Dimer Mixtures

The following four systems have been extensively studied 

We shall limit ourselves here to a discussion of the first of these four mixtures (a discussion of the others on the basis of the cell model for solutions can be found in a paper by Bellemans and Naar-Colin [1955]). 

The comparison will be given for g , A , s and The theoretical expressions are (17.6.1) and (17.7.1) (A and s are easily deduced from (17.6.1) by differentiation). Here again we shall use argon at 87.3 K as reference component, the correspondence with benzene being given roughly by the factor 

From the heat of vaporization (diphenyl Montillon, Rohreach and Badger [1931]), benzene Everett [1952a]), we find using a method analogous to that described in Ch. XI, 2, 

6 cannot be determined from the data of pure components. There is at present no theoretical reason to use the geometric mean rule because of the rather arbitrary cutting of the dimer into two s pients. We shall adjust d from the experimental excess enthalpy and see whether the value thus obtained will produce a satisfactory agreement for the other excess functions. 

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In 1977 De Santis et al. (J5) as well as Heidemann et al. ( ) calculated the gas-phase fugacities in the systems HjO-air and H2O-N2-CO2 by equation of state in these calculations the liquid phase was not included. One of the authors (7J showed in 1978 that aqueous systems with some inert gases and alkanes as well as H2S and C02 could be represented by an equation of state if the molecular weight of water was artificially increased. An extension of this method applied to alcohols was found to be only partially successful. Gmehling et al. (8) treated polar fluids such as alcohols, ketones and water as monomer-dimer mixtures using Donohue s equation of state (9) various systems including water-methanol and water-ethanol were succussfully represented. 

Liquid TIBA exists as an equilibrium monomer-dimer mixture. For example, at 40° TIBA is 16% associated. In hydrocarbon solution, the dimer is more extensively dissociated with increasing dilution (ISO). 

Streitwieser and coworkers recently reported that the lithium enolate of p-(phenylsul-fonyl)isobutyrophenone exists as a monomer-dimer mixture iu THE, with the equilibrium constant = (5.0 0.1) x 10 M . The rate of the reaction of the enolate with p-tert-butylbenzyl bromide was measured spectrophotometricaUy at the enolate concentration range of 7 x 10 to 5 x 10 M, where the percentage of the monomer was 4.5 to 11%. The logarithmic plot of the rate vs the enolate concentration was linear with a slope of 0.50 0.04, indicating that the reacting species is the monomer that exists as a minor component in the equilibrium with the dimer. 

Lithiation of 2-biphenylcyclohexanone gave two enolate species in THF one an unconjugated secondary enolate and the other a conjugated tertiary enolate. The former exists dominantly as the tetramer and the latter as a monomer-dimer mixture. In both cases the monomer reacts faster than the aggregates with alkylating substrates. ... 

The reduction of NAD+ at a mercury electrode at —1.1 V versus SCE gives a 90% yield of dimers220 three stereoisomers of 4,4 -dimers were found to account for 90% of the dimer mixture, and three 4,6 -dimers were responsible for the remaining 10%. The dimers were separated, using reverse-phase HPLC and gel filtration on Sephadex G-15 no 6,6 -dimers were detected. Reduction at —1.8 V resulted in 50% 1,4-NADH, 30% 1,6-NADH, and 20% dimers. In other investigations221 the three stereoisomeric 4,4 -dimers have also been obtained as the main products. 

In order to understand the thermal oxidation of C2F4, it is first necessary to understand the reactions in the absence of 02. Below about 280°C, C2F4 is thermally stable. Above this temperature, it dimerizes to c-C4Fe, but otherwise undergoes no other reactions below about 550°C. Between 550 and 800°C, the monomer-dimer mixture decomposes to other fluorocarbons via CF2 as an intermediate. At even more elevated temperatures the CF2-C2F4 equilibrium is achieved and ultimately forms carbon and CF4 via CF radicals and fluorine atoms. 

More satisfactory separations were obtained when reverse-phase liquid chromatography was used. The separation of the standard dimeric mixture was carried out on an LC-18 column with refractometry as the mode of detection and acetonitrile/acetone (1 1) as the mobile phase (system II). Separation proceeded according to the polarity of the various dimers, and complete separation of all dimers, except those of the thermal dimer of methyl linoleate and the dehydrodimer of methyl oleate, were obtained at a flow rate of 0.5 ml/min. The resolution of the two unresolved peaks would be increased by using another LC-18 column in series, but a sacrifice in the analysis time would have to be made. 

Fortunately, ( )-3 was readily in 76% yield from If following the Singaram, Cole, Brown method" which takes full advantage of the insolubility of dialkoxyalanes in ether. The addition of TMSC1 (1.0 equiv) to the borohydride (nB NMR 5 -20 (t, J = 66 Hz)) releases the free borane which distills as a monomer, but exists in solution as a 3 1 monomer/dimer mixture at 25°C in CDC13 (Scheme 1). In separate experiments, it was confirmed that the reaction of TMSCHN2 was... 

In the absence of kinetic effects in the dimerization step the yield of diastereo-meric dimers of Scheme 4 reflects directly the R,S) microcomposition (m n) of each parent crystal (Scheme 5) this ratio could be measured by n.m.r. of the dimer mixtures (Figure 4 h) and proved to be 61 39 in the case of the eutectic, but to approach 50 50 for the pure metastable phase. The metastable phase is therefore the one we need for absolute asymmetric synthesis however, due to its metastability, it is difficult to crystallize it in the form of a pure homochiral phase. Nevertheless, when large crystals were grown rapidly from the melt, there resulted phases at least partially enriched with one of the two enantiomorphous forms and which upon irradiation yielded net asymmetric syntheses. The optical yields of dimerization and polymerization ranged from 0—35%, while the recovered monomer was always racemic, in keeping with an asymmetric synthesis due only to the crystalline environment. 

To obtain Binor-S the reaction must be conducted in nonpolar solvents such as heptane or toluene. In coordinating solvents such as tetrahydrofuran, dioxane, f-butyl alcohol, ethylene glycol or glycol dimethyl ether dimer mixtures are formed. The effect of these solvents is similar to that of bases (see below). Alcohols cause solvolysis in addition to forming solvates with the zinc atom (52). 

The cobalt carbonylates of Cd and In gave mixtures of dimers at the same concentrations at which Zn[Co(CO)4]2 also gave dimer mixtures. However, both catalysts also responded to Lewis acid cocatalysis. The mercury compound in itself was also unselective, but gave Binor-S in the presence of BPa O(C2H6)2. The catalyst In[Co(CO)4]3 did not produce a norbornadiene trimer. Concerted trimerization, though not completely unlikely, would require all three norbornadiene molecules in... 

The enol-lactam (30), which has occupied a central role in the synthesis of Erythrina alkaloids, has been converted in an unprecedented reaction into the dimeric isomers [31 C(6)-a-0] and [31 C(6)-/ -OJ.15 This reaction may be effected in benzene, pyridine, or acetic acid solution in the presence of lead tetra-acetate. The structures of the products were elucidated by spectral and chemical means. As enol ethers, these compounds were found to exhibit surprising stability to mineral acids. However, catalytic reduction of [31 C(6)-a-OJ under neutral conditions gave the starting enol-lactam (30) and the 7/Miydroxylactam (32 RJ = OH, R2 = R3 = H). The dimer [3 l C(6)-/i-0] yielded only compound (32 R1 = OH, R2 = R3 = H). Similarly, sodium borohydride reduction of the dimer mixture in hot isopropanol led to cleavage products (32 R1 = OH, R2 = R3 = H)and(32 RJ = R3 = H, R2 = OH). Besides the dimeric products, compound (32 R1 + R2 = O, R3 = OAc) was also isolated from the lead tetraacetate oxidation in low yields. Attempts to discover conditions for the formation of preparative amounts of (32 R1 + R2 = O, R3 = OAc), a compound of more potential usefulness for alkaloid synthesis, were fruitless. The other question of interest, whether or not the trans-dimer [31 C(6)-/i-0] could be converted into a monomeric trans-erythrinane system, remains to be answered. 

In solution, organolithium compounds exist as aggregates, with the degree of aggregation depending on the structure of the organic group and the solvent. The nature of the species present in solution can be studied by low-temperature NMR. n-Butyllithium in THF, for example, is present as a tetramer-dimer mixture. The tetrameric species is dominant. 

The electrolytic oxidation of the sodium salt of (+ )-N-ethoxycarbonyl-Al-norarmepavine (46) led to the dimeric mixtures (47) and (48). The latter mixture was converted into a mixture of dauricine analogues (49) via O-benzylation, reduction with lithium aluminium hydride, and hydrogenolytic debenzylation. This transformation represents the first preparation of an analogue of a natural bis-benzylisoquinoline by oxidation of a phenolic monomeric benzylisoquinoline. Detailed studies on the mass-spectral cleavage patterns of bisbenzylisoquinolines have appeared. 

Compounds HXB(NMe2), where X = C1 or Br, are prepared from (H2BNMe2)2 and HgX2 at 110 °C. Vapour-pressure and gas-phase molecular weight data show that they are present as equilibrium monomer/dimer mixtures in the gaseous phase, with a preponderance of the latter at 25 °C. When X = Br, the dimer is the more stable. 

The conversion of (4-)-8 to R-lf was accomplished through the intermediate B-allyl derivative 9R followed by methanolysis (Scheme 4) (79). As for the racemic material (Scheme 2), 3R was readily generated as a monomer-dimer mixture in THF solution. Because this material as well as the racemic 3 were not obtained as crystalline solids, a general protocol was developed (Scheme 4) for the hydrobcH aticms with 3, its being generated in situ from If in either racemic or optically pure form. 

McCabe, C., Gfl-VHegas, A., and Jackson, G., 1999. Gibbs ensemble computer simulation and SAFT-VR theory of non-conformal square-well monomer-dimer mixtures. Chem. Phys. Lett., 303 27. 

IR (neat, in cavity cells) 2113 cm (s, Si-H, trimer and dimer mixture). 

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