Dimerization, vapor phase

Monomer, vapor phase Dimer, vapor phase Infinite chain, crystal ... 

Equation (12), applicable at low or moderate pressures, is used in this monograph for typical vapor mixtures. However, when the vapor phase contains a strongly dimerizing component such as carboxylic acid. Equation (7) is not applicable and... 

Two additional illustrations are given in Figures 6 and 7 which show fugacity coefficients for two binary systems along the vapor-liquid saturation curve at a total pressure of 1 atm. These results are based on the chemical theory of vapor-phase imperfection and on experimental vapor-liquid equilibrium data for the binary systems. In the system formic acid (1) - acetic acid (2), <() (for y = 1) is lower than formic acid at 100.5°C has a stronger tendency to dimerize than does acetic acid at 118.2°C. Since strong dimerization occurs between all three possible pairs, (fij and not... 

As discussed in Chapter 3, at moderate pressures, vapor-phase nonideality is usually small in comparison to liquid-phase nonideality. However, when associating carboxylic acids are present, vapor-phase nonideality may dominate. These acids dimerize appreciably in the vapor phase even at low pressures fugacity coefficients are well removed from unity. To illustrate. Figures 8 and 9 show observed and calculated vapor-liquid equilibria for two systems containing an associating component. 

Figure 4-9. Vapor-liquid equilibria for a binary system where one component dimerizes in the vapor phase. Activity coefficients show only small deviations from liquid-phase ideality.

Enthalpies are referred to the ideal vapor. The enthalpy of the real vapor is found from zero-pressure heat capacities and from the virial equation of state for non-associated species or, for vapors containing highly dimerized vapors (e.g. organic acids), from the chemical theory of vapor imperfections, as discussed in Chapter 3. For pure components, liquid-phase enthalpies (relative to the ideal vapor) are found from differentiation of the zero-pressure standard-state fugacities these, in turn, are determined from vapor-pressure data, from vapor-phase corrections and liquid-phase densities. If good experimental data are used to determine the standard-state fugacity, the derivative gives enthalpies of liquids to nearly the same precision as that obtained with calorimetric data, and provides reliable heats of vaporization. 

VPLQFT is a computer program for correlating binary vapor-liquid equilibrium (VLE) data at low to moderate pressures. For such binary mixtures, the truncated virial equation of state is used to correct for vapor-phase nonidealities, except for mixtures containing organic acids where the "chemical" theory is used. The Hayden-0 Connell (1975) correlation gives either the second virial coefficients or the dimerization equilibrium constants, as required. 

System type refers to whether vapor phase contains strongly associating (dimerizing) components or not. 

Dimeric ion. An ion formed either when a chemical species exists in the vapor phase as a dimer and can be detected as such, or when a molecular ion can attach to a neutral molecule within the ion source to form an ion such as (2M)+, where M represents the molecule. 

Reactions with Aldehydes and Ketones. The base-catalyzed self-addition of acetaldehyde leads to formation of the dimer, acetaldol [107-89-1/, which can be hydrogenated to form 1,3-butanediol [107-88-0] or dehydrated to form crotonaldehyde [4170-30-3]. Crotonaldehyde can also be made directiy by the vapor-phase condensation of acetaldehyde over a catalyst (53). 

The vapor density of acetic acid suggests a molecular weight much higher than the formula weight, 60.06. Indeed, the acid normally exists as a dimer (4), both in the vapor phase (5) and in solution (6). This vapor density anomaly has important consequences in engineering computations, particularly in distillations. 

In the vapor phase formic acid forms a hydrogen-bonded dimer ... 

Propylene Dimer. The synthesis of isoprene from propjiene (109,110) is a three-step process. The propjiene is dimeri2ed to 2-methyl-1-pentene, which is then isomeri2ed to 2-methyl-2-pentene in the vapor phase over siUca alumina catalyst. The last step is the pyrolysis of 2-methyi-2-pentene in a cracking furnace in the presence of (NH 2 (111,112). Isoprene is recovered from the resulting mixture by conventional distillation. 

Bismuth Penta.fIuoride, Bismuth(V) fluoride consists of long white needles that have been shown to have the same stmcture as the body-centered, tetragonal a-polymorph of uranium hexafluoride. The density of the soHd is 5.4 g/mL at 25°C. The soHd consists of infinite chains of trans-bridged BiF polyhedra dimers and trimers are present in the vapor phase (22). Bismuth pentafluoride may be prepared by the fluorination of BiF or... 

Tables 13-1, 13-2, and 13-4 include data on formic acid and acetic acid, two substances that tend to dimerize in the vapor phase according to the chemical-equilibrium expression...

As shown by Marek and Standart [Collect. Czech. Chem. Commun., 19, 1074 (1954)], it is preferable to correlate and utilize hquid-phase activity coefficients for the dimerizing component by considering separately the partial pressures of the monomer and dimer. For example, for a binary system of components 1 and 2, when only compound 1 dimerizes in the vapor phase, the following equations apply if an ideal gas is assumed ... 

Hexafluoropropene and sulfur react at 425 °C in the vapor phase to give hexafluorothioacetone and its dimer [/]. In dimethylformamide, it reacts with potassium fluoride and sulfur to give hexafluorothioacetone dimer, which further reacts with hexafluoropropene to give the E and Z isomers of perfluoro-2,4,6-tris(trifluoromethyl)-5-thia-3-heptene [2] (equation 1). 

The photolytic reaction of a perfluoro anhydride in the vapor phase at 50 °C results in the elimination of not only carbon monoxide but also of carbon dioxide [92] The tetrafluorocyclobutadiene formed is not stable and dimerizes easily (equation 58)... 

The efficacy of RF in stabilizing small rings is well illustrated by the fact that the azete from trifluoro-l,2,3-triazine is considerably more reactive. Trapping experiments were unsuccessful and a polymer was isolated at room temperature. The dimer (41) forms an observable anion with CsF, which confirmed the endo structure [87CC1699 90JCS(P1)975, 90JCS(P1)983], In contrast, trifluoro-l,2,4,-triazine is resistant to vapor phase photoysis and flow pyrolysis [87JCS(P1) 1251]. 

The hydroxo complexes [Au(6-Rbipy)(OH)2], postulated as intermediates in the formation of 44c-i, are not isolated nevertheless, in all the mass spectra (FAB conditions) a weak peak is found corresponding to these species. Unchanged 44c-i are quantitatively recovered from the reaction with aqueous solutions of HX (X = BF4 or PFfi) neither dihydroxo complexes similar to those observed in the vapor phase, nor hydroxo bridged dimers are obtained [25]. 

Effect of molecular diffusion and vapor-phase chemical reactions Liquid metal vapors consist of molecules and gaseous atoms. Working with alkali metals, Ewing et al. (1967) found that the molecules are principally dimers and tetramers. The... 

The most extensively studied of these complexes for use in CVD are the diethyldichalcogeno-carbamates [M(E2CNEt2)2] (M = Zn, Cd E = Se, S). These complexes are air-stable solids, and are dimeric in the solid state175,176 but monomeric in the vapor phase, as judged from mass spectra. Thin films of CdS, CdSe, ZnS, and ZnSe have been grown by a number of different MOCVD techniques using these complexes as single-source precursors they are listed in Table 11. 

The association of acetic acid in the vapor phase occurs so that the molecular weight of the gas indicates that it exists as dimers ... 

FIG. 3 The vapor phase water dimer structure. Polar covalent bonds are shown as solid lines and the hydrogen bond as a dashed line (adapted from Ludwig, 2001). 

We shall examine the simplest association in the vapor phase, dimerization. Any conclusions drawn may be extended to higher polymerizations. For the comparison salt AX, which has a characteristic size parameter d0, the association constant for the equilibrium... 

Elemental sulfur is found in the flames of all the sulfur-bearing compounds discussed in the previous subsections. Generally, this sulfur appears as atoms or the dimer S2. When pure sulfur is vaporized at low temperatures, the vapor molecules are polymeric and have the formula Sg. Vapor-phase studies of pure sulfur oxidation around 100°C have shown that the oxidation reaction has the characteristics of a chain reaction. It is interesting to note that in the explosive studies the reaction must be stimulated by the introduction of O atoms (spark, ozone) in order for the explosion to proceed. 

Mass spectrometric studies on iron(II) chloride and iron(II) bromide showed that monomers are the major species in the vapor phase, while concentration of the dimer, Fe2X4, increases with temperature in its certain interval. The electron diffraction data on iron(II) chloride could be well approximated by monomers only, while the data on the bromide indicated the presence of a detectable amount of dimeric species. This can be seen on the radial distribution of Fig. 12. It was found that there was about 10% dimeric qjecies present under the experimental conditions (nozzle temperature around 625 °C). As regards the relative scattering power, this constituted about 20%, and allowed only the determination of a limited amount of structural information. The electron diffraction date were consistent with a bridge stmcture characterized by the same Fe-Br, bond length as that of the monomeric... 

Many inorganic molecules are present as dimers (or higher associates) in the vapor phase at the pressure necessary for the electron diffraction experiment. The reaction... 

Photolytic. Based on data for structurally similar compounds, acenaphthylene may undergo photolysis to yield quinones (U.S. EPA, 1985). In a toluene solution, irradiation of acenaphthylene at various temperatures and concentrations all resulted in the formation of dimers. In water, ozonation products included 1,8-naphthalene dialdehyde, 1,8-naphthalene anhydride, 1,2-epoxyacenaphthylene, and 1-naphthoic acid. In methanol, ozonation products included 1,8-naphthalene dialdehyde, 1,8-naphthalene anhydride, methyl 8-formyl-1-naphthoate, and dimethoxyacetal 1,8-naphthalene dialdehyde (Chen et al., 1979). Acenaphthylene reacts with photochemically produced OH radicals and ozone in the atmosphere. The rate constants and corresponding half-life for the vapor-phase reaction of acenaphthylene with OH radicals (500,000/cm ) at 25 °C are 8.44 x lO " cmVmolecule-sec and 5 h, respectively. The rate constants and corresponding half-life for the vapor-phase reaction of acenaphthylene with ozone at 25 °C are... 

Formula FeCls MW 162.21 occurs as a dimer Fe2Cl6 in vapor phase. Synonym ferric chloride... 

Formula M0CI5 MW 273.20 monomeric in vapor phase but occurs as dimeric M02CI10 in solid phase (Mo— Mo bond 3.84 A)... 

Vapor-phase flash pyrolysis of 4-phenyl-1,2,3-benzotriazine (8, R = Ph) at 420°-450° gives a mixture of biphenylene, 9-phenylacridine, unchanged triazine, and the 2-phenylbenzazete (158). Compound 158, which is stable at -80°, dimerizes when warmed to room temperature and reacts readily with nucleophiles and 1,3-dienes. The thermally more... 

The mechanism of this reaction is obscure. One suggested mechanism, analogous to the vapor phase reaction, involves concerted decarboxylation of the pyruvic acid to yield a triplet hydroxy carbene which can either dimerize or attack another molecule of pyruvic acid to yield the observed product.91 Dimerization seems to be the less likely process since the carbene can rearrange to acetaldehyde or react with water. Further, this mechanism predicts that acetoin will be formed when pyruvic acid is irradiated in any solvent that does not possess readily abstractable hydrogen atoms, such as benzene, a solvent in which no reaction is observed. One possible explanation of this discrepancy is that the solvation of the pyruvic acid is extremely different in benzene and in water. However, the specific role that the water plays in the reaction has not been determined. 

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