View source for Chemistry:Solvated electron

{{Short description|Free electron in a solution, often liquid ammonia}}
A '''solvated electron''' is a [[Physics:Particle|free]] [[Physics:Electron|electron]] in a [[Chemistry:Solution|solution]], in which it behaves like an anion.<ref>{{cite journal |author=Dye, J. L. |title=Electrons as Anions |journal=Science |year=2003 |volume=301 |pages=607–608 |doi=10.1126/science.1088103 |pmid=12893933 |issue=5633 |s2cid=93768664}}</ref> An electron's being solvated in a solution means it is bound by the solution.<ref>{{Cite journal |doi=10.1002/anie.196801901 |title=Formation and Properties of Solvated Electrons |journal=Angewandte Chemie International Edition in English |volume=7 |issue=3 |pages=190–203 |year=1968 |last1=Schindewolf |first1=U.}}</ref> The notation for a solvated electron in formulas of chemical reactions is "e<sup>−</sup>". Often, discussions of solvated electrons focus on their solutions in ammonia, which are stable for days, but solvated electrons also occur in water and many other solvents{{snd}} in fact, in any solvent that mediates outer-sphere electron transfer.  The solvated electron is responsible for a great deal of [[Chemistry:Radiation chemistry|radiation chemistry]].

== Ammonia solutions ==
Liquid ammonia will dissolve all of the [[Chemistry:Alkali metal|alkali metal]]s and other [[Chemistry:Electronegativity|electropositive]] metals such as [[Chemistry:Calcium|Ca]],<ref>{{cite encyclopedia|title=Calcium–Ammonia|author=Edwin M. Kaiser|encyclopedia=Encyclopedia of Reagents for Organic Synthesis|year=2001|doi=10.1002/047084289X.rc003|isbn=978-0471936237}}</ref> [[Chemistry:Strontium|Sr]], [[Chemistry:Barium|Ba]], [[Chemistry:Europium|Eu]], and [[Chemistry:Ytterbium|Yb]] (also [[Chemistry:Magnesium|Mg]] using an electrolytic process<ref>{{cite journal|doi=10.1016/S0022-0728(00)00504-0|title=Solutions of solvated electrons in liquid ammonia|journal=Journal of Electroanalytical Chemistry|volume=499|pages=144–151|year=2001|last1=Combellas|first1=C|last2=Kanoufi|first2=F|last3=Thiébault|first3=A}}</ref>), giving characteristic blue solutions. For alkali metals in liquid ammonia, the solution is blue when dilute and copper-colored when more concentrated (> 3 [[Physics:Molar concentration|molar]]).<ref name="c&w">{{cite book |last1=Cotton |first1=F. A. |last2=Wilkinson |first2=G. |title=Advanced Inorganic Chemistry |year=1972 |publisher=John Wiley and Sons Inc |isbn=978-0-471-17560-5}}</ref> These solutions [[Physics:Conductivity (electrolytic)|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 [[Physics:Chronoamperometry|chronoamperometry]].<ref>{{cite journal |doi=10.1016/S0022-0728(80)80115-X |title=The diffusion coefficient of solvated electrons in liquid ammonia |journal=Journal of Electroanalytical Chemistry and Interfacial Electrochemistry |volume=109 |issue=1–3 |pages=167–177 |year=1980 |last1=Harima |first1=Yutaka |last2=Aoyagui |first2=Shigeru }}</ref>

Solvated electrons in ammonia are the anions of salts called [[Physics:Electride|electride]]s. 
:Na  +  6 NH<sub>3</sub>  →  [Na(NH<sub>3</sub>)<sub>6</sub>]<sup>+</sup> + e<sup>−</sup>
The reaction is reversible: evaporation of the ammonia solution produces a film of metallic sodium.

===Case study: Li in NH<sub>3</sub>===
[[File:Li-NH3.jpg|200px|right|thumb|Solutions obtained by dissolution of [[Chemistry:Lithium|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.|alt=Photos of two solutions in round-bottom flasks surrounded by dry ice; one solution is dark blue, the other golden.]]

A lithium–ammonia solution at −60&nbsp;°C is saturated at about 15&nbsp;mol% metal (MPM). When the concentration is increased in this range [[Physics:Electrical conductivity|electrical conductivity]] increases from 10<sup>−2</sup> to 10<sup>4</sup>&nbsp;[[Ohm|Ω]]<sup>−1</sup>[[Centimetre|cm]]<sup>−1</sup> (larger than liquid [[Chemistry:Mercury (element)|mercury]]). At around 8&nbsp;MPM, a "transition to the metallic state" (TMS) takes place (also called a "metal-to-nonmetal transition" (MNMT)). At 4&nbsp;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 [[Physics:Density|density]] decreases by 30%.

==Other solvents==
Alkali metals also dissolve in some small primary amines, such as [[Chemistry:Methylamine|methylamine]] and ethylamine<ref>{{Greenwood&Earnshaw2nd}}</ref> and [[Chemistry:Hexamethylphosphoramide|hexamethylphosphoramide]], forming blue solutions.  [[Chemistry:Tetrahydrofuran|THF]] dissolves alkali metal, but a [[Chemistry:Birch reduction|Birch reduction]] (see {{Slink||Applications}}) analogue does not&nbsp;proceed without a [[Chemistry:Diamine|diamine]] [[Chemistry:Ligand|ligand]].<ref>{{Cite journal |last1=Burrows |first1=James |last2=Kamo |first2=Shogo |last3=Koide |first3=Kazunori |date=2021-11-05 |title=Scalable Birch reduction with lithium and ethylenediamine in tetrahydrofuran |url=https://doi.org/10.1126/science.abk3099 |journal=Science |volume=374 |issue=6568 |pages=741–746 |doi=10.1126/science.abk3099 |pmid=34735232 |s2cid=243761715 |issn=0036-8075}}</ref> Solvated electron solutions of the alkaline earth metals magnesium, calcium, strontium and barium in ethylenediamine have been used to [[Chemistry:Intercalation|intercalate]] graphite with these metals.<ref>{{cite journal | doi=10.1021/acs.chemmater.8b03421 | title=A New and Facile Route Using Electride Solutions to Intercalate Alkaline Earth Ions into Graphite | year=2018 | last1=Xu | first1=Wei | last2=Lerner | first2=Michael M. | journal=Chemistry of Materials | volume=30 | issue=19 | pages=6930–6935 | s2cid=105295721 }}</ref>

==Water==
Solvated electrons are involved in the reaction of alkali metals with water, even though the solvated electron has only a fleeting existence.<ref>{{cite journal |doi=  10.1139/v66-336|title=Production of hydrated electron |journal=Canadian Journal of Chemistry |volume=44 |issue=  18|pages=2226–  |year=1966 |last=Walker |first=D.C. |doi-access=free }}</ref> Below pH = 9.6 the hydrated electron reacts with the [[Chemistry:Hydronium|hydronium]] ion giving atomic hydrogen, which in turn can react with the hydrated electron giving hydroxide ion and usual molecular hydrogen H<sub>2</sub>.<ref>{{cite journal |doi=10.1021/j100875a026 |title=Some Thermodynamic Properties of the Hydrated Electron |journal=The Journal of Physical Chemistry |volume=70 |issue=3 |pages=770–774 |year=1966 |last1=Jortner |first1=Joshua |last2=Noyes |first2=Richard M. }}</ref>

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 [[Chemistry:Aerosol|aerosol]] formation.<ref>{{cite journal | doi=10.1038/294732a0 | title=Solvated electrons in the upper atmosphere | year=1981 | last1=Arnold | first1=F. | journal=Nature | volume=294 | issue=5843 | pages=732–733 | s2cid=4364255 }}</ref>

Its [[Chemistry:Standard electrode potential|standard electrode potential]] value is -2.77 V.<ref>{{cite journal | jstor=3583572 | title=Effects of Oxygen and pH in the Radiation Chemistry of Aqueous Solutions | last1=Baxendale | first1=J. H. | journal=Radiation Research Supplement | year=1964 | volume=4 | pages=114–138 | doi=10.2307/3583572 }}</ref> The equivalent conductivity of 177 Mho cm<sup>2</sup> is similar to that of [[Chemistry:Hydroxide ion|hydroxide ion]]. This value of equivalent conductivity corresponds to a diffusivity of 4.75 <math>\times 10^{-5}</math> cm<sup>2</sup>s<sup>−1</sup>.<ref>{{cite journal |doi=10.1016/B978-0-12-395706-1.50010-8 |title=The Hydrated Electron |journal=Survey of Progress in Chemistry |volume=5 |pages=129–184 |year=1969 |first=Edwin J. |last=Hart|isbn=9780123957061 |s2cid=94713398 }}</ref>

== Reactivity==
Although quite stable, the blue ammonia solutions containing solvated electrons degrade rapidly in the presence of catalysts to give colorless solutions of [[Chemistry:Sodium amide|sodium amide]]:
:2 [Na(NH<sub>3</sub>)<sub>6</sub>]<sup>+</sup>e<sup>−</sup>  →  H<sub>2</sub>  +  2 NaNH<sub>2</sub>  +  10 NH<sub>3</sub>

Electride salts can be isolated by the addition of macrocyclic [[Chemistry:Ligand|ligand]]s such as [[Chemistry:Crown ether|crown ether]] and [[Chemistry:Cryptand|cryptand]]s to solutions containing solvated electrons.  These ligands strongly bind the cations and prevent their re-reduction by the electron.
:[Na(NH<sub>3</sub>)<sub>6</sub>]<sup>+</sup>e<sup>−</sup>  +  cryptand →  [Na(cryptand)]<sup>+</sup>e<sup>−</sup>+  6 NH<sub>3</sub>

The solvated electron reacts with [[Chemistry:Oxygen|oxygen]] to form a [[Chemistry:Superoxide|superoxide]] [[Biology:Radical|radical]] (O<sub>2</sub><sup>.−</sup>).<ref>{{cite journal |doi=10.1021/acs.chemrev.5b00407 |pmid=26875845 |title=Superoxide Ion: Generation and Chemical Implications |journal=Chemical Reviews |volume=116 |issue=5 |pages=3029–3085 |year=2016 |last1=Hayyan |first1=Maan |last2=Hashim |first2=Mohd Ali |last3=Alnashef |first3=Inas M. |doi-access=free }}</ref> With [[Chemistry:Nitrous oxide|nitrous oxide]], solvated electrons react to form hydroxyl radicals (HO<sup>.</sup>).<ref>{{cite journal |doi=10.1021/j100208a035 |title=Rate constant for scavenging eaq- in nitrous oxide-saturated solutions |journal=The Journal of Physical Chemistry |volume=86 |issue=11 |pages=2078–2084 |year=1982 |last1=Janata |first1=Eberhard |last2=Schuler |first2=Robert H. }}</ref>

==Applications ==
Solvated electrons are involved in electrode processes, a broad area with many technical applications ([[Chemistry:Electrosynthesis|electrosynthesis]], [[Engineering:Electroplating|electroplating]], [[Physics:Electrowinning|electrowinning]]).

A specialized use of sodium-ammonia solutions is the [[Chemistry:Birch reduction|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 [[Chemistry:Bouveault–Blanc reduction|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.<ref>{{cite journal |last1=Cullen |first1=Patrick L. |last2=Cox |first2=Kathleen M. |last3=Bin Subhan |first3=Mohammed K. |last4=Picco |first4=Loren |last5=Payton |first5=Oliver D. |last6=Buckley |first6=David J. |last7=Miller |first7=Thomas S. |last8=Hodge |first8=Stephen A. |last9=Skipper |first9=Neal T. |last10=Tileli |first10=Vasiliki |last11=Howard |first11=Christopher A. |title=Ionic solutions of two-dimensional materials |journal=Nature Chemistry |date=March 2017 |volume=9 |issue=3 |pages=244–249 |doi=10.1038/nchem.2650 |pmid=28221358 |url=https://www.nature.com/articles/nchem.2650 |language=en |issn=1755-4349|hdl=1983/360e652b-ca32-444d-b880-63aeac05f6ac |hdl-access=free }}</ref> 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. <ref>{{cite journal |last1=Angel |first1=Gyen Ming A. |last2=Mansor |first2=Noramalina |last3=Jervis |first3=Rhodri |last4=Rana |first4=Zahra |last5=Gibbs |first5=Chris |last6=Seel |first6=Andrew |last7=Kilpatrick |first7=Alexander F. R. |last8=Shearing |first8=Paul R. |last9=Howard |first9=Christopher A. |last10=Brett |first10=Dan J. L. |last11=Cullen |first11=Patrick L. |title=Realising the electrochemical stability of graphene: scalable synthesis of an ultra-durable platinum catalyst for the oxygen reduction reaction |journal=Nanoscale |date=6 August 2020 |volume=12 |issue=30 |pages=16113–16122 |doi=10.1039/D0NR03326J |pmid=32699875 |language=en |issn=2040-3372|doi-access=free }}</ref>

== History ==
The observation of the color of metal-electride solutions is generally attributed to [[Biography:Humphry Davy|Humphry Davy]]. In 1807–1809, he examined the addition of grains of potassium to gaseous ammonia (liquefaction of ammonia was invented in 1823).<ref>{{cite journal |last1=Thomas |first1=Sir John Meurig |last2=Edwards |first2=Peter |last3=Kuznetsov |first3=Vladimir L. |title=Sir Humphry Davy: Boundless Chemist, Physicist, Poet and Man of Action |journal=ChemPhysChem |date=January 2008 |volume=9 |issue=1 |pages=59–66 |doi=10.1002/cphc.200700686 |pmid=18175370 |quote=An entry from Humphry Davy′s laboratory notebook of November 1808. It reads “When 8 Grains of potassium were heated in ammoniacal gas—it assumed a beautiful metallic appearance & gradually became of a fine blue colour”.}}</ref> James Ballantyne Hannay and J. Hogarth repeated the experiments with sodium in 1879–1880.<ref>{{cite journal |last1=Hannay |first1=J. B. |last2=Hogarth |first2=James |title=On the solubility of solids in gases |journal=Proceedings of the Royal Society of London |date=26 February 1880 |volume=30 |issue=201 |pages=178–188 |url=https://www.biodiversitylibrary.org/item/139575#page/202/mode/1up}}</ref> W. Weyl in 1864 and C.&nbsp;A. Seely in 1871 used liquid ammonia, whereas Hamilton Cady in 1897 related the ionizing properties of ammonia to that of water.<ref>{{cite journal |last1=Weyl |first1=W. |title=Ueber Metallammonium-Verbindungen |journal=Annalen der Physik und Chemie |date=1864 |volume=121 |pages=601–612 |url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015065833884&view=1up&seq=621&skin=2021 |trans-title=On metal-ammonium compounds |language=German}}
* See also: {{cite journal |last1=Weyl |first1=W. |title=Ueber die Bildung des Ammoniums und einiger Ammonium-Metalle |journal=Annalen der Physik und Chemie |date=1864 |volume=123 |pages=350–367 |url=https://babel.hathitrust.org/cgi/pt?id=coo.31924066254446&view=1up&seq=368&skin=2021 |trans-title=On the formation of ammonium and of some ammonium metals |language=German}}</ref><ref>{{cite journal |last1=Seely |first1=Charles A. |title=On ammonium and the solubility of metals without chemical action |journal=The Chemical News |date=14 April 1871 |volume=23 |issue=594 |pages=169–170 |url=https://babel.hathitrust.org/cgi/pt?id=uc1.$c193335&view=1up&seq=177&skin=2021}}</ref><ref>{{cite journal |last1=Cady |first1=Hamilton P. |title=The electrolysis and electrolytic conductivity of certain substances dissolved in liquid ammonia |journal=The Journal of Physical Chemistry |date=1897 |volume=1 |issue=11 |pages=707–713 |doi=10.1021/j150593a001 |url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015026388507&view=1up&seq=737&skin=2021}}</ref>  Charles A. Kraus measured the [[Physics:Electrical conductance|electrical conductance]] of metal ammonia solutions and in 1907 attributed it to the electrons liberated from the metal.<ref>{{cite journal | last1 = Kraus | first1 = Charles A. | year = 1907 | title = Solutions of metals in non-metallic solvents; I. General properties of solutions of metals in liquid ammonia | url =https://zenodo.org/record/1428868 | journal = J. Am. Chem. Soc. | volume = 29 | issue = 11| pages = 1557–1571 | doi = 10.1021/ja01965a003 }}</ref><ref>{{cite journal | last1 = Zurek | first1 = Eva | year = 2009 | title = A molecular perspective on lithium–ammonia solutions | journal = Angew. Chem. Int. Ed. | volume = 48 | issue = 44 | pages = 8198–8232 | doi = 10.1002/anie.200900373 | pmid = 19821473 }}</ref> In 1918, G. E. Gibson and W. L. Argo  introduced the solvated electron concept.<ref>{{cite journal | last1 = Gibson | first1 = G. E. | last2 = Argo | first2 = W. L. | year = 1918 | title = The absorption spectra of the blue solutions of certain alkali and alkaline earth metals in liquid ammonia and methylamine | url = https://zenodo.org/record/1429048| journal = J. Am. Chem. Soc. | volume = 40 | issue = 9| pages = 1327–1361 | doi = 10.1021/ja02242a003 }}</ref> They noted based on [[Physics:Absorption spectrum|absorption spectra]] that different metals and different solvents ([[Chemistry:Methylamine|methylamine]], ethylamine) produce the same blue color, attributed to a common species, the solvated electron.  In the 1970s, solid salts containing [[Physics:Electride|electrons as the anion]] were characterized.<ref>{{cite journal | author = Dye, J. L. | title = Electrons as anions | journal = Science | year = 2003 | volume = 301 | pages = 607–608 | doi = 10.1126/science.1088103 | pmid = 12893933 | issue = 5633| s2cid = 93768664 }}</ref>

== References ==
{{reflist}}

== Further reading ==
{{refbegin|30em}}
* {{cite journal | last1 = Sagar | first1 = D. M. | last2 = Colin | last3 = Bain | first3 = D. | last4 = Verlet | first4 = Jan R. R. | year = 2010 | title = Hydrated Electrons at the Water/Air Interface | journal = J. Am. Chem. Soc. | volume = 132 | issue = 20| pages = 6917–6919 | doi = 10.1021/ja101176r | pmid = 20433171 | s2cid = 207049708 }}
* {{cite journal|last1=Martyna|first1=Glenn|title=Electronic states in metal-ammonia solutions|journal=Physical Review Letters|volume=71|issue=2|pages=267–270|doi=10.1103/physrevlett.71.267 |pmid=10054906|bibcode = 1993PhRvL..71..267D |year=1993}}
* {{cite journal|last1=Martyna|first1=Glenn|title=Quantum simulation studies of singlet and triplet bipolarons in liquid ammonia|journal=Journal of Chemical Physics|volume=98|issue=1|pages=555–563|doi=10.1063/1.464650|bibcode = 1993JChPh..98..555M |year=1993}}
* {{cite book |doi=10.1021/ba-1965-0050 |title=Solvated Electron |volume=50 |series=Advances in Chemistry |year=1965 |isbn=978-0-8412-0051-7 }}
* {{cite book|doi=10.1021/ba-1965-0050.ch006|chapter=Reactions of the Hydrated Electron|title=Solvated Electron|volume=50|pages=55–81|series=Advances in Chemistry|year=1965|last1=Anbar|first1=Michael|isbn=978-0-8412-0051-7}}
* {{cite journal|doi= 10.1039/C1CP21803D |pmid= 22075842 |title= On the nature and signatures of the solvated electron in water |journal= Phys. Chem. Chem. Phys. |volume= 14 |issue= 1 |pages= 22–34 |year= 2012 |last1= Abel |first1= B. |last2= Buck |first2= U. |last3= Sobolewski |first3= A. L. |last4= Domcke |first4= W. |bibcode= 2012PCCP...14...22A }}
* {{cite journal |doi=10.1016/S0022-0728(81)80027-7 |title=Determination of the chemical solvation energy of the solvated electron |journal=Journal of Electroanalytical Chemistry and Interfacial Electrochemistry |volume=129 |issue=1–2 |pages=349–352 |year=1981 |last1=Harima |first1=Y. |last2=Aoyagui |first2=S. }}
* {{cite book |doi=10.1016/B978-0-12-395706-1.50010-8 |chapter=The Hydrated Electron |title=Survey of Progress in Chemistry Volume 5 |volume=5 |pages=129–184 |series=Survey of Progress in Chemistry |year=1969 |last1=Hart |first1=Edwin J. |isbn=9780123957061 |s2cid=94713398 }}
* [https://pure.tue.nl/ws/files/2089318/388448.pdf The electrochemistry of the solvated electron]. Technische Universiteit Eindhoven.
* [http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/07/254/7254831.pdf IAEA On the Electrolytic Generation of Hydrated Electron].
* Fundamentals of Radiation Chemistry, chapter 6, [https://books.google.com/books?id=TudUOdz8SIwC&dq=sodium+reaction+with+water+Walker+1966&pg=PA148 p. 145–198], Academic Press, 1999.
* [https://doi.org/10.1016/0020-708X(65)90176-6 Tables of bimolecular rate constants of hydrated electrons, hydrogen atoms and hydroxyl radicals with inorganic and organic compounds], International Journal of [[Physics:Applied Radiation and Isotopes|Applied Radiation and Isotopes]] Anbar, Neta
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{{Sourceattribution|Solvated electron}}