Solvated electron

Source: Wikipedia, the free encyclopedia.

A solvated electron is a

outer-sphere electron transfer. Solvated electrons are frequent objects of study in radiation chemistry. Salts containing solvated electrons are known as electrides
.

Ammonia solutions

Liquid

liquid ammonia, the solution is blue when dilute and copper-colored when more concentrated (> 3 molar).[5] 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.[6]

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

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 Ω−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[7] and hexamethylphosphoramide, forming blue solutions. Tetrahydrofuran (THF) dissolves alkali metal, but a Birch reduction (see § Applications) analogue does not proceed without a diamine ligand.[8] Solvated electron solutions of the alkaline earth metals magnesium, calcium, strontium and barium in ethylenediamine have been used to intercalate graphite with these metals.[9]

Water

Solvated electrons are involved in the reaction of alkali metals with water, even though the solvated electron has only a fleeting existence.[10] 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.[11]

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.[12]

Its

hydroxide ion
. This value of equivalent conductivity corresponds to a diffusivity of 4.75 cm2s−1.[14]

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.−).[15] With nitrous oxide, solvated electrons react to form nitroxyl radicals (NO.).[16]

Uses

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.[17] 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. [18]

History

The observation of the color of metal-electride solutions is generally attributed to

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.[27]

References

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  8. S2CID 243761715
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  10. doi:10.1139/v66-336
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  11. doi:10.1021/j100875a026
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  13. JSTOR 3583572
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  15. PMID 26875845
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  16. doi:10.1021/j100208a035
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  17. PMID 28221358
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  18. PMID 32699875
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  19. PMID 18175370
    . 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".
  20. ^ 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.
  21. doi:10.1002/andp.18641970407
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  22. ^ Seely, Charles A. (14 April 1871). "On ammonium and the solubility of metals without chemical action". The Chemical News. 23 (594): 169–170.
  23. doi:10.1021/j150593a001
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  24. doi:10.1021/ja01965a003
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  25. PMID 19821473
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  26. doi:10.1021/ja02242a003
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  27. S2CID 93768664
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Further reading

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