Radical anion
{{Short description|Free radical species}}
File:Sodium naphthalenide.svg, a salt containing the radical anion of naphthalene as the anion]]
In organic chemistry, a radical anion is a free radical species{{GoldBookRef|title=radical ion|file= R05073}} 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 .
Polycyclic radical anions
Many aromatic compounds can undergo one-electron reduction by alkali metals. The electron is transferred from the alkali metal ion to an unoccupied antibonding p-p п* orbital of the aromatic molecule. This transfer is usually only energetically favorable if the aprotic solvent efficiently solvates the alkali metal ion. Effective solvents are those that bind to the alkali metal cation: diethyl ether < THF < 1,2-dimethoxyethane < HMPA. In principle any unsaturated molecule can form a radical anion, but the antibonding orbitals are only energetically accessible in more extensive conjugated systems. Ease of formation is in the order benzene < naphthalene < anthracene < pyrene, etc. Salts of the radical anions are often not isolated as solids but used in situ. They are usually deeply colored.
- Naphthalene in the form of
- Lithium naphthalene is obtained from the reaction of naphthalene with lithium.
- Sodium naphthalene is obtained from the reaction of naphthalene with sodium.
- Sodium 1-methylnaphthalene and 1-methylnaphthalene are more soluble than sodium naphthalene and naphthalene, respectively.{{cite journal|last1=Liu |first1=X. |last2=Ellis |first2=J. E. |title=Hexacarbonylvanadate(1−) and Hexacarbonylvanadium(0) |journal=Inorg. Synth.|date=2004 |volume=34 |pages=96–103 |doi=10.1002/0471653683.ch3| isbn=0-471-64750-0}}
- biphenyl as its lithium salt.{{cite journal|title=Highly Reactive Calcium for the Preparation of Organocalcium Reagents: 1-Adamantyl Calcium Halides and Their Addition to Ketones: 1-(1-Adamantyl)cyclohexanol
|first1=Reuben D. |last1=Rieke |first2=Tse-Chong |last2=Wu |first3=Loretta I. |last3=Rieke |name-list-style=amp |journal=Org. Synth.|year=1995|volume=72|page=147|doi=10.15227/orgsyn.072.0147}}
- acenaphthylene is a milder reductant than the naphthalene anion.
- anthracene in the form of its alkali metal salts.
- pyrene as its sodium salt.{{cite journal |doi=10.1107/S2053229614015290|title=Bis(pyrene)metal complexes of vanadium, niobium and titanium: Isolable homoleptic pyrene complexes of transition metals|year=2014|last1=Kucera|first1=Benjamin E.|last2=Jilek|first2=Robert E.|last3=Brennessel|first3=William W.|last4=Ellis|first4=John E.|journal=Acta Crystallographica Section C: Structural Chemistry|volume=70|issue=8|pages=749–753|pmid=25093352}}
- Perylene in the form of its alkali metal (M = Li, Na, Cs) etherates.{{cite journal |doi=10.1021/om970610g|title=Solvent-Shared and Solvent-Separated Ion Multiples of Perylene Radical Anions and Dianions: An Exemplary Case of Alkali Metal Cation Solvation|year=1998|last1=Näther|first1=Christian|last2=Bock|first2=Hans|last3=Havlas|first3=Zdenek|last4=Hauck|first4=Tim|journal=Organometallics|volume=17|issue=21|pages=4707–4715}}
Other examples
Cyclooctatetraene is reduced by elemental potassium to the dianion. The resulting dianion is a 10-pi electron system, which conforms to the Huckel rule for aromaticity. Quinone is reduced to a semiquinone radical anion. Semidiones are derived from the reduction of dicarbonyl compounds.
Reactions
=Redox=
The pi-radical anions are used as reducing agents in specialized syntheses. Being soluble in at least some solvents, these salts act faster than the alkali metals themselves. The disadvantages are that the polycyclic hydrocarbon must be removed. The reduction potential of alkali metal naphthalene salts is about 3.1 V (vs Fc+/0). The reduction potentials of the larger systems are lower, for example acenaphthalene is 2.45 V.{{cite journal|doi=10.1021/cr940053x|title=Chemical Redox Agents for Organometallic Chemistry|year=1996|last1=Connelly|first1=Neil G.|last2=Geiger|first2=William E.|journal=Chemical Reviews|volume=96|issue=2|pages=877–910|pmid=11848774}} Many radical anions are susceptible to further reduction to dianions.
| class=wikitable style="float:left; text-align:center
|+ reduction potentials for various M(18-crown-6)+hydrocarbon−{{cite journal |doi=10.1039/C5NJ02841H|title=Isolation of Gravimetrically Quantifiable Alkali Metal Arenides Using 18-Crown-6|year=2016|last1=Castillo|first1=Maximiliano|last2=Metta-Magaña|first2=Alejandro J.|last3=Fortier|first3=Skye|journal=New Journal of Chemistry|volume=40|issue=3|pages=1923–1926}} !hydrocarbon!!M+!!E1/2!!comments |
| naphthalene
| Li+ | -3.09 V | can be reduced to dianion |
| naphthalene
| Na+ | -3.09 V | |
| biphenyl
| Li+ | -3.18 V | |
| anthracene
| Na+ | -2.53 V | |
| perylene
| Na+ | -2.19 V | includes dme solvate |
{{clear}}
=Protonation=
Addition of a proton source (even water) to a radical anion results in protonation, i.e. the sequence of reduction followed by protonation is equivalent to hydrogenation. For instance, the anthracene radical anion forms mainly (but not exclusively) 9,10-dihydroanthracene. Radical anions and their protonation are central to the Birch reduction.
=Coordination to metal ions=
Radical anions of polycyclic aromatic compounds function as ligands in organometallic chemistry.{{cite journal |doi=10.1039/C8DT05029E|title=The Chatt Reaction: Conventional Routes to homoleptic Arenemetalates of d-Block Elements|year=2019|last1=Ellis|first1=John E.|journal=Dalton Transactions|volume=48|issue=26|pages=9538–9563|pmid=30724934|s2cid=73436073 }}