Dimerization, alkenes

In collaboration with Prof. Herman Ammon (University of Maryland), MOLPAK/WMIN computational methods [6-12] have been employed successfully to predict the crystal densities of (i) several isomerically pure monomethylated PCU alkene dimers [13] and (ii) several polycyclic epoxides [14]. The crystal densities calculated from X-ray crystal structure data are compared with the corresponding calculated values. Attempts have been made to address discrep>-ancies that may exist between computed densities and those derived from A-ray data by using advanced theoretical methods. 

Initially, Prof. Segal and members of his research group at the University of Florida evaluated the combustion characteristics of mixtures of isomeric PCU alkene dimers (2a) as solid fuels in compressible flows. Subsequently, these studies were extended to include evaluation of 2a as a solid fuel under conditions of high-shear flow. Samples of the mixture of isomeric PCU alkene dimers were cured with a styrene-polybutadiene copolymer (10% w/w) binder on the test... 

The results of this study have been published [37]. Evaluation samples of several candidate energetic hydrocarbon fuel systems have been sent to Prof. Segal for his combustion studies (see Scheme 14). More recently. Prof. Segal and his co-workers have studied rheological properties and burning rates of a mixture of isomeric methylated PCU alkene dimers (2b). A stable 18% w/w solution of 2b in JP-10 was achieved. More concentrated solutions (up to 25% w/w) were unstable and produced sediments after standing for ca. 2 weeks under ambient conditions. An 18% w/w solution of 2b in JP-10 increased the kinematic viscosity of JP-10 by 1.3 centistokes at 30 °C and by 0.65 centistokes at 70 °C, thereby effectively matching the viscosity of RJ-4. 

The combustion characteristics of mixtures of PCU alkene dimers as solid fuels in compressible flows have been studied by Prof. Corin Segal and his coworkers at the University of Florida. The results of his combustion studies demonstrate that fuels formed by the addition of mixtures of methylated PCU alkene dimers (18% w/w solutions) to JP-10 have a significant accelerated burning rate relative to that of pure JP-10. In addition, a new candidate hydrocarbon fuel, i.e., compound 5, was found by Prof. Segal to burn rapidly (i.e., 2.9 mm /s) and to release a relatively large quantity of heat during combustion. 

Methylated PCU Alkene Dimer — C24H28, is a low-melting point (about 55 °C) mixture of up to 64 isomers. In contrast to the base PCU alkene dimer, the methylated PCU alkene dimer dissolved in a large proportion in JP-10, which was the selected fuel system used in this study and stable solutions in proportion of up to 18% have been obtained. The addition of methylated PCU alkene dimers to JP-10 induced droplet boiling and increased the fuel heat output. These mixtures have been studied by means of suspended droplet combustion and spectrometric analyses. 

Spirocyclic Alkene Dimer — C22H26, is a mixture of two isomers. This material is a powder under normal conditions. It was analyzed as a solid sample by the same procedure as the PCU alkene dimers. 

Mass Spectrometric Analysis of Methylated PCU Alkene Dimer... 

Solutions of Solid Fuels in Liquid Hydrocarbons — Methylated PCU Alkene Dimers... 

The methylated PCU alkene dimer was dissolved in JP-10 and stable solutions to 18% have been obtained. Higher concentrations have been achieved, but it... 

Figure 5.2 Images of burning droplets (a) initial (t = 0) JP-10 droplet size (6) JP-10 droplet at t = 0.997 s (c) JP-10 droplet close to complete combustion at t = 1.673 s shows no indication of internal vaporization (d) initial droplet of a 18% mixture of JP-10 and methylated PCU alkene dimer (e) initial formation of internal vapors in the 18% mixture at t = 0.713 s (/) strong effervescence in the 18% mixture at t = 1.230 s...

Images of pure JP-10 and the mixtures are shown in Figs. 5.2a to 5.2/. As shown in Figs. 5.2a to 5.2c, recorded at t = 0, 0.997, and 1.673 s, respectively, the kerosene droplet did not indicate internal vaporization until close to complete combustion. The situation changed when mixtures of kerosene with methylated PCU alkene dimer were used, as shown in Figs. 5.2d to 5.2/ for a 18% mixture. For the 18% mixture the first internal vapors appeared at 1 = 0.713 s (Fig. 5.2e) and indication of strong effervescence appeared at t = 1.23 s (Fig. 5.2/). 

Burning rates of mixtures of PCU alkene dimers in JP-10 are shown in Fig. 5.3. The burning rate for the pure fuel is 0.757 mm /s. The solution vaporization rate is 0.409 mm /s and combustion proceeds with a rate of 1.971 mm /s, significantly higher than the base JP-10. 

Stable mixtures to 18% of methylated PCU alkene dimers in JP-10 have been obtained. This concentration is sufficiently large to increase the density of the fuel to a significant degree and also to augment the exothermicity of the droplet combustion. 

Under proper conditions a carbocation (R" ), formed by adding an electrophile such as H or BF, to an alkene, may add to the C==C bond of another alkene molecule to give a new dimeric R here, R" acts as an electrophile and the tt bond of 0=C acts as a nucleophilic site. R " may then lose an to give an alkene dimer. 

R.S. Smith, Alkene dimerization, US Patent 5 243119, assigned to Ethyl Corporation (Richmond, VA), September 7,1993. 

Nickel,40 41 like almost all metal catalysts (e.g., Ti and Zr) used for alkene dimerization, effects the reaction by a three-step mechanism.12 Initiation yields an organometallic intermediate via insertion of the alkene into the metal-hydrogen bond followed by propagation via insertion into the metal-carbon bond [Eq. (13.8)]. Intermediate 11 either reacts further by repeated insertion [Eq. (13.9)] or undergoes chain transfer to yield the product and regenerate the metal hydride catalyst through p-hydrogen transfer [Eq. (13.10)] ... 

The most commonly observed dimerization is that of alkenes to form cyclobutane derivatives. Nonconjugated alkenes such as cyclopentene and norbornene are dimerized in the presence of a sensitizer, whereas conjugated alkenes dimerize directly dimerizations of the second type have been observed in dienes, phenyl-ethylenes, and a,/9-unsaturated carbonyl, cyano, and nitro derivatives. The precise structure and stereochemistry of many of these dimers is uncertain, although it is known to be influenced by the solvent, by the presence and nature of substituents, and by the use of a sensitizer. The structures of dimers formed in solid-state irradiations are often determined by crystal structure. 

Attempts have recently been made to prepare solid acids by loading triflic acid into various inert oxides including silica,184 titania,185,186 and zirconia.187,188 Silica functionalized with anchored aminopropyl groups was also used to immobilize triflic acid.189 These new catalysts have been tested in a variety of organic transformations, such as alkane-alkene alkylation, Friedel-Crafts acylation, alkene dimerization, and acetalization. Silica nanoboxes prepared by dealumination of Na-X- and Ca-A-type zeolites were also loaded with triflic acid up to 32 wt%.190 The materials were thoroughly characterized but have not been tested as catalysts. 

This carbocation then reacts with a molecule of styrene in the manner we have seen earlier (Chapter 6) for alkene dimerization. 

The possibility of the intermediacy of the triplet state of benzene itself has been discussed by Atkins et al. [108], Photoaddition of alkenes to arenes is often accompanied by the formation of dimers of the alkene, a reaction sensitized by triplet benzene. With methyl acrylate and methyl vinyl ketone, however, it was found that the ratio of ortho cycloadducts to alkene dimers increased with the concentration of benzene. Because the yield of T, benzene increases with benzene concentration, these results might indicate that ortho photocycloaddition of aery-... 

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