Radical anion

In

molecule. Radical anions are typically indicated by ${\displaystyle M^{\bullet -}}$.

Polycyclic radical anions

Many

• 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.^[2]
• biphenyl as its lithium salt.^[3]
• acenaphthylene is a milder reductant than the naphthalene anion.
• anthracene in the form of its alkali metal salts.^[4]
• pyrene as its sodium salt.^[5]
• Perylene in the form of its alkali metal (M = Li, Na, Cs) etherates.^[6]

Other examples

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.[7] Many radical anions are susceptible to further reduction to dianions.

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.^[8]

Radical cations

Cationic radical species are much less common than the anions. Denoted ${\displaystyle M^{+\bullet }}$, they appear prominently in mass spectrometry.

Some compounds containing the dioxygenyl cation can be prepared in bulk.^[10]

Organic conductors

Radical cations figure prominently in the chemistry and properties of

n C₄H₄NH + 2 FeCl₃ → (C₄H₂NH)_n + 2 FeCl₂ + 2 HCl

Once formed, these polymers become conductive upon oxidation.^[11] Polarons and bipolarons are radical cations encountered in doped conducting polymers.

References