Solvent-free reaction media

Microwave irradiation has been shown to be synergistic with a solvent-free reaction medium. Lactam 51, upon microwave irradiation in the presence of alumina, was found to undergo the Maimich reaction to afford 53 using formaldehyde 2 and amine 52. 

The refined grade s fastest growing use is as a commercial extraction solvent and reaction medium. Other uses are as a solvent for radical-free copolymerization of maleic anhydride and an alkyl vinyl ether, and as a solvent for the polymerization of butadiene and isoprene usiag lithium alkyls as catalyst. Other laboratory appHcations include use as a solvent for Grignard reagents, and also for phase-transfer catalysts. 

There are different ways in which the nanoparticles prepared by ME-technique can be used in catalysis. The use of ME per se [16,17] implies the addition of extra components to the catalytic reaction mixture (hydrocarbon, water, surfactant, excess of a metal reducing agent). This leads to a considerable increase of the reaction volume, and a catal5fiic reaction may be affected by the presence of ME via the medium and solubilization effects. The complex composition of ME does not allow performing solvent-free reactions. 

In solvent-free reaction systems, the absence of solvent facilitates downstream processing as fewer components are present in the reaction mixture moreover, the elimination of solvent from the production step offers significant cost savings. In some ways, solvent-free reaction systems are similar to solvent systems in that reactants serve as a reaction medium. With the absence of solvent, high purity products (solvent-free) often can be obtained. One drawback to solvent-free systems is the possibility of high media viscosity, which may result in poor mixing and, hence, slow reaction rates. 

Very efficient Michael addition reactions of amines, thiophenoi and acetylacetate to chalcone in a water suspension medium have been developed as completely organic solvent-free reactions. 

Ionic polymerizations commonly involve two types of propagating species— an ion pair (II-IV) and a free ion (V)—coexisting in equilibrium with each other. The relative concentrations of these two types of species, as also the identity of the ion pair (that is, whether of type II, ID, or IV), depend on the particular reaction conditions and especially the solvent or reaction medium, which has a large effect in ionic polymerizations. Loose ion pairs are more reactive than tight ion pairs, while free ions are significantly more reactive than ion pairs. In general, more polar media favor solvent-separated ion pairs or free solvated ions. In hydrocarbon media, jffee solvated ions do not exist, though other equilibria may occur between ion pairs and clusters of ions (Rudin, 1982). 

Solar light is of no use for the above reaction, because anisoles do not absorb solar light, but the related anilines absorb and react in a fully analogous way (Scheme 12.3) [13]. In general, the abovementioned insensitivity to the medium makes possible a green choice [14]. Examples are solvent-free reactions (see the photooxygenation of neat benzene by water in Scheme 12.4 [15]), solid-state processes (see the cyclization of the carboxylate 4 that occurs with high enantiomeric excess from the salt with proline. Scheme 12.5 [16]), and reaction in water or... 

The photolysis of chlorinated aromatic compounds occurs by several processes which follow predictable routes 13). They frequently undergo photochemical loss of chlorine by dissociation of the excited molecule to free radicals or, alternatively, through a nucleophilic displacement reaction with a solvent or substrate molecule. Either mechanism is plausible, and the operation of one or the other may be influenced by the reaction medium and the presence of other reagents. 

A mixture of 1,4-dioxane and water is often used as the solvent for the conversion of aldehydes and ketones by H2Se03 to a-dicarbonyl compounds in one step (Eq. 8.117).331 Dehydrogenation of carbonyl compounds with selenium dioxide generates the a, (i-unsaturated carbonyl compounds in aqueous acetic acid.332 Using water as the reaction medium, ketones can be transformed into a-iodo ketones upon treatment with sodium iodide, hydrogen peroxide, and an acid.333 Interestingly, a-iodo ketones can be also obtained from secondary alcohol through a metal-free tandem oxidation-iodination approach. 

Figure 4.11 Stripping in solvent-free medium Synthesis of myristyl myristate from myristyric acid and myristyl alcohol applying a bubble column reactor stripping the reaction water...

To illustrate these trends, we now present some typical illustrative examples. These have been selected because strict comparisons of microwave and classical heating activation were made under similar conditions (time, temperature, pressure, etc.. ..) for the same reaction medium and using, preferably, a monomode system equipped with stirring. They mostly involve reactions performed under solvent-free conditions or, occasionally, in a nonpolar solvent, because these conditions are also favorable for observation of microwave effects. 

Based on this approach Schouten et al. [254] attached a silane-functionalized styrene derivative (4-trichlorosilylstyrene) on colloidal silica as well as on flat glass substrates and silicon wafers and added a five-fold excess BuLi to create the active surface sites for LASIP in toluene as the solvent. With THF as the reaction medium, the BuLi was found to react not only with the vinyl groups of the styrene derivative but also with the siloxane groups of the substrate. It was found that even under optimized reaction conditions, LASIP from silica and especially from flat surfaces could not be performed in a reproducible manner. Free silanol groups at the surface as well as the ever-present impurities adsorbed on silica, impaired the anionic polymerization. However, living anionic polymerization behavior was found and the polymer load increased linearly with the polymerization time. Polystyrene homopolymer brushes as well as block copolymers of poly(styrene-f)lock-MMA) and poly(styrene-block-isoprene) could be prepared. 

Mechanical synthesis by cold mastication of rubber and monomers depends on the reaction condition (monomer concentration, temperature, solvent concentration, atmosphere, presence of transfer agents, or catalyst) and on the physical and chemical properties of the rubbers, the monomers and the product interpolymers. A critical factor is the shear stress developed in the system rather than instrumentally-defined shear rates. The degree of reaction of polymer and consequently also the concentration of free macroradicals depends on stress. As a consequence, the influence of the above parameters may be connected to their influence on the viscosity of the reaction medium since an increase in viscosity causes an increase in stress at constant shear rate. 

This, however, can be an oversimplification if the reaction conditions (solvent, temperature eta) are such that the propagating ion pair is either partially dissociated or more highly aggregated. Usually the reaction medium is a moderately polar solvent, e.g. dichloromethane or 1,2-dichloro-ethane, and the concentrations of active centres employed are sufficiently small to discount contributions from more highly aggregated species. The problem, therefore, with few exceptions revolves around a fairly simple equilibrium involving only ion pairs and free solvated ions, each with its own reactivity, e.g. 

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