Solvated electron

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A solvated electron is a free electron in (solvated in) a solution, and is the smallest possible anion. Solvated electrons occur widely. [1] Often, discussions of solvated electrons focus on their solutions in ammonia, which are stable for days, but solvated electrons also occur in water and other solvents – in fact, in any solvent that mediates outer-sphere electron transfer. The solvated electron is responsible for a great deal of radiation chemistry.

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Ammonia solutions

Liquid ammonia will dissolve all of the alkali metals and other electropositive metals such as Ca, [2] Sr, Ba, Eu, and Yb (also Mg using an electrolytic process [3] ), giving characteristic blue solutions. For alkali metals in liquid ammonia, the solution is blue when dilute and copper-colored when more concentrated (> 3 molar). [4] These solutions conduct electricity. The blue colour of the solution is due to ammoniated electrons, which absorb energy in the visible region of light. The diffusivity of the solvated electron in liquid ammonia can be determined using potential-step chronoamperometry. [5]

Solvated electrons in ammonia are the anions of salts called electrides.

Na + 6 NH3 → [Na(NH3)6]+e−

The reaction is reversible: evaporation of the ammonia solution produces a film of metallic sodium.

Case study: Li in NH3

Solutions obtained by dissolution of lithium in liquid ammonia. The solution at the top has a dark blue color and the lower one a golden color. The colors are characteristic of solvated electrons at electronically insulating and metallic concentrations, respectively. Li-NH3.jpg
Solutions obtained by dissolution of lithium in liquid ammonia. The solution at the top has a dark blue color and the lower one a golden color. The colors are characteristic of solvated electrons at electronically insulating and metallic concentrations, respectively.

A lithium–ammonia solution at −60 °C is saturated at about 15 mol% metal (MPM). When the concentration is increased in this range electrical conductivity increases from 10−2 to 104  Ω −1 cm −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-colored phase becomes immiscible from a denser blue phase. Above 8 MPM the solution is bronze/gold-colored. In the same concentration range the overall density decreases by 30%.

Other solvents

Alkali metals also dissolve in some small primary amines, such as methylamine and ethylamine [6] and hexamethylphosphoramide, forming blue solutions. THF dissolves alkali metal, but a Birch reduction (see § Applications) analogue does not proceed without a diamine ligand. [7] Solvated electron solutions of the alkaline earth metals magnesium, calcium, strontium and barium in ethylenediamine have been used to intercalate graphite with these metals. [8]

Water

Solvated electrons are involved in the reaction of alkali metals with water, even though the solvated electron has only a fleeting existence. [9] Below pH = 9.6 the hydrated electron reacts with the hydronium ion giving atomic hydrogen, which in turn can react with the hydrated electron giving hydroxide ion and usual molecular hydrogen H2. [10]

Solvated electrons can be found even in the gas phase. This implies their possible existence in the upper atmosphere of Earth and involvement in nucleation and aerosol formation. [11]

Its standard electrode potential value is -2.77 V. [12] The equivalent conductivity of 177 Mho cm2 is similar to that of hydroxide ion. This value of equivalent conductivity corresponds to a diffusivity of 4.75 cm2s−1. [13]

Reactivity

Although quite stable, the blue ammonia solutions containing solvated electrons degrade rapidly in the presence of catalysts to give colorless solutions of sodium amide:

2 [Na(NH3)6]+e− → H2 + 2 NaNH2 + 10 NH3

Electride salts can be isolated by the addition of macrocyclic ligands such as crown ether and cryptands to solutions containing solvated electrons. These ligands strongly bind the cations and prevent their re-reduction by the electron.

[Na(NH3)6]+e− + cryptand → [Na(cryptand)]+e−+ 6 NH3

The solvated electron reacts with oxygen to form a superoxide radical (O2.−). [14] With nitrous oxide, solvated electrons react to form hydroxyl radicals (HO.). [15]

Applications

Solvated electrons are involved in electrode processes, a broad area with many technical applications (electrosynthesis, electroplating, electrowinning).

A specialized use of sodium-ammonia solutions 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.

Work by Cullen et al. showed that metal-ammonia solutions can be used to intercalate a range of layered materials, which can then be exfoliated in polar, aprotic solvents, to produce ionic solutions of two-dimensional materials. [16] An example of this is the intercalation of graphite with potassium and ammonia, which is then exfoliated by spontaneous dissolution in THF to produce a graphenide solution. [17]

History

The observation of the color of metal-electride solutions is generally attributed to Humphry Davy. In 1807–1809, he examined the addition of grains of potassium to gaseous ammonia (liquefaction of ammonia was invented in 1823). [18] James Ballantyne Hannay and J. Hogarth repeated the experiments with sodium in 1879–1880. [19] W. Weyl in 1864 and C. A. Seely in 1871 used liquid ammonia, whereas Hamilton Cady in 1897 related the ionizing properties of ammonia to that of water. [20] [21] [22] Charles A. Kraus measured the electrical conductance of metal ammonia solutions and in 1907 attributed it to the electrons liberated from the metal. [23] [24] In 1918, G. E. Gibson and W. L. Argo introduced the solvated electron concept. [25] 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. [26]

Related Research Articles

<span class="mw-page-title-main">Alkali metal</span> Group of highly reactive chemical elements

The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). Together with hydrogen they constitute group 1, which lies in the s-block of the periodic table. All alkali metals have their outermost electron in an s-orbital: this shared electron configuration results in their having very similar characteristic properties. Indeed, the alkali metals provide the best example of group trends in properties in the periodic table, with elements exhibiting well-characterised homologous behaviour. This family of elements is also known as the lithium family after its leading element.

<span class="mw-page-title-main">Ammonia</span> Chemical compound (NH₃)

Ammonia is an inorganic compound of nitrogen and hydrogen with the formula NH3. A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous waste, particularly among aquatic organisms, and it contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to 45% of the world's food and fertilizers. Around 70% of ammonia is used to make fertilisers in various forms and composition, such as urea and diammonium phosphate. Ammonia in pure form is also applied directly into the soil.

<span class="mw-page-title-main">Acid–base reaction</span> Chemical reaction

An acid–base reaction is a chemical reaction that occurs between an acid and a base. It can be used to determine pH via titration. Several theoretical frameworks provide alternative conceptions of the reaction mechanisms and their application in solving related problems; these are called the acid–base theories, for example, Brønsted–Lowry acid–base theory.

In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively charged sodium ions and negatively charged chloride ions.

<span class="mw-page-title-main">Solvation</span> Association of molecules of a solvent with molecules or ions of a solute

Solvation describes the interaction of a solvent with dissolved molecules. Both ionized and uncharged molecules interact strongly with a solvent, and the strength and nature of this interaction influence many properties of the solute, including solubility, reactivity, and color, as well as influencing the properties of the solvent such as its viscosity and density. If the attractive forces between the solvent and solute particles are greater than the attractive forces holding the solute particles together, the solvent particles pull the solute particles apart and surround them. The surrounded solute particles then move away from the solid solute and out into the solution. Ions are surrounded by a concentric shell of solvent. Solvation is the process of reorganizing solvent and solute molecules into solvation complexes and involves bond formation, hydrogen bonding, and van der Waals forces. Solvation of a solute by water is called hydration.

<span class="mw-page-title-main">Ammonium</span> Polyatomic ion (NH₄, charge +1)

The ammonium cation is a positively charged polyatomic ion with the chemical formula NH+4 or [NH4]+. It is formed by the protonation of ammonia. Ammonium is also a general name for positively charged (protonated) substituted amines and quaternary ammonium cations, where one or more hydrogen atoms are replaced by organic or other groups.

<span class="mw-page-title-main">Base (chemistry)</span> Type of chemical substance

In chemistry, there are three definitions in common use of the word "base": Arrhenius bases, Brønsted bases, and Lewis bases. All definitions agree that bases are substances that react with acids, as originally proposed by G.-F. Rouelle in the mid-18th century.

<span class="mw-page-title-main">Cryptand</span> Cyclic, multidentate ligands adept at encapsulating cations

In chemistry, cryptands are a family of synthetic, bicyclic 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, thus launching the now flourishing field of supramolecular chemistry. The term cryptand implies that this ligand binds substrates in a crypt, interring the guest as in a burial. These molecules are three-dimensional analogues of crown ethers but are more selective and strong as complexes for the guest ions. The resulting complexes are lipophilic.

<span class="mw-page-title-main">Radical anion</span> Free radical species

In organic chemistry, a radical anion is a free radical species that carries a negative charge. Radical anions are encountered in organic chemistry as reduced derivatives of polycyclic aromatic compounds, e.g. sodium naphthenide. An example of a non-carbon radical anion is the superoxide anion, formed by transfer of one electron to an oxygen molecule. Radical anions are typically indicated by .

<span class="mw-page-title-main">Hexamethylphosphoramide</span> Chemical compound

Hexamethylphosphoramide, often abbreviated HMPA, is a phosphoramide (an amide of phosphoric acid) with the formula [(CH3)2N]3PO. This colorless liquid is a useful reagent in organic synthesis.

<span class="mw-page-title-main">Electride</span> Ionic compound with electrons as the anion

An electride is an ionic compound in which an electron is the anion. Solutions of alkali metals in ammonia are electride salts. In the case of sodium, these blue solutions consist of [Na(NH3)6]+ and solvated electrons:

The chemical element nitrogen is one of the most abundant elements in the universe and can form many compounds. It can take several oxidation states; but the most oxidation states are -3 and +3. Nitrogen can form nitride and nitrate ions. It also forms a part of nitric acid and nitrate salts. Nitrogen compounds also have an important role in organic chemistry, as nitrogen is part of proteins, amino acids and adenosine triphosphate.

Sodium atoms have 11 electrons, one more than the stable configuration of the noble gas neon. As a result, sodium usually forms ionic compounds involving the Na+ cation. Sodium is a reactive alkali metal and is much more stable in ionic compounds. It can also form intermetallic compounds and organosodium compounds. Sodium compounds are often soluble in water.

An alkalide is a chemical compound in which alkali metal atoms are anions with a charge or oxidation state of −1. Until the first discovery of alkalides in the 1970s, alkali metals were known to appear in salts only as cations with a charge or oxidation state of +1. These types of compounds are of theoretical interest due to their unusual stoichiometry and low ionization potentials. Alkalide compounds are chemically related to the electrides, salts in which trapped electrons are effectively the anions.

In chemistry, a Zintl phase is a product of a reaction between a group 1 or group 2 and main group metal or metalloid. It is characterized by intermediate metallic/ionic bonding. Zintl phases are a subgroup of brittle, high-melting intermetallic compounds that are diamagnetic or exhibit temperature-independent paramagnetism and are poor conductors or semiconductors.

<span class="mw-page-title-main">Chloroauric acid</span> Chemical compound

Chloroauric acid is an inorganic compound with the chemical formula H[AuCl4]. It forms hydrates H[AuCl4]·nH2O. Both the trihydrate and tetrahydrate are known. Both are orange-yellow solids consisting of the planar [AuCl4]− anion. Often chloroauric acid is handled as a solution, such as those obtained by dissolution of gold in aqua regia. These solutions can be converted to other gold complexes or reduced to metallic gold or gold nanoparticles.

<span class="mw-page-title-main">Birch reduction</span> Organic reaction used to convert arenes to cyclohexadienes

The Birch reduction is an organic reaction that is used to convert arenes to 1,4-cyclohexadienes. The reaction is named after the Australian chemist Arthur Birch and involves the organic reduction of aromatic rings in an amine solvent with an alkali metal and a proton source. Unlike catalytic hydrogenation, Birch reduction does not reduce the aromatic ring all the way to a cyclohexane.

<span class="mw-page-title-main">Caesium auride</span> Chemical compound

Caesium auride is the inorganic compound with the formula CsAu. It is the Cs+ salt of the unusual Au− anion.

<span class="mw-page-title-main">Hydration number</span> Measure of solvency/solution

The hydration number of a compound is defined as the number of molecules of water bonded to a central ion, often a metal cation. The hydration number is related to the broader concept of solvation number, the number of solvent molecules bonded to a central atom. The hydration number varies with the atom or ion of interest.

<span class="mw-page-title-main">Azanide</span> Anion derived from deprotonation of ammonia

Azanide is the IUPAC-sanctioned name for the anion NH−2. The term is obscure; derivatives of NH−2 are almost invariably referred to as amides, despite the fact that amide also refers to the organic functional group –C(=O)−NR2. The anion NH−2 is the conjugate base of ammonia, so it is formed by the self-ionization of ammonia. It is produced by deprotonation of ammonia, usually with strong bases or an alkali metal. Azanide has a H–N–H bond angle of 104.5°.

References

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  15. ↑ Janata, Eberhard; Schuler, Robert H. (1982). "Rate constant for scavenging eaq- in nitrous oxide-saturated solutions". The Journal of Physical Chemistry. 86 (11): 2078–2084. doi:10.1021/j100208a035.
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