be a useful solvent but has only recently been used for quatemization reactions. ... 

Reversible formation of ionic intermediates in halogenated solvents has been suggested to be due to the weakly nucleophilic character of the counteranion, the tribromide ion, which should dissociate into nucleophilic bromide and free bromine before reacting with the bromonium ion (refs. 11,25,26). In order to check this hypothesis the product distribution of the c/s-stilbene bromination in chloroform was investigated (ref. 27). In the latter solvent the formation constant of Br3 is considerably lower than in DCE, Kf = 2.77 (0.13) x 10 against > 2 x 107 M 1. (ref. 28). As a consequence, at 10 3 M [Br2] relevant amounts of bromide ions are present as counteranion of the bromonium intermediate. Nevertheless, the same trend for the isomerization of cis- to rran -stilbene, as well as an increase of... 

Table 1 Formation constant and spectral characteristics of bromide (charge-transfer) complexes with various acceptorsa...

Quantitative analysis of the absorption intensity affords values of the formation constants (Kda) and extinction coefficients (act) listed in Table 1 for comparison with the corresponding characteristics of the bromide complexes with tetrabromomethane. 

A similar dependence of the first-order rate constants with respect to the quantity of added water has been reported for the reaction of sodium formate with 1,4-dichlorobutane and related displacement reactions, In these studies tetra- n-butylammonium hydrogen sulphate and tetra- n-butylammonium bromide were used as catalysts and chlorobenzene as the solvent. 

The early stage of electrophilic bromination of tetraisobutylethylene (TTBE) has been examined to detect the formation constant (K[) for the 1 1 charge-transfer complex (in CH2CI2, AcOH and MeOH). Based on the Kf values, the thermodynamic parameters for CTC formation from TTBE + Br2 are AH = —4.2( 0.2) kcalmol-1 and AS = —9.5( 0.7) e.u. TTBE reacts with Br2 to afford substitution products, two of which are an initially formed allylic bromide and a more slowly formed diene bromide127. 

The data for bromo complexes were obtained in aqueous methanolic solutions and outer-sphere bromo complexes with K = 1.3-1.9 were obtained for Pr, Nd, Sm, which are larger than the values of Ho (0.97) and Er (0.70). Chloro and bromo complex formation in dimethyl formamide studied by titration calorimetry [122] showed the evidence for MC12+, MCC. MCI3, and MCI4 species in solutions. In the case of bromide, monobromo and dibromo inner sphere complexes have been detected. The stepwise formation constants could not be determined for iodo complexes due to the small value of enthalpies of reaction. The stability constants data obtained in DMF are given in Table 4.8. 

Assume that log P4, for chloride, bromide, and iodide with Zn is — 1, — 0.74, and — 1.25 and with Cd" " is 0.9, 2.53, and 6.1. (a) Calculate the masking index for Zn" " and Cd" " in 1.0 Af chloride, 1.0 Af bromide, and 0.1 Af and 1.0 Af iodide. Assume activity coefficients of unity and that the metal ions are present predominately as their highest complexes, (b) Assuming that the conditional formation constants of Zn" " and Cd with EDTA are the same (Figure 11-3), which condition is most favorable for the titration of zinc in the presence of cadmium ... 

The complexing of zirconium and hafnium ions by fluoride ions is quite extensive compared to chloro complexing, while complex ion formation w ith bromide and iodide ions is negligible. Formation constants for fluoride complexing with zirconium(IV) and hafnium(IV) calculated from the data of Connick (126), Buslaev (94), and Hume (574), have been summarized graphically by Goldstein (213). Slightly different values have been published by Bukhsh (92). Noren (15a, 401-403) has redetermined the equilibrium constants for the reaction. 

The precipitate has to be separated from the original test solution since the ions present here, e.g., the cations of the substance to be examined, can disturb the complexation. The solubility of silver bromides is intermediate compared to that of silver chloride and silver iodide. So the complex formation constant of the diammninoargentate complex overrules the solubility constant of silver chloride easily, and with difficulty the solubility constant of silver bromide, but it is not able to do so in the case of silver iodide. 

The extent of the ionization produced by a Lewis acid is dependent on the nature of the more inert solvent component as well as on the Lewis acid. A trityl bromide-stannic bromide complex of one to one stoichiometry exists in the form of orange-red crystals, obviously ionic. But as is. always the case with crystalline substances, lattice energy is a very important factor in determining the stability and no quantitative predictions can be made about the behaviour of the same substance in solution. Thus the trityl bromide-stannic bromide system dilute in benzene solution seems to consist largely of free trityl bromide, free stannic bromide, and only a small amount of ion pairs.187 There is not even any very considerable fraction of covalent tfityl bromide-stannic bromide complex in solution. The extent of ion pair and ion formation roughly parallels the dielectric constant of the solvents used (Table V). The more polar solvent either provides a... 

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