Larry 1. ~ i l l e r ' Colorado State University
Fort Collins, Colorado 80521
Organic Ion Radical Chemistry
The reaetive intermediate concept ha.s proven invaluable in correlating and understanding organic reactions. The scientific utility of carbonium ions, carbenes, etc., can be clearly seen in the extensive literature of physical organic chemistry. This concept also has great pedagogical utility since i t unifies large numbers of apparently unrelated reactions and reveals the dynamics of chemical conversions. This paper will briefly introduce ion radicals as a relatively new class of reactive intermediates. Ion radicals are those species formed by one electron transfer to or from neutral molecules. A: - c - e A = e - + A + . Anion Radical
Cation Radical
They are involved in a variety of processes including electrochemical and photochemical reactions as well as purely chemical t,ransformations. It is, therefore, of some importance to understand the struct,ure and chemistry of these species. This paper will introduce general concepts and give examples of characteristic solution phase ion radical reactions. A large amount of data has been acquired on vapor phase cation radical (molecular ion) fragmentations t,hrough the use of mass spectromet,ry. Treatment of t,hese fragmentations is beyond the scope of this article. We wish to emphasize, however, that correlations of maadspectral and solut,ion phase chemistry may be possible when the latter involves intermediate cation radicals. A simple and successful application of this idea is the correlat,ion of electrochemical oxidat,ion potentials and ionization potentials of aromatic hydrocarbons ( 1 , 2 ) . A correlation results because the most important factor in determining both pot,entials is the stability of the cation radical.
The chemical reactions of this species are also consistent with this formulation and will be discussed. Electrochemical evidence is of some importance, but, can be summarized by saying that reduction gives a stable species which can be reconverted to naphthalene by oxidation. A fully reversible one-electron t,ransfer is all that. tales place (4). Very similar evidence has been obtained in a few cases for stable cation radicals. The term ion radical arises because these species are charged and have an unpaired electron. I t is to be emphasized t,hat t,hey are dist,inct from the widely studied cation, anion, or radical species which are the more usual organic intermediates. These, in fact, form a separate redox series. This may be illustrated by comparing the phenyl radical, anion and cation with benzene and the benzene cation radical and anion radical.
Examination of the anion, radical, cation trio shows that they differ in the number of electrons held in a formally nonbonding orbital. The ion radical sequence, however, involves addition or deletion of electrons from 7 bonding or anti-bonding orbitals. Illustrated below are the electronic configurationsinvolved. Anion Radical
-
Neutral Molecule
-
Cation Radical
-
Structure of Ion Radicals
Let us first review the evidence for format,ion of stable ion radicals. Naphthalene when treated with .one epuivalent of sodium metal in tetrahydrofuran forms a green homogeneous solution. The solution conducts electricity and gives an electron spin resonance (esr) spectrum. This indicates that the species formed are charged radicals. The number of radicals formed can be estimated from the intensity of t,he esr signal and shows that the reaction is a one-electron reduction. The esr signal shows hyperfine splitting due to interact,ion of the unpaired electron spin with the nuclear spins of four a hydrogens and four 6 hydrogens on the naphthalene (3). This confirms the idea that an anion radical, sodium salt is formed and the odd electron is spread about the naphthalene molecule. Visiting Professor, University of Southampton, England.
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The benzene anion radical is formed by adding an electron to the lowest unfilled molecular orbital of benzene. That orbital is one of the pair of degenerate a antibonding orbitals. Similarly the cation radical is formed by removal of m electron from the highest filled n molec'ular orbital. Describing the benzene anion radical with the resonance convention is less satisfactory. The negative charge and odd electron can be placed a t any of the six carbon atoms.
-
These structures, however, falsely suggest that the negative charge and odd electron are somehow separable. This can lead to misunderstanding of the structure and reactions of ion radicals. As an example authors have wrjtten structures which seem to indicate that in anion radicals the aromatic hydrogens are moved out of the plane of the ring. This structure is not correct. I n the benzene radical anion all of the hydrogens are equivalent and lie in the plane of the benzene ring.
Illolecular orbital theory not only provides an accurate picture of ion radical st,ructure, but also allows accurate predictions of some ion radical chemistry. Since electron t,ransfer reduction involves addit,ion of one elect,ronto the lowest.vacant molecular orbital, the ease of this react,ion is largely determined by the energy of that vacant orbital (5). The energy of interest is readily extracted from molecular orbital calculations. Quantitative measures of the ease of reduction are electron affinit,y (vapor phase) or E,/, (solut,ion phase). I t is found that for polycyclic aromatic hydrocarbons there is a direct relationship between t,he orbital energy calculated by molecular orbital techniques and both the electron affinity and E l / , (6). There is also an extremely valuable relationship between molecular orbital theory and esr data. The hyperfine splitting from any proton is directly related to the electron spin density at that nucleus. The magnitude of the hyperfine splitting from the various protons, therefore, provides a map of the spin density (7). Such data can provide a test for theoretically derived molecular wave funct,ions or t,heoretically derived unpaired electron spin densities can be compared wit,h experimentally determined values in order to elucidate the structure of ion radicals. Several recent reviews deal with this subject and, indeed, a monograph ent,itled "Radical Ions" is primarily devoted to discussions of recent advances in the application of esr to ion radical problems (8).
Numerous examples exist of polycyclic aromatics which are reduced to the radical anion or dianion (10). Quenching with water, ammonium chloride, or alcohol will then give dihydro-aromatic. Benzene and many substituted benzenes generally do not reduce easily enough to achieve a significant amount of anion radical. This has been overcome by Birch who used sodiumammonia and alcohol as a hydrogenating medium (11). The sodium-ammonia produces solvated electrons ~vhich in turn produces anion radicals. Although the anion radicals do not build up in solution, the alcohol gives rapid protonation and this drives the reduction to completion.
Anion Radical Chemistry
In this reaction it is found that carbons which carry the most electron density are most easily protonated. More specifically, the electron density distribution (from esr or MO calculation) of the highest occupied orbital (T antibonding) of the radical anion correctly predicts the position of predominant protonation (12). Anthracene reduction, for example, produces mainly 9,10-dihydroanthracene not 1,4-dihydroanthracene.
We have already discussed evidence for the formation of stable anion radicals. Some of these species are stable if formed in polar-aprotic solvents like tet,rahydrofuran (THF). A wide variety of reducing methods have been used, but the most common are alkali metal and electrolytic reduct,ione. The complete absence of wat,erand of oxygen is necessary since t,hesecommon impurities will rapidly dest,roymost anion radicals. One of the most charact,eristic reactions of anion radicals is indeed protonation, i.e., they act as bases. An example is the quenching of pre-formed naphthalene radical anion with water (9). This leads to a 41% yield of dihydronaphthalenes, 58y0 yield of naphthalene, and 1Y0 yield of tetralin. The first step of this reaction is protonation and the following mechanism is one consistent wit,h t,he results of an isotopic labeling experiment. Closson and Bank added tritiated water to a solution of t,he anion radical in tetrahydrofuran. The t,rit,iumwas incorporated only in the dihydronaphthalenes and tetralin.
a+* \
H H
Hydrogenation has also been accomplished with a wide variety of conjugated dienes and carbonyl compounds (IS), e.g.
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I t may be noted that radical anion mechanisms explain why the unsaturated products do not undergo further reduction. This seems to be a result of the more neg* tive reduction potential of the products (less extensive T system) than the reactants uhich kinetically inhibits over-reduction. Although ammonia solutions are widely used, a number of improvements in the utility of these reductions have been achieved (14). In particular, the use of metals in an aliphatic amine solvent leads to a very powerful reducing system (15). A second characteristic reaction of anion radicals is simple electron transfer. An example is reaction (2) in uhich an anion radical donates an electron to a free radical. A "reaction" of considerable theoretical interest is the transfer of an electron from an anion radical to the corresponding neutral molecule. Such no reaction-reactions can be revealed by esr studies Nnph-
+ Naph
-
+
N ~ p h Naph-
The reduction of organic halides is a very general reaction which formally involves radical anions. These radical anions, however, dissociate immediately and the process is in fact termed dissociative electron transfer. It has been performed in the gas phase using alkali metals, in the gas and solution phase using ionizing radiation, and in solution using alkali metals, electrodes and aromatic radical anions as electron sources. I n solution the process usually leads to hydrocarbons.
The Wurtz and Wurt5Fittig reactions, so special to sophomore organic students, must involve this cleavage. Elect,rochemical reduction has generally been carried out a t a mercury electrode and ordinarily gives high yields of hydrocarbon (RH) (23). The reactions (eqns. (8), (9), and (10)) may, however, be complicated in some cases by organomercurial formation (24). The mechanisms of reduction of akyl halides by the naphthalene radical anion has recently received much attention. The following scheme is indicated (25).
since rapidly transferring the electron to various naphthalene molecules washes out the hyperfine splitting (16). I t is found that the rates of these reactions are of the order 108 1 mole-' sec-I, i.e., close to diffusion controlled (17). A related reaction is the electronic disproportionation of two anion radicals. 2 Naph7 =t N a p h
+ NaphS-
The equilibria for such reactions ordinarily lie to t,he left because of the extra electron repulsions in the diamagnetic dianion (18). It. is interesting to note, however, that a small equilibrium concentration of highly reactive diions could be responsible for many of t,he "ion radical reactions" reviewed here. Tetraphenylethylene presents an anomalous and, therefore, intriguing example of a radical anion-dianion equilibrium. Preferential dianion formation was discovered chemically and confirmed by esr studies (19). I t apparently results from two effects (20) : the non-planarity of t,he dianion and ion pairing between the dianion and alkali metal cations. Rotation about the ethylenic bond is a low activation energy process in radical ions and diions, and the dianion in question can minimize electronic repulsion and steric interactions between the phenyls by a 90' rotation from planarity. This produces a pair of benz'ylhydryl anions
Ion pairing seems to stabilize dianions much more than radical anions (21) and will be specially effective with this geometry. Most interestingly, the equilibrium shifts back to~vardradical anions in hexamethylphosphoramide solvent. In this very polar solvent free ions are favored over the ion pairs found in etherial solvents (22). 170 / Journal of Chemical Education
Radical anions are also involved in several chemical reactions which do not formally involve reduction. One of the most intriguing is the nucleophilic substitution of p-nit,robenzyl halides. The anomolous nature of these reactions compared with other benzyl halide substitutions was first discovered by Kornblum (26) and coworlcers who found that p-nitrobenzyl chloride reacted unexpectedly fast and unexpectedly gave carbon allcylation.
NO.
CH,Cl
I
I
CH2-C-CH,
I
I
They suggested radical anion intermediates in the anomolous reaction and this was confirmed in studies by Russell (27) and by Kornhlum (28). The following chain reaction involving coupling of radicals and anions to form radical anions (eqn. (9)) is suggested.
Cohalt(II1) (36) and manganese(II1) (37) have both been implicated as good o n e d x t r o n oxidants and with certain organic substrates can produce cation radicals. Cohaltic acetate in glacial acetic acid will produce a variety of stable cation radicals, e.g., triphenylamine and 1,4dimethoxyhenzene monocations. Reaction of this reagent with substituted toluenes also involves cation radicals as intermediates. The final products are benzylic acetates. Co"11'
+ CaHICH*= Co(I1) + CaHsCHlt.
--
(11)
C6HaCHl' + Elt + CbHaCHz. Cou11 + CsHsCHat CSH5CHlf + OAcC8HsCH20Ac CaHsCHlt'
Co""'
(12) (13) (14)
Reactions (a), (9), and (10) constitute the propagation steps which can he diverted by oxygen or by very good The reader will note that there is a mechanism common electron acceptors like dinitrobeuzene. to many of the redox reactions mentioned. It involves A final important reaction of radical anions is dielectron transfer-chemical reaction-electron transfer. merization. The dimerization of styrene radical anElectrochemists have tagged this an ECE mechanism ions and 1,l-diphenylethylene radical anions has been (38). One reason that it must often occur is that an carefully studied by Szwarc and coworkers (89). These oxidant or reductant powerful enough to transfer the reactions are quite facile and are of importance in underfirst electron will transfer the second even more rapidly. standing the mechanism of alkali metal catalyzed polyThis is predicted from molecular orbital theory. As merization. applied to oxidations the first reaction removes an elec~(c,IL),C=CH,~ (C~I~,)~C-CH~CH~-C-(COH.)Z tron from a bonding orbital, e.g., reactions (11) and (15), the second from a formally nonhonding orbital, Baizer and coworkers have studied a related electroe.g., reactions (13) and (17). chemical hydrodimerization of activated alkeues, e.g., The oxidation most thoroughly studied to date is @unsaturated nitriles, ketones and esters (SO). This chlorine dioxide oxidation of aliphatic amines for which process is used for the industrial preparation of adian ECE type mechanism usually holds (39). ponitrile.
-
+ 2e- + 2H+
2CHs=CH-C=N
-
NC-CH2CH1CH2CH2-CN
Hydrodimerization is also useful for the synthesis of certain cyclic compounds (31).
+ + H1O
RCH=NR:
The most familiar hydrodimerization is pinacol forma, tion from ketones (38). 0
OH OH
All of these reductive processes involve radical anion intermediates under most conditions. I n each case, however, there is a paucity of mechanistic information. Cation Radical Chemistry
The central role of cation radicals in mass spectrometry has already been mentioned. It is interesting to note that the distinction between cations and cation radicals has only recently been appreciated by many mass spectrometrists. It is, however, becoming clear that in the gas phase as well as in solution the reactions of cations and cation radicals are often not comparable. In solution stable cation radicals have been produced in a variety of media including concentrated sulfuric acid (SS), antimony pentachloride-methylene chloride (34) and aluminum chloride in nitromethane (36). Direct oxidations have utilized electrochemistry (usually in acetonitrile), metal ions, and other chemical oxidants. There are also a few reports of cation radical intermediates in non-electron transfer reactions.
-
RCHO
+ HNR:
In water the stoichiometry is as indicated. To briefly summarize the mechanistic information: (1) the rate is first order in amine and inverse first order in chlorite, (2) kA/kD for kl is 1.3 - 1.8, (3) the oxidation rates tertiary > secondary > primary are consistent with ionization potential and there is a direct correlation between CIOz oxidation rate and electro-oxidation potential. This last point is most interesting as it shows the correlation between electrochemistry and homogeneous chemistry. The above mechanism may also he applicable to permanganate (40) and other amine oxidations. In each case it appears that the most characteristic aliphatic amine cation radical reaction in nonacidic solutions is loss of an a-hydrogen from the cation radical. This process also has been implicated in the iron catalyzed dealkylation of tertiary amine oxides (41).
Electro-oxidation has been the most general t,echnique for the generation of both stable and unstable cation radicals. Stable cation radicals can be produced if (1) a non-nucleophilic, hut polar solvent is used (2) the orbital from which an electron is lost is of high energy, (3) the "reactive positions" are substituted. An example is diphenyl anthracene (48). In acetonitrile, methylene chloride, nitrobenzene, henzonitrile, or sulVolume 48, Number 3, March 1971
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fur dioxide, a stable cation radical is formed by electrooxidat,iou at a platinum electrode. This can be demonstrated by a cyclic voltammetric experiment. I n this experiment (38) the voltage is cycled first anodically, then cathodically a t a constant rate. As diphenylantbracene is oxidized an anodic current is passed and cation radical is formed. When the electrode potential is then cycled cathodically the cation radical which was formed is reduced giving rise to a cathodic current. The shape of the current-potential curves and the integrated amount of current passed indicate that a stable one electron oxidation product is formed and this is confirmed by esr. Cation radicals are discreet intermediates in a number of organic electrode processes. The oxidation of aromatic amines is a simple example. It bas been shown that triarylamines oxidize to the corresponding cation radicals which then dimerize (45).
Electro-oxidation of other. aromatic amines is not so simple. These reactions have received study, but mechanistic complications including adsorption and short cation lifetimes make interpretation more difficult (38,44). The oxidation of dimethylaniline in methanol is an example.
A number of oxidations of benzenoid aromatics have been performed (46). 'These reactions may all involve discreet cat,ion radicals, but all that has been proven is that electron transfer from the aromatic is the initial reaction (46).
Thianthrene cation radical is relatively stable and has been investigated by Shine and coworkers (47). In concentrated sulfuric acid either thianthrene or thianthrene oxide will produce the cation radical.
Thianthrene cation radical salts can actually be isolated and mechanistic studies of their reactivity have proven very interesting. It has been shown that the thianthrene cation radical reacts with water to disproportionate (48). The corresponding oxygen compound, contrastingly, oxidizes water (49). That ion radicals might be photolytically produced was first postulated by Lewis (50). Photolysis of triphenylamine in the glassy state gave electronic spectra interpreted as arising from cation radical formation. This proposal has been followed by the careful mechanistic studies of Albrecht and by extension t o radiolysis by Hammill. Photolytic ionization of tetramethylphenylenediamine has been shown to be a two photon process (51).
This is feasible since the energy from two phot,ons is sufficient for ionization and the triplet state has a sufficient lifetime a t the low temperatures used to be photolyzed before decaying. As the name implies, a primary effect of ionizing radiation is the ionization of neutral molecules. At low temperatures in the glassy state numerous cation radicals have been so produced and spectroscopically characterized (52). One useful technique which has been perfected is electron scavenging. Recombination of electrons and cation radicals (holes) ordinarily occurs, but can be prevented if the electrons are trapped by suitable scavengers like alkyl halides or aromatic hydrocarbons (58).
Recent photochemical studies in solution have demonstrated the importance of charge transfer processes between organic solutes. Amines have been generally used as electron donors and aromatic hydrocarbons (55), ketones (54), and organic halides (55) as electron acceptors.
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