Organic liquids, solvated electron

Solutions of alkali metals in liquid ammonia are used in organic chemistry as reducing agents. The deep blue solutions effectively contain solvated electrons (p. 126), for example... 

The alkali metals also release their valence electrons when they dissolve in liquid ammonia, but the outcome is different. Instead of reducing the ammonia, the electrons occupy cavities formed by groups of NH3 molecules and give ink-blue metal-ammonia solutions (Fig. 14.14). These solutions of solvated electrons (and cations of the metal) are often used to reduce organic compounds. As the metal concentration is increased, the blue gives way to a metallic bronze, and the solutions begin to conduct electricity like liquid metals. 

The properties of organic liquids relevant to their use as solvating agents have also been reviewed [76]. The ability of liquids to solvate a solute species depends mainly on their polarity and polarizability properties, ability to hydrogen bond, and cohesive electron density. These molecular properties are best measured by the Kamlet-Taft solvatochromic parameters, and the square of Hildebrand s solubility parameter. 

It is logical to consider the nncleophile, Nu-, as a source of the electron to be transferred onto the snbstrate molecnle, RX. However, in most cases, the nucleophile is such a poor electron donor that electron transfer from Nn- to RX is extremely slow, if it is possible at all. These reactions reqnire an external stimulation in which a catalytic amount of electrons is injected. Such kinds of assistance to the reactions from photochemical and electrochemical initiations or from solvated electrons in the reaction mediums have been pointed out earlier. Alkali metals in liquid ammonia and sodium amalgam in organic solvents can serve as the solvated electron sources. Light initiation is also used widely. However, photochemical initiation complicates the reaction performance. 

The opticol absorption spectra of the solvated electron have now been reported for a number of organic liquids. The chemical reactivity in the aliphatic alcohols has been studied by the pulse radiolysis method. The absorption maxima for a series of five aliphatic alcohols are in the visible to near infra-red. These maxima show a red shift with decrease in the static dielectric constant. The solvated electron undergoes reactions of electron-ion combination, electron attachment, and dissociative electron attachment. Absolute rate constants have been determined for these reactions. 

The solvated electron has been studied in a number of organic liquids, among which are the aliphatic alcohols (27, 28, 3, 2d, 2, 27), some ethers (25, 5), and certain amines (9, 22, 2). Of these systems, it is only in the alcohols, to which this paper is principally but not exclusively directed, that both the chemical reactivity and the optical absorption spectrum of the solvated electron have been investigated in detail. The method used in these studies is that of pulse radiolysis (22, 22), developed some five years ago. The way was shown for such investigations of the solvated electron by the observation of the absorption spectrum of the hydrated electron (6, 28, 19) and by the subsequent kinetic studies (2d, 22, 20) which are being discussed in other papers in this symposium. 

It would appear at this stage that a good deal of useful information has yet to be obtained by the pulse radiolysis method concerning the absorption spectra of the solvated electron in various organic liquids. Such data would help to remove uncertainties regarding the assignment of bands and would serve as criteria for the validity of developing models. 

The solvated electron has also been shown to be produced in the pulse radiolysis of polar organic liquids, viz. amines, ethers and alcohols53. Some spectra in aliphatic alcohols are shown in Fig. 6. Decrease in the static dielectric constant shifts A towards the red. 

Deep blue solutions of solvated electrons are formed when Li, Na, K, Ca, or other group I and group II metals are dissolved into liquid ammonia (Eq. (1)). These media have long been used to reduce organic compounds. The widely used Birch reduction [11-18], known for 80 years, is employed... 

Solvated Electrons in Organic Media. There was, of course, no reason to suppose that water was the only liquid capable of solvating the electron and over a very few years pulse radiolysis experiments demonstrated the existence of solvated electrons, e", in many liquids. 

Another occasion when this definition is of some importance to an organic chemist occurs in the Birch reduction, which is performed in liquid ammonia. Sodium metal is dissolved in liquid ammonia to give solvated electrons, which may be represented, rather simplistically, as NH /p-. 

A wealth of information on the reduction of metal ions in aqueous solutions has been obtained and a compilation was published in 1988 [20], However, alkali or alkaline earth metal ions such as Li Na or cannot be reduced by the hydrated electron in aqueous solution but can form an ion pair with the solvated electron in polar liquids. Among the various reactions of the solvated electron, the reduction of halogenated hydrocarbons is often used in radiation chemistry to produce well-defined radicals because of the selective cleavage of the carbon-halogen bond by the attack ofthe solvated electron. This reaction produces the halide ion and a carbon-centered radical, and is of great interest for environmental problems related to the destruction of halogenated organic contaminants in water and soil [21,22]. 

Solvated electrons obtained by dissolving alkali metals in liquid anunonia and similar solvents are now extensively used for the reduction of organic compounds (Birch reaction) To this end, the application of electrochemically obtained solvat-... 

Another factor ensuring high current density is the rapid removal of solvated electrons from the electrode due to intense convection in the solution, which is caused by a decrease in its density at the cathode surface. This phenomenon is associated with an increase in solution volume caused by the introduction of electrons into it. Unlike electrostriction that accompagies the solvation of ordinary ions, the formation of solvated electrons increases the volume by 65-96 ml/mol for liquid ammonia and by about 80 ml/mol for hexamethylphosphotriamide . As a result, according to Avaca and Bewick the current densities for the generation of solvated electrons can by 2500 or more times exceed the rate of mass transfer of organic compounds to the electrode. 

Electron-metal+ ion-pairs are more reactive than the free solvated electrons in reactions yielding alkoxide or amides. Apparently, the charge transfer required for the formation of those anions is facilitated by the proximity of Met+ cation which becomes coordinated with the oncomming X-OR reagent. Since, in contrast to liquid ammonia, ion-pairs are the abundant species in organic amine or ether solutions, these directly... 

Solutions of sodium metal in liquid ammonia are blue and have high electrical conductivities the main current carrier of such solutions is the solvated electron. Such solutions are used in both organic and inorganic chemistry as efficient reducing agents. Sodium also forms a number of alkyl and aryl derivatives by reaction with the appropriate mercury compound, e.g. ... 

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