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== Ammonia solutions ==
== Ammonia solutions ==
Lquid ammonia will dissolve all of the [[alkali metal]]s and other [[electronegativity|electropositive]] metals such as [[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|chapter=Calcium-Ammonia|isbn=978-0471936237}}</ref> [[strontium|Sr]], [[barium|Ba]], [[europium|Eu]], and [[ytterbium|Yb]] (also [[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 [[Molar concentration|molar]]).<ref name="c&w">{{cite book |last=Cotton |first=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 [[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 [[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>
Lquid ammonia will dissolve all of the [[alkali metal]]s and other [[electronegativity|electropositive]] metals such as [[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|chapter=Calcium-Ammonia|isbn=978-0471936237}}</ref> [[strontium|Sr]], [[barium|Ba]], [[europium|Eu]], and [[ytterbium|Yb]] (also [[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 [[Molar concentration|molar]]).<ref name="c&w">{{cite book |last=Cotton |first=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 [[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 [[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 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.

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

Revision as of 16:46, 9 December 2021

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.

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).[2] James Ballantyne Hannay and J. Hogarth repeated the experiments with sodium in 1879–1880.[3] 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.[4][5][6] Charles A. Kraus measured the electrical conductance of metal ammonia solutions and in 1907 attributed it to the electrons liberated from the metal.[7][8] In 1918, G. E. Gibson and W. L. Argo introduced the solvated electron concept.[9] 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.[10]

Ammonia solutions

Lquid ammonia will dissolve all of the alkali metals and other electropositive metals such as Ca,[11] Sr, Ba, Eu, and Yb (also Mg using an electrolytic process[12]), giving characteristic blue solutions. For alkali metals in liquid ammonia, the solution is blue when dilute and copper-colored when more concentrated (> 3 molar).[13] 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.[14]

Solvated electrons produced by dissolution of reducing metals in ammonia and amines are the anions of salts called electrides. Such salts can be isolated by the addition of macrocyclic ligands such as crown ether and cryptands. These ligands bind strongly the cations and prevent their re-reduction by the electron.

Case study: Li in NH3

Photos of two solutions in round-bottom flasks surrounded by dry ice; one solution is dark blue, the other golden.
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 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-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[15] and hexamethylphosphoramide, forming blue solutions. THF solutions of diamines are effective.[16] Solvated electron solutions of the alkaline earth metals magnesium, calcium, strontium and barium in ethylenediamine have been used to intercalate graphite with these metals.[17]

Water

Solvated electrons are involved in the reaction of alkali metals with water, even thought the solvated electron has only a fleeting existence.[18] 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.[19]

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.[20]

Its standard electrode potential value is -2.77 V.[21] Equivalent conductivity 177 Mho cm2 is similar to that of hydroxide ion. This value of equivalent conductivity corresponds to a diffusivity of 4,75*10−5 cm2s−1.[22]

Reactivity and applications

Solvated electrons are also involved in electrode processes.

The solvated electron reacts with oxygen to form a superoxide radical (O2.−).[23] With nitrous oxide, solvated electrons react to form hydroxyl radicals (HO.).[24] The solvated electrons can be scavenged from both aqueous and organic systems with nitrobenzene or sulfur hexafluoride[citation needed].

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.

References

  1. ^ Schindewolf, U. (1968). "Formation and Properties of Solvated Electrons". Angewandte Chemie International Edition in English. 7 (3): 190–203. doi:10.1002/anie.196801901.
  2. ^ Thomas, Sir John Meurig; Edwards, Peter; Kuznetsov, Vladimir L. (January 2008). "Sir Humphry Davy: Boundless Chemist, Physicist, Poet and Man of Action". ChemPhysChem. 9 (1): 59–66. 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".
  3. ^ Hannay, J. B.; Hogarth, James (26 February 1880). "On the solubility of solids in gases". Proceedings of the Royal Society of London. 30 (201): 178–188.
  4. ^ Weyl, W. (1864). "Ueber Metallammonium-Verbindungen" [On metal-ammonium compounds]. Annalen der Physik und Chemie (in German). 121: 601–612.
  5. ^ Seely, Charles A. (14 April 1871). "On ammonium and the solubility of metals without chemical action". The Chemical News. 23 (594): 169–170.
  6. ^ Cady, Hamilton P. (1897). "The electrolysis and electrolytic conductivity of certain substances dissolved in liquid ammonia". The Journal of Physical Chemistry. 1: 707–713.
  7. ^ Kraus, Charles A. (1907). "Solutions of metals in non-metallic solvents; I. General properties of solutions of metals in liquid ammonia". J. Am. Chem. Soc. 29 (11): 1557–1571. doi:10.1021/ja01965a003.
  8. ^ Zurek, Eva (2009). "A molecular perspective on lithium–ammonia solutions". Angew. Chem. Int. Ed. 48 (44): 8198–8232. doi:10.1002/anie.200900373. PMID 19821473.
  9. ^ Gibson, G. E.; Argo, W. L. (1918). "The absorption spectra of the blue solutions of certain alkali and alkaline earth metals in liquid ammonia and methylamine". J. Am. Chem. Soc. 40 (9): 1327–1361. doi:10.1021/ja02242a003.
  10. ^ Dye, J. L. (2003). "Electrons as anions". Science. 301 (5633): 607–608. doi:10.1126/science.1088103. PMID 12893933.
  11. ^ Edwin M. Kaiser (2001). "Calcium-Ammonia". Calcium–Ammonia. Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rc003. ISBN 978-0471936237.
  12. ^ Combellas, C; Kanoufi, F; Thiébault, A (2001). "Solutions of solvated electrons in liquid ammonia". Journal of Electroanalytical Chemistry. 499: 144–151. doi:10.1016/S0022-0728(00)00504-0.
  13. ^ Cotton, F. A.; Wilkinson, G. (1972). Advanced Inorganic Chemistry. John Wiley and Sons Inc. ISBN 978-0-471-17560-5.
  14. ^ Harima, Yutaka; Aoyagui, Shigeru (1980). "The diffusion coefficient of solvated electrons in liquid ammonia". Journal of Electroanalytical Chemistry and Interfacial Electrochemistry. 109 (1–3): 167–177. doi:10.1016/S0022-0728(80)80115-X.
  15. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  16. ^ . doi:10.1126/science.abk3099. {{cite journal}}: Cite journal requires |journal= (help); Missing or empty |title= (help)
  17. ^ W. Xu and M. M. Lerner, "A New and Facile Route Using Electride Solutions To Intercalate Alkaline Earth Ions into Graphite", Chem. Mater. 2018, 30, 19, 6930–6935. https://doi.org/10.1021/acs.chemmater.8b03421
  18. ^ Walker, D.C. (1966). "Production of hydrated electron". Canadian Journal of Chemistry. 44 (18): 2226–. doi:10.1139/v66-336.
  19. ^ Jortner, Joshua; Noyes, Richard M. (1966). "Some Thermodynamic Properties of the Hydrated Electron". The Journal of Physical Chemistry. 70 (3): 770–774. doi:10.1021/j100875a026.
  20. ^ F. Arnold, Nature 294, 732-733, (1981)
  21. ^ Baxendale, J. H. (1964), Radiation Res. Suppl., 114 and 139
  22. ^ Hart, Edwin J. (1969). "The Hydrated Electron". Survey of Progress in Chemistry. 5: 129–184. doi:10.1016/B978-0-12-395706-1.50010-8. ISBN 9780123957061.
  23. ^ Hayyan, Maan; Hashim, Mohd Ali; Alnashef, Inas M. (2016). "Superoxide Ion: Generation and Chemical Implications". Chemical Reviews. 116 (5): 3029–3085. doi:10.1021/acs.chemrev.5b00407. PMID 26875845.
  24. ^ 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.

Further reading

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