Solvated electron
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A solvated electron is an free electron held in (solvated in) a solution. It can occur in both water and liquid ammonia and turns the solute blue. While it is stable in anhydrous liquid ammonia for days it is very short lived when in water. A solution of metal ions and solvated electrons in ammonia can be made by dissolving an alkali metal such as sodium in liquid ammonia.
Birch reduction
The solvated electron solution formed by adding sodium to liquid ammonia is used with a proton source such as ethanol in the Birch reduction.
The Birch reduction is the organic reduction of aromatic rings by sodium in liquid ammonia invented by Arthur Birch. The reaction product is a 1,4-cyclohexadiene. The metal can also be lithium or potassium and the hydrogen atoms are supplied by an alcohol such as ethanol or tert-butanol. Sodium in liquid ammonia gives an intense blue color.
The first step of a Birch reduction is a one-electron reduction of the aromatic ring to a radical anion. Sodium is oxidized to the sodium ion Na+. This intermediate is able to dimerize to the dianion. In the presence of an alcohol the second intermediate is a free radical which takes up another electron to form the carbanion. This carbanion abstracts another proton from the alcohol to form the cyclohexadiene.
In the presence of an alkyl halide the carbanion can also engage in nucleophilic substitution with carbon-carbon bond formation. In substituted aromatics an electron withdrawing substituent such as a carboxylic acid stabilizes a carbanion and the least-substituted olefin is generated. With an electron donating substituent the opposite effect is obtained.The non-conjugated 1,4-addition product is preferred over the conjugated 1,3-diene which is explained in terms of the principle of least motion. Experimental alkali metal alternatives that are safer to handle, such as the M-SG reducing agent, also exist.
External links
- reduction of o-anisic acid to 2-heptyl-2-hexenone in Organic Syntheses [Article]
- reduction of naphthalene to 1,4,5,8-Tetrahydronaphthalene (isotetralin) in Organic Syntheses [Article].
- reduction of o-xylene to 1,2-Dimethyl-1,4-cyclohexadiene in Organic Syntheses [Article]
- reduction of benzoic acid to 2,5-Cyclohexadiene-1-carboxylic acid in Organic Syntheses [Article]
References
- A. J. Birch, J. Chem. Soc. 1944, 430.
Radiation chemistry
The solvated electron is a reactive species which is responsible for a great deal of radiation chemistry. One possible industrial application of radiation generated solvated electrons is the treatment of PCBs, the polychlorobiphenyls are converted to biphenyl and inorganic chloride.
The solvated electron can react with dissolved oxygen to form a type of oxygen radical (O2.-), which is a potent oxidant, also the solvated electron can react with nitrous oxide to form hydroxyl radicals (HO.). The solvated electrons can be scavenged from both aqueous and organic systems by adding nitrobenzene or sulfur hexafluoride.
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