Solvated electron
Encyclopedia
A solvated electron is a free electron in (solvated in) a solution
. Solvated electrons occur widely although they are often not observed directly. The deep colour of solutions of alkali metals in ammonia
arises form the presence of solvated electrons: blue when dilute and copper-colored when more concentrated (> 3 molar). Classically, discussions of solvated electrons focus on their solutions in ammonia, which are stable for days, but solvated electrons occur in water and other solvents, in fact any solvent that mediates outer-sphere electron transfer. The solvated electron is responsible for a great deal of radiation
chemistry.
and ethylamine
.
A lithium ammonia solution at -60 °C is saturated at about 16 mol% metal (16 MPM in the local jargon). When the concentration is increased in this range electrical conductivity increases from 10−2 to 104 ohm
−1cm
−1 (larger than liquid mercury
). At around 8 MPM, a "transition to the metallic state" (TMS) takes place (also called a "metal to nonmetal transition" (MNMT)). At 4 MPM a liquid-liquid phase separation takes place: the less dense gold-color phase becomes immiscible from a more dense blue phase. Above 8 MPM the solution is bronze/gold-colored. In the same concentration range the overall density
decreases by 30%.
Dilute solutions are paramagnetic and at around 0.5 MPM all electrons are paired up
and the solution becomes diamagnetic. Several models exist to describe the spin-paired species: as an ion trimer, or as an ion-triple—a cluster of two single-electron solvated-electron species in association with a cation, or as a cluster of two solvated electrons and two solvated cations.
Solvated electrons produced by dissolution of reducing metals in ammonia and amines are the anions of salts called electride
s. Such salts can be isolated by the addition of macrocyclic ligand
s such as crown ether
and cryptand
s. These ligands bind strongly the cations and prevent their re-reduction by the electron.
to form a superoxide
radical
(O2.-), which is a potent oxidant. With nitrous oxide
, solvated electrons react to form hydroxyl
radicals (HO.). The solvated electrons can be scavenged from both aqueous and organic systems with nitrobenzene
or sulfur hexafluoride
.
A common use of sodium dissolved in liquid ammonia is the Birch reduction
. Other reactions where sodium is used as a reducing agent also are assumed to involve solvated electrons, e.g. the use of sodium in ethanol as in the Bouveault–Blanc reduction.
and J. Hogarth repeated the experiments with sodium in 1879-1880. W. Weyl in 1844 and C.A. Seely in 1871 were the first to use liquid ammonia. Hamilton Cady
in 1897 was the first to relate the ionizing properties of ammonia to that of water Charles A. Kraus
measured the electrical conductance of metal ammonia solutions and in 1907 was the first to attribute it the electrons liberated from the metal. In 1918, G. E. Gibson and W. L. Argo introduced the solvated electron concept. They noted based on absorption spectra that different metals and different solvents (methylamine
, ethylamine
) produce the same blue color, attributed to a common species, the solvated electron. In the 1970s, solid salts containing electrons as the anion were characterized.
Solution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
. Solvated electrons occur widely although they are often not observed directly. The deep colour of solutions of alkali metals in ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
arises form the presence of solvated electrons: blue when dilute and copper-colored when more concentrated (> 3 molar). Classically, discussions of solvated electrons focus on their solutions in ammonia, which are stable for days, but solvated electrons occur in water and other solvents, in fact any solvent that mediates outer-sphere electron transfer. The solvated electron is responsible for a great deal of radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
chemistry.
Properties
Focusing on ammonia solutions, all of the alkali metals as well as Ca, Sr, Ba, Eu, and Y, dissolve to give the characteristic blue solutions. Other amines are also suitable solvents, such as methylamineMethylamine
Methylamine is the organic compound with a formula of CH3NH2. This colourless gas is a derivative of ammonia, but with one H atom replaced by a methyl group. It is the simplest primary amine. It is sold as a solution in methanol, ethanol, THF, and water, or as the anhydrous gas in pressurized...
and ethylamine
Ethylamine
Ethylamine is an organic compound with the formula CH3CH2NH2. This colourless gas has a strong ammonia-like odor. It is miscible with virtually all solvents and is considered to be a weak base, as is typical for amines. Ethylamine is widely used in chemical industry and organic...
.
A lithium ammonia solution at -60 °C is saturated at about 16 mol% metal (16 MPM in the local jargon). When the concentration is increased in this range electrical conductivity increases from 10−2 to 104 ohm
Ohm
The ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...
−1cm
CM
- Places :* Cameroon, which has the ISO and FIPS country code "CM"** .cm, the country code top-level domain for Cameroon* Chelmsford, which has the British post code "CM"- Science :* Centimetre a unit of length equal to one hundredth of a metre...
−1 (larger than liquid mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
). At around 8 MPM, a "transition to the metallic state" (TMS) takes place (also called a "metal to nonmetal transition" (MNMT)). At 4 MPM a liquid-liquid phase separation takes place: the less dense gold-color phase becomes immiscible from a more dense blue phase. Above 8 MPM the solution is bronze/gold-colored. In the same concentration range the overall density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
decreases by 30%.
Dilute solutions are paramagnetic and at around 0.5 MPM all electrons are paired up
Electron pair
In chemistry, an electron pair consists of two electrons that occupy the same orbital but have opposite spins.Because electrons are fermions, the Pauli exclusion principle forbids these particles from having exactly the same quantum numbers. Therefore the only way to occupy the same orbital, i.e....
and the solution becomes diamagnetic. Several models exist to describe the spin-paired species: as an ion trimer, or as an ion-triple—a cluster of two single-electron solvated-electron species in association with a cation, or as a cluster of two solvated electrons and two solvated cations.
Solvated electrons produced by dissolution of reducing metals in ammonia and amines are the anions of salts called electride
Electride
An electride is an ionic compound in which an electron is the anion. The first electrides to be studied in depth were solutions of alkali metals in ammonia. When sodium metal dissolves in ammonia, the result is a blue solution consisting of [Na6]+ and solvated electrons. Such solutions are powerful...
s. Such salts can be isolated by the addition of macrocyclic ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s such as crown ether
Crown ether
Crown ethers are cyclic chemical compounds that consist of a ring containing several ether groups. The most common crown ethers are oligomers of ethylene oxide, the repeating unit being ethyleneoxy, i.e., -CH2CH2O-. Important members of this series are the tetramer , the pentamer , and the hexamer...
and cryptand
Cryptand
Cryptands are a family of synthetic bi- and polycyclic multidentate ligands for a variety of cations. The Nobel Prize for Chemistry in 1987 was given to Donald J. Cram, Jean-Marie Lehn, and Charles J. Pedersen for their efforts in discovering and determining uses of cryptands and crown ethers,...
s. These ligands bind strongly the cations and prevent their re-reduction by the electron.
Reactivity and applications
The solvated electron reacts with oxygenOxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
to form a superoxide
Superoxide
A superoxide, also known by the obsolete name hyperoxide, is a compound that possesses the superoxide anion with the chemical formula O2−. The systematic name of the anion is dioxide. It is important as the product of the one-electron reduction of dioxygen O2, which occurs widely in nature...
radical
Radical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
(O2.-), which is a potent oxidant. With nitrous oxide
Nitrous oxide
Nitrous oxide, commonly known as laughing gas or sweet air, is a chemical compound with the formula . It is an oxide of nitrogen. At room temperature, it is a colorless non-flammable gas, with a slightly sweet odor and taste. It is used in surgery and dentistry for its anesthetic and analgesic...
, solvated electrons react to form hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
radicals (HO.). The solvated electrons can be scavenged from both aqueous and organic systems with nitrobenzene
Nitrobenzene
Nitrobenzene is an organic compound with the chemical formula C6H5NO2. It is a water-insoluble pale yellow oil with an almond-like odor. It freezes to give greenish-yellow crystals. It is produced on a large scale as a precursor to aniline. Although occasionally used as a flavoring or perfume...
or sulfur hexafluoride
Sulfur hexafluoride
Sulfur hexafluoride is an inorganic, colorless, odorless, and non-flammable greenhouse gas. has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule. Typical for a nonpolar gas, it is poorly soluble in water but soluble in...
.
A common use of sodium dissolved in liquid ammonia is the Birch reduction
Birch reduction
The Birch Reduction is an organic reaction which is particularly useful in synthetic organic chemistry. The reaction was reported in 1944 by the Australian chemist Arthur Birch working in the Dyson Perrins Laboratory in the University of Oxford, building on earlier work by Wooster and Godfrey in...
. Other reactions where sodium is used as a reducing agent also are assumed to involve solvated electrons, e.g. the use of sodium in ethanol as in the Bouveault–Blanc reduction.
History
The first to note the colour of metal-electride solutions was Sir Humphry Davy. In 1807-1809, he examined the addition of grains of potassium to gaseous ammonia (liquefaction of ammonia was invented in 1823). James Ballantyne HannayJames Ballantyne Hannay
James Ballantyne Hannay was a Scottish chemist who believed he had synthesized diamond in 1880. However, modern testing showed that the surviving samples from his experiments were natural diamond, not synthetic...
and J. Hogarth repeated the experiments with sodium in 1879-1880. W. Weyl in 1844 and C.A. Seely in 1871 were the first to use liquid ammonia. Hamilton Cady
Hamilton Cady
Hamilton Perkins Cady, , was an American chemist who in 1907 in collaboration with David McFarland discovered that helium could be extracted from natural gas.-Early life:...
in 1897 was the first to relate the ionizing properties of ammonia to that of water Charles A. Kraus
Charles A. Kraus
Charles August Kraus was an American chemist. He was professor of chemistry and director of the chemical laboratories at Clark University, where he directed the Chemical Warfare Service during World War I...
measured the electrical conductance of metal ammonia solutions and in 1907 was the first to attribute it the electrons liberated from the metal. In 1918, G. E. Gibson and W. L. Argo introduced the solvated electron concept. They noted based on absorption spectra that different metals and different solvents (methylamine
Methylamine
Methylamine is the organic compound with a formula of CH3NH2. This colourless gas is a derivative of ammonia, but with one H atom replaced by a methyl group. It is the simplest primary amine. It is sold as a solution in methanol, ethanol, THF, and water, or as the anhydrous gas in pressurized...
, ethylamine
Ethylamine
Ethylamine is an organic compound with the formula CH3CH2NH2. This colourless gas has a strong ammonia-like odor. It is miscible with virtually all solvents and is considered to be a weak base, as is typical for amines. Ethylamine is widely used in chemical industry and organic...
) produce the same blue color, attributed to a common species, the solvated electron. In the 1970s, solid salts containing electrons as the anion were characterized.