Side products, removal

High conversions and molecular weights are obtained because reversible reactions and side-product removal is not an issue. In RIM processes, liquid monomers are mixed using impinging jets and must quickly flow into the mold before the reacting mixture becomes too viscous. The complete cycle time for injection, reaction and umnolding (so that the mold can be used to make the next product) is often only 30-60 s. RIM is used to produce rigid molded polyurethane automobile parts and polyurethane foam seat cushions [3j. 

Example 2 (Effect of escaping monomer) Consider an AA and BB polycondensation system that uses a slightly volatile AA monomer (e.g., HMD used in nylon 6,6 polymerization) that escapes from the reaction mixture during the early stages of the polycondensation, when monomer concentrations are high and side-product removal is rapid. To compensate for the loss of volatile AA monomer, assume that the recipe is designed so that 105 moles of AA are... 

Applying an adequate mixture of catalysts during reforming reaction, methane is produced solely as a side-product. Removing hydrogen with a selective membrane, the formation of methane decrease, and the amount of produced hydrogen is increased (Basile et al., 2008). 

The lack of significant vapor pressure prevents the purification of ionic liquids by distillation. The counterpoint to this is that any volatile impurity can, in principle, be separated from an ionic liquid by distillation. In general, however, it is better to remove as many impurities as possible from the starting materials, and where possible to use synthetic methods that either generate as few side products as possible, or allow their easy separation from the final ionic liquid product. This section first describes the methods employed to purify starting materials, and then moves on to methods used to remove specific impurities from the different classes of ionic liquids. 

Kister [94, 95] examines binary distillation systems with multiple feeds, one or more side products, one or more points of heat removal or addition, and various combinations. 

In the horizontal plate freezer (Figure 7.9a), the plates are arranged in a stack on slides, so that the intermediate spaces can be opened and closed. Trays, boxes or cartons of the product are loaded between the plates and the stack is closed to give good contact on both sides. When the necessary cooling is complete, the plates are opened and the product removed. 

The temperature is controlled throughout the reaction by intermittently adding additional pieces of dry ice to the dry ice-acetone bath. Strict temperature control throughout the bromination reaction is important to obtain high regioselectivity and purity of the product as the side products cannot be removed with ease. 

The reaction rates cannot be set as high as intrinsically possible by the kinetics, because otherwise heat removal due to the large reaction enthalpies (500-550 kj mol ) will become a major problem [17, 60, 61]. For this reason, the hydrogen supply is restricted, thereby controlling the reaction rate. Otherwise, decomposition of nitrobenzene or of partially hydrogenated intermediates can occur ]60], The reaction involves various elemental reactions with different intermediates which can react with each other ]60], At short reaction times, the intermediates can be identified, while complete conversion is achieved at long reaction times. The product aniline itself can react further to give side products such as cyclohexanol, cyclohexylamine and other species. 

Recently, iodobenzoates anchored onto an ionic liquid support (6.4) were coupled to various aryl boronic acids (6.5) in aqueous media using Pd(OAc)2 as the catalyst at 80°C to give the coupled product 6.6 (Scheme 6.3). Compounds 6.6 were purified simply by washing the reaction mixture with ether, which removed the unreacted starting materials and the side product 6.7 without the need of chromatography. Compounds 6.6 were then cleaved from the ionic liquid support... 

Presently, there is a general consensus that heterogeneous catalytic processes play an important role in environmental issues regarding their high selectivity towards the removal of undesired side products, such as atmospheric pollutants, in comparison with that obtained from non-catalysed processes. However, such a benefit could be disputed in the future with the implementation of severe restrictions on standard emission of those atmospheric pollutants, particularly nitric oxide, which is a very challenging aspect. 

Modem shuttle machines can be set up with two complete moulds so that approximately 2/3 of the cure time is under full clamp pressure in the main station. The clamp pressure is then reduced and the closed mould is shuttled out to continue the cure in the side station where it is opened for product removal, mould cleaning and insert loading. While one mould is in the side station the other is injected and receives the first part of its cure. With a special mould opening device on the shuttle table the machine becomes as effective as a two-station machine. 

Even with the simple laboratory equipment used in these experiments, the CESS procedure allowed quantitative recovery of the product free of solvent, and with rhodium contents ranging from 0.36-1.94 ppm (determined by atomic absorption measurements). Furthermore, using this approach removal of unreacted starting material or side products from the product is possible during extraction from the catalyst, since even small structural differences can result in significant differences in... 

Substitution of the 4-nitro group in 3,4-dinitrofuroxan 1176 by ammonia occurs readily, even at low temperature. Subsequent treatment of the obtained amine, product 1177, with r-butylamine results in formation of 4-amino-2-(/-butyl)-5-nitro-l,2,3-triazole 1-oxide 1178. However, there must be some additional side products in the reaction mixture, as the isolated yield of compound 1178 is only 17%. Upon treatment with trifluoroperacetic acid, the r-butyl group is removed. The obtained triazole system can exist in two tautomeric forms, 1179 and 1180 however, the 1-oxide form 1179 is strongly favored (Scheme 195) <2003CHE608>. 

The organometallic side-product MAr is conveniently removed by selective reaction with a proton source such as Bu Cl or NH4C1. This reaction is extremely effective for certain symmetrical triarylphos-phines and for mixed aralkylphosphines. However, detailed investigations of P-C cleavage in functionalized and/or unsymmetrical triaryl-phosphines (17-19) indicate that such reactions are far from straightforward. The products obtained on treatment of triarylphos-phines of type IV or V with alkali metals depend both on the nature of the substituents X and on the alkali metal. 

OCTENAR [Octane enhancement by removing aromatics] A process for removing aromatic hydrocarbons from petroleum reformate by extractive distillation with N-formyl mor-phylane. The product can be blended with gasoline to increase its octane number — hence the name. A paraffin mixture is obtained as a side-product. Developed by Krupp Koppers from its MORPHYLANE and MORPHYLEX processes. 

A picture of the last step, the casting, can be seen in Figure 5.1. The processing steps are done under strictly controlled temperatures (up to roughly 1220 °C). During these, several chemical reactions take place and these enable the separation of copper from other materials. A typical reaction during the conversion is to remove the ferrous sulfides (FeS) by oxidization to iron oxides (FeO), which react with silica and can be thereafter removed as slag that is collected on the top of a ladle. As a side product, sulfur dioxide is formed and it is quite common to reuse it in a combined sulfuric acid plant. These two reactions are shown in (A) and (B) ... 

Hydrogen transfer reactions are highly selective and usually no side products are formed. However, a major problem is that such reactions are in redox equilibrium and high TOFs can often only be reached when the equilibria involved are shifted towards the product side. As stated above, this can be achieved by adding an excess of the hydrogen donor. (For a comparison, see Table 20.2, entry 8 and Table 20.7, entry 3, in which a 10-fold increase in TOF, from 6 to 60, can be observed for the reaction catalyzed by neodymium isopropoxide upon changing the amount of hydrogen donor from an equimolar amount to a solvent. Removal of the oxidation product by distillation also increases the reaction rate. When formic acid (49) is employed, the reduction is a truly irreversible reaction [82]. This acid is mainly used for the reduction of C-C double bonds. As the proton and the hydride are removed from the acid, carbon dioxide is formed, which leaves the reaction mixture. Typically, the reaction is performed in an azeotropic mixture of formic acid and triethylamine in the molar ratio 5 2 [83],... 

The side products of the reactions, e. g. bis(trimethylsilyl)acetylene, THF, pyridine, acetone, etc., are soluble and volatile and are thus easy to remove. 

The reagent is obtained as a fairly pure oil by reaction of phenylsilane with iodine catalyzed by a trace of an organic ester. After removal of volatile side products (benzene and HI), the oil can be purified by fractional distillation. 

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