Polar solvents spontaneous

A number of reductive procedures have found general applicability. a-Azidoketones may be reduced catalytically to the dihydropyrazines (80OPP265) and a direct conversion of a-azidoketones to pyrazines by treatment with triphenylphosphine in benzene (Scheme 55) has been reported to proceed in moderate to good yields (69LA(727)23l). Similarly, a-nitroketones may be reduced to the a-aminoketones which dimerize spontaneously (69USP3453279). The products from this reaction are pyrazines and piperazines and an intermolecular redox reaction between the initially formed dihydropyrazines may explain their formation. Normally, if the reaction is carried out in aqueous acetic acid the pyrazine predominates, but in less polar solvents over-reduction results in extensive piperazine formation. 

Hydrogenolysis of aromatic carbonyls occurs mainly by conversion to the benzyl alcohol and its subsequent loss. If hydrogenolysis is desired, the usual catalyst is palladium 38). Hydrogenolysis is facilitated by polar solvent and by acid (55). For instance, hydrogenation of 3,3-dicarbethoxy-5,8-dimethoxy-l-tetralone (5) over 5% Pd-on-C gave 6 quantitatively 54) when hydrogen absorption ceased spontaneously. 

Some halogenometalate species have been observed to have formed spontaneously during spectroelectrochemical studies in ionic liquids. For example, [MoCl ] (which is hydrolyzed in water, is coordinated by solvent in polar solvents, and has salts that are insoluble in non-polar solvents) can only be observed in basic (X(A1C13) < 0.5 chloroaluminate ionic liquids [1]. FFowever, this work has been directed at the measurement of electrochemical data, rather than exploitation of the ionic liquids as solvents for synthesis [2]. It has been shown that the tetrachloroa-luminate ion will act as a bidentate ligand in acidic X(A1C13) > 0.5 chloroaluminate ionic liquids, forming [M(AlCl4)3] ions [3]. This was also the result of the spontaneous formation of the complexes, rather than a deliberate attempt to synthesize them. 

This is the most common route to vinylidene complexes and occurs in reactions of the 1 -alkynes with metal complexes, preferably with labile neutral or anionic ligands, which give neutral or cationic complexes, respectively. In the latter case, halide is commonly extracted, either by spontaneous displacement by a polar solvent, or by using sodium, silver or thallium salts. 

The use of methanol or ethanol as solvent (or sometimes the molecule of water resulting from the spontaneous dehydration) often leads to the isolation of a Fischer-type alkoxy- or hydroxy-carbene [M]=C(OR)CH=CR R instead of the desired allenylidene. Addition of nucleophiles to allenylidenes dominates the reactivity of these electrophilic groups (see below). Nevertheless, in some cases, the use of silver (I) salts Ag[X] (X = PFg, TfO, BF4 ) results in a more practical and flexible synthetic method since the use of nucleophilic polar solvents can be avoided. 

Spontaneous oxidation of amines by one-electron transfer has been reported as a key process in polar solvents (35). It is not easy to distinguish the spontaneous and initiated mechanisms, because these pathways have a common intermediate (XI, Fig. 9). Thus, potassium hexacyanoferrate (III), a one-electron oxidant, gives electron transfer oxidation of amines (56) yielding the classical radical autoxidation products. 

Computer simulations have provided further insight into the model of random fluctuations as a prerequisite for e.t. in polar solvents [60], It has been shown that spontaneous local polarity fluctuations of the magnitude envisaged by the Marcus model are so improbable as to be statistically insignificant and it was necessary to assume that the solvent could adjust continuously in order to follow the position of the electron in the course of e.t., as if e.t. would be slow enough to be the rate-determining kinetic step. To what extent such a modification of the model... 

First, we note that the charge ordering of the solvent can impose itself on the distribution of products in reaction. Chiappe and Pieraccini [40] report that in their study of electron transfer between Micheler s ketone and tetracyanoethene, they observed that the formation of a radical ion pair to be preferred over formation of a single, neutral complex. Such a preference is only observed for the most highly polar molecular liquids, and is analogous to the spontaneous ionization of metal atoms in fused salts noted above. This represents a novel phenomenon for moderately polar solvents, though its generality is unclear at this time. 

Reactive electrodes refer mostly to metals from the alkaline (e.g., lithium, sodium) and the alkaline earth (e.g., calcium, magnesium) groups. These metals may react spontaneously with most of the nonaqueous polar solvents, salt anions containing elements in a high oxidation state (e.g., C104 , AsF6 , PF6 , SO CF ) and atmospheric components (02, C02, H20, N2). Note that ah the polar solvents have groups that may contain C—O, C—S, C—N, C—Cl, C—F, S—O, S—Cl, etc. These bonds can be attacked by active metals to form ionic species, and thus the electrode-solution reactions may produce reduction products that are more stable thermodynamically than the mother solution components. Consequently, active metals in nonaqueous systems are always covered by surface films [46], When introduced to the solutions, active metals are usually already covered by native films (formed by reactions with atmospheric species), and then these initial layers are substituted by surface species formed by the reduction of solution components [47], In most of these cases, the open circuit potentials of these metals reflect the potential of the M/MX/MZ+ half-cell, where MX refers to the metal salts/oxide/hydroxide/carbonates which comprise the surface films. The potential of this half-cell may be close to that of the M/Mz+ couple [48],... 

These macromolecular compounds when dissolved in a suitable polar solvent (generally water) instantly acquire or can be made to acquire large number of elementary electrical charges distributed along the macromolecular chain. In the former case and when the charge that appears spontaneously has its maximum value, these macromolecules are termed low molar-mass (LMM) electrolytes and in other cases weak polyelectrolytes. 

Acetylene HCsCH (dj 1.089 bpi.oia = 87.8 Q is a flammable gas, soluble in acetone and many polar solvents. It is an unstable compound whose explosive decomposition can occur spontaneously if its partial pressuie in a mixture exceeds 0.14.10 Pa... 

In the first two parts of this chapter, electron transfer (ET) from atomic donors, e.g., alkali metals or the iodine anion, to an accepting unit composed of simple molecular or atomic solvents was discussed. It was demonstrated that even for a molecule without a stable anionic state or large dipole moment, e.g., water and ammonia, an ensemble of a relatively small number of the molecules can act as an electron acceptor. In the case of the solvated alkali metal atom clusters, ET takes place spontaneously as the number of solvent molecules increases, while the ET in the solvated 1 clusters is induced by photoexcitation into the diffuse electronic excited states just below the vertical detachment thresholds. These ET processes in isolated supermolecular systems resemble the charge delocalization phenomena in condensed phases, e.g., excess-electron ejection from alkali metals into polar solvents and the charge transfer to solvent in a solution of stable anions. 

Surfactants are amphiphilic molecules which, when dispersed in a solvent, spontaneously self-assemble to form a wide variety of structures, including spherical and asymmetric micelles, hexagonal, lamellar, and a plethora of cubic phases. With the exception of the lamellar phase, each of these phase structures can exist in both normal and reverse orientations with the hydrophobic chains on the exterior of the aggregate, in contact with solvent or vice versa orientation. The range of structures a particular surfactant forms and the concentration range over which they form, depends upon the molecular architecture of the surfactant, its concentration, and the solvent in which it is dispersed. For example, some solvents such as ethanol do not support the formation of aggregates. As most pharmaceutical systems use water as their solvent, this entry will concentrate on aqueous-based systems, although other solvent systems, particularly other non-aqueous polar systems, will be mentioned where appropriate. 

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