Molecular addition compounds

The boron trihalides form a great many molecular addition compounds with molecules... 

J. B. Ott and J. R. Goates. "Summary of Melting and Transition Temperatures of Pure Substances and Congruent and Incongruent Melting Temperatures of Molecular Addition Compounds," J. Chem. Eng. Data. 41. 669-677 (1996). 

Stability relationships among analogous molecular addition compounds of group III elements. 

Graham, W. A. G., and F.G. A. Stone The Relative Stabilities of some Molecular Addition Compounds of Boron. J. Inorg. Nucl. Chem. 3, 164 (1956). 

Sisler, H. H., and J. C. Cory Molecular addition compounds of the group... 

The greek letters given in brackets indicate the different crystal lattices observed for one and the same molecule, a and P-Sig, on the other hand, consist of different molecules since the conformations of the two rings are different. The molecular addition compound Sg Sjg is a unique case among the many allotropes of the non-metallic elements in so far as it consists of two different molecules of the same element. 

Hphe formation of molecular or addition compounds by organic sub-stances is well known, and many handbooks cite as an example the mixture of benzophenone and diphenylamine to produce a stable equi-molecular addition compound. Another example is the molecular compound formed by benzene and picric acid. This is stable only in the presence of an excess of picric acid. 

It also forms molecular addition compounds with a-trinitrotoluene and 2,4,6-tri-nitroanisole (T. Urbadski [6, 7]). 

From these facts it seems beyond doubt that the crystal lattice of nitrocellulose undergoes a marked deformation as the result of absorbing a solvent or gelatinizer. For the time being it is difficult to say whether a new chemical compound is formed. Nevertheless the formation of molecular addition compounds appears likely. It is also without question that addition compound are formed by nitrocellulose and camphor. According to Hess et al. nitrocellulose of 13.7% N combines with camphor in a 1 1 mole ratio (one molecule camphor to one anhydroglucose unit). 

Partial Miscibility in the Solid State So far, we have described (solid + liquid) phase equilibrium systems in which the solid phase that crystallizes is a pure compound, either as one of the original components or as a molecular addition compound. Sometimes solid solutions crystallize from solution instead of pure substances, and, depending on the system, the solubility can vary from small to complete miscibility over the entire range of concentration. Figure 14.26 shows the phase diagram for the (silver + copper) system.22 It is one in which limited solubility occurs in the solid state. Line AE is the (solid -I- liquid) equilibrium line for Ag, but the solid that crystallizes from solution is not pure Ag. Instead it is a solid solution with composition given by line AC. If a liquid with composition and temperature given by point a is... 

Figure 14.29 shows the (solid + liquid) phase diagram for (benzene + hexafluoro-benzene). A congruently melting solid molecular addition compound with the formula QFU-CeFe ) is evident in this system.26 The rounded top of the freezing curve (solid line) for the addition compound results from almost complete dissociation of the addition compound in the liquid mixture. In other words, benzene and hexafluorobenzene act as independent molecular species in the liquid state and combine together as the addition compound only in the solid state. 

J. Boerio-Goates, S. R. Goates, J. B. Ott, and J. R. Goates, Enthalpies of Formation of Molecular Addition Compounds in Tetrachloromethane+ p-Xylene,+Toluene, and + Benzene from (Solid + Liquid) Phase Equilibria , J. Chem. Thermodyn., 17,665-670 (1985). 

The nature of the bonds forming molecular addition compounds has been investigated by several workers, but up to now it has not been made sufficiently clear. Pfeiffer [112] held that complex formation was due to the mutual saturation of residual valencies , but Briegleb [113,114] advanced the theory that addition compounds should be regarded as polarization aggregates which owed their stability to electrostatic interactions, possibly due to polarization of one component by the other. Thus, in the case of polynitro compounds, their strongly polar mole cules influence the non polar molecules of the hydrocarbon. Further, no covalent bonds exist between these two kinds of molecules. This hypothesis has received considerable support and a new development. 

Molecular addition compounds are readily formed and decomposed, e.g. by acting with a suitable solvent, which would dissolve one of the components and extract it. For example addition compound formed between picric acid and a hydrocarbon can be split into its components by extracting the picric acid with alcohol while the hydrocarbon remains undissolved. Another fact also indicative of the instability of the addition products is that those of picric acid may be decomposed by treating their solutions with saturated solution of potassium chloride. Potassium picrate is then precipitated, while the other component remains in solution (Taben and Kosak [117]). Generally speaking, the organic addition compounds... 

It is important to add that some molecular addition compounds of sym- trinitrobenzene with hydrocarbons can serve for identification and purification of hydrocarbons (Veibel [141]). Trinitrofluorenone seems to be of particular value (Orchin and Woolfolk [142], Lepley [154]). 

As shown by T. Urbanski [143] between 1933 and 1937, there is a group of molecular addition compounds whose existence cannot be explained by all the points mentioned above. They are addition compounds of certain nitro compounds with esters of nitric acid. Thus many aromatic mononitro compounds form addition compounds with mannitol hexanitrate, and some aromatic trinitro compounds do so with erythritol tetranitrate (Vol. II). On the basis of these facts the author suggests that two main reasons are responsible for the formation of these addition compounds ... 

It has been used as a solvent and forms molecular addition compounds with a great variety of nitrogen, oxygen, and aromatic donor compounds, e.g., N204-THF. 

Molecular addition compounds are indexed under the formulae of their components (except that entries are not made for a few common components). Thus, the 1 1 addition compound of ethanol with sulflnylbis(methane) (dimethyl sulfoxide (DMSO)) appears at C2HgO (ethanol) and at C2H( OS (sulflnylbismethane) and not at C4H12O2S. 

Secondary aliphatic phosphines are prepared by other methods, which have been described. Secondary phosphines form molecular addition compounds with boron halides. Dehydrohalogenation of diphenylphosphine-boron triiodide (or diphenylphosphine-boron tribromide) gives the dimeric phosphinoborane. The following method of preparation makes isolation of the intermediate addition compound unnecessary. Reaction of dibutyl(trimethy 1-silyl)phosphine with boron tribromide and ehmination of bromotrimethylsilane gives a dimeric phosphinoborane also. ... 

This review deals with studies performed at the Oak Ridge National Laboratory concerning the chemical fractionation of boron isotopes between BF3 and its molecular addition compounds. This research resulted in the development of a new separation process which was superior to methods previously employed. The work also led to a theoretical explanation of the exchange reaction which accounted for the anomalous concentration of boron-10 in the molecular addition compound, and the observed variations of the isotopic equilibrium constants as a function of different donors. The model predicted maximum isotopic equilibrium constants for the exchange reaction which were consistent with the experimental data. It also predicted the behavior of the other boron halides. 

Potentially interesting donors were first screened on the basis of the saturation pressure of the adduct. (The term saturation pressure refers to the total pressure of vapor in equilibrium with a sample of molecular addition compound. The vapor may consist of free acid, base, undissociated complex, or a combination of all of these constituents.) Measured quantities of BF3 and the donor were equilibrated at a given temperature in the apparatus shown in Reference 14. Manometric observations (corrected for the free volume of the equipment) were made over that part of the liquid range which lay between room temperature and the freezing point of the complex. Estimates of the heat of association of the complexes were obtained from the temperature dependence of the saturation... 

Table I. Saturation Pressures of Some 1 1 Molecular Addition Compounds from the Freezing Point to Room Temperature...

The equilibrium constants for a particular donor were determined by stirring appropriate quantities of the donor and BF3 for several hours in a suitable reaction vessel (24). Replicate aliquots of BF3 before and after equilibration were analyzed for boron-10 by means of a 6-inch, 60°-sector ratio mass spectrometer. In our experiments the amount of boron trifluoride in the gas phase was deliberately kept small, compared with the amount of boron trifluoride in the liquid phase. For this condition, the ratio of boron-10 to boron-11 in the gas before and after equilibration approximated the true single-stage fractionation factor, B/ B(liquid)/ B/ B(gas). When corrected for the BF3 present in the liquid phase in excess of the 1 1 mole ratio required by the molecular addition compound (Table II), these single-stage fractionation factors represented the isotopic equilibrium constants for Reaction 2. Equilibrium constants for the exchange of boron between BF3 gas and fifteen addition compounds of BF3 are shown in Table III. Curves of the form, log Keq = (b/T) — a, were fitted to the data by means of the least squares technique. From the slopes of these curves and the values of the isotopic... 

The second objective of the present investigation was to understand the mechanism by which boron isotopes are fractionated in Reaction 1. We wished to explain why boron-10 concentrated preferentially in the molecular addition compound, contrary to the usual expectation that the heavy isotope should be preferred by the complexed species. We also wished to explain the variations of the isotopic equilibrium constants for Reaction 1 for various donor molecules. Further, it was desirable to compute theoretical maxima for the isotopic equilibrium constants of Reaction 1 to serve as guides for the practical work. Finally, we wished to evaluate the potential usefulness of other boron trihalides in the fractionation of boron isotopes by Reaction 1. 

Theory of Isotopic Fractionation in the Exchange of Boron Between BFs and Its Molecular Addition Compounds (25)... 

When a molecular addition compound is formed from BF3 and a Lewis base, energy associated with the 7r-bond in the BF3 molecule is absorbed, and energy associated with the coordinate covalent bond in... 

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