Stabilities of Radicals

Stability in chemistry is not an absolute, but a relative concept. Let us consider the standard heats of reaction AH0 of the homolytic dissociation reaction R—H — R + H. It reflects, on the one hand, the strength of this C—H bond and, on the other hand, the stability of the radical R produced. So the dissociation enthalpy of the R—H bond depends in many ways on the structure of R. But it is not possible to tell clearly whether this is due to an effect on the bond energy of the broken R—H bond and/or an effect on the stability of the radical R that is formed. 

How do we explain, for example, the fact that the dissociation enthalpy of a Cs[fl —H bond essentially depends on n alone and increases in the order n = 3,2, and 1, that homolytic cleavage of a C-H bond of an sp3-hybridized carbon requires a lot less energy than that of an sp-hybridized carbon  

Overall, however, the homolytic bond dissociation energy of every Cy,-element bond increases in the order n = 3,2, and 1. This is due to the fact that Gy,-element bonds become shorter in this order, i.e. n = 3, 2, and 1, which, in turn, is due to the fact that the s character of the Cy-element bond increases in the same order. Other things being equal, the shorter the bond, the stronger the bond. 

An immediate consequence of the different ease with which Cy,-element bonds dissociate is that in radical substitution reactions, alkyl radicals are more easily formed. Vinyl and aryl radicals are less common, but can be generated productively. Alkynyl radicals do not appear at all in radical substitution reactions. In the following, we therefore limit ourselves to a discussion of substitution reactions that take place via radicals of the general structure R1 R2R3C.  

Hydrogen and chlorine atoms have an odd number of electrons and are radicals. The methyl radical is the simplest organic radical. It has one more electron than a carbocat-ion and one fewer than a carbanion. The last example is a radical cation, which results from the loss of one electron from a normal molecule. Radical cations are important in mass spectrometry (see Chapter 15). 

Both nitrogen oxide (NO) and nitrogen dioxide (N02) are radicals. Show Lewis structures for these compounds. 

Because most radicals have an odd number of electrons on an atom, the octet rule cannot be satisfied at that atom. It is no surprise, then, that most radicals are unstable species and are quite reactive. They are most often encountered, like carbocations, as transient intermediates in reactions. However, alkyl radicals tend to have longer lifetimes than carbocations because they are less electron deficient, and therefore more stable. In fact, the lifetime of a radical can be appreciable in an environment where nothing is available with which to react. For example, hydrogen atoms are the principal type of matter in interstellar space. And die methyl radical has a lifetime of about 10 min when frozen in a methanol matrix at 77 K. 

A comparison of the stabilities of different carbon radicals is provided by the bond dissociation energies of the bond between the carbon and a hydrogen. This is the energy that must be added when the reaction shown in the following equation occurs  

Bond dissociation energies for some carbon-hydrogen bonds are shown in Table 21.1. 

Stability in chemistry is not an absolute but a relative concept It always refers to a stability difference with respect to a reference compound. Radicals reported in the literature range from extremely unstable, short-lived species to relatively stable one that could be isolated as pure substances. 

Not aU radicals show the same reactivity, broadly speaking the character of radicals is affected by (i) the nature of the atom that is the radical center and (ii) the electronic properties of the groups attached to the radical. The importance of the atom bearing the unpaired electron is nicely illustrated by the different reactivities of group 6 radicals, and is rationalized by the hard/soft nature of the radical center. Thus, alkoxy radicals (RO ) are small and hard they typically undergo H-atom abstraction and p-scission reactions. However, they rarely add to C=C. Thiyl (RS ) and selenyl (RSe ) radicals, however, are larger and softer. They do not usually abstract H, but they do readily add to C=C. This is useful for (Z/E) isomerization of alkenes. 

Carbon-hydrogen bonds decrease in strength in R- H when R goes from primary to secondary to tertiary. Tertiary alkyl radicals are therefore the most stable and methyl radicals are the least stable. As C-H bonds next to conjugating groups such as allyl or benzyl are particularly weak, allyl and benzyl radicals are more stable. 

There are two reasons why some radicals are more persistent than others  

EPR experiments on carbon-centred radicals with either a- or ) -boronic ester substituents have been reported. While the a-substituted radicals were modestly thermodynamically stable, the ) -substituted radicals imderwent easy ) -elimination. An EPR experiment on the photo-oxidation of phenohc compoimds containing at least one free ortho position has indicated the formation of persistent secondary radicals derived from dimerization or pofymerization from C-O coupling.The structure of the succinimidyl radical has been re-examined using density functional theory with a variety of basis sets. The electronic ground state was foimd to be of (X-symmetry allowing for facile )S-scission. These conclusions were also predicted using MP2 but 

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Other functional groups provide sufficient stabilization of radicals to permit successful chain additions to alkenes. Acyl radicals are formed by abstraction of the formyl hydrogen from aldehydes. As indicated in Table 12.7, the resulting acyl radicals are... 

The most evident of these is the marked stability of radical cations formed in an aprotic medium by the oxidation of compounds where the first ionization potential (in the sense of photoelectron spectroscopy) is for the removal of an electron from a non-bonding orbital, e. g. thianthrene... 

A central theme in our approach, which we believe to be different from those of others, is to focus on the changing chemistry associated with higher, middle and lower oxidation state compounds. The chemical stability of radical species and open-shell Werner-type complexes, on the one hand, and the governance of the 18-electron rule, on the other, are presented as consequences of the changing nature of the valence shell in transition-metal species of different oxidation state. 

For a discussion concerning the difference between thermodynamic stability of radicals and their kinetic persistence e.g., due to steric effects see Ref.9)... 

B Homolytic Bond Dissociation Energies and the Relative Stabilities of Radicals ... 

Bond dissociation energies can be used to eatimate the relative stabilities of radicals. 

Numerous reports published in recent years have focused on carbon-centered radicals derived from compounds with selected substitution patterns such as alkanes [40,43,47], halogenated alkanes [43,48,49,51-57], alkenes [19], benzene derivatives [43,47], ethers [51,58], aldehydes [48], amines [10,59], amino acids [23,60-67] etc. Particularly significant advances have been made in the theoretical treatment of radicals occurring in polymer chemistry and biological chemistry. The stabilization of radicals in all of these compounds is due to the interaction of the molecular orbital carrying the unpaired electron with energetically and spatially adjacent molecular orbitals, and four typical scenarios appear to cover all known cases [20]. 

While Eqs. 1-4 may represent the most consistent approach for the definition of radical stability for a selected subclass of radicals, there may still be the need (or desire) to compare the stabilities of radicals characterized through different reference systems. The following three hydrogen transfer reactions... 

In connection with the captodative effect, Riichardt (Zamkanei et al., 1983) has determined the BDE of the tertiary C—H bond in [20] and compared it with the tertiary bond in isobutane. He concludes that the stabilization of 12.8 kcal mol which he derives from this comparison falls 4kcal mol short of the value of 16.5 kcal mol which he calculates for the sum of the substituent effects for phenyl (9 kcal mol ), cyano- (5.5 kcal moP ) and methoxyl (1.5kcal mol ) groups. The latter values were derived from studies on C—C BDEs. Not even additivity of the substituent effects is observed. The existence of a captodative stabilization of radical [21] is denied (see, however, the studies on the thermolysis of [24]). 

The stability of radical anions of disulfides [RS—SR] and their ease of dissociation into thiolate anions and thiyl radical were studied as a function of pH with alkyl substituents of different structures ... 

Knowledge of bond dissociation enthalpies (DH) has always been considered fundamental for understanding kinetics and mechanisms of free radicals. DHs offer an interesting window through which to view stability of radicals. Indeed, based on Reaction (2.1) the bond dissociation enthalpy of silanes D/f(R3Si—H) is related to enthalpy of formation of silyl radicals, A//f (RsSi ), by Equation (2.2). 

The formation of a radical-anion with a very short lifetime on the surface of a sodium-potassium alloy during the reduction of thieno[3,2- ]-thiophene (2) at —100° was established by ESR (theoretical and experimental spectra are presented). The formation of the thieno[2,3-61-thiophene (1) radical-anion even under such extreme conditions was not observed. The difference in the stability of radical-anions of thienothiophenes 1 and 2 was accounted for by a greater degree of conjugation in thienothiophene 2 molecule as compared to 1. The spectrum of the thienothiophene 2 radical-anion distinctly exhibits two types of hydrogen atoms with coupling constants 4.87 and 0.52 Gauss. The... 

The relative stabilities of radicals follow the same trend as for carhoca-tions. Like carbocations, radicals are electron deficient, and are stabilized by hyperconjugation. Therefore, the most substituted radical is most stable. For example, a 3° alkyl radical is more stable than a 2° alkyl radical, which in turn is more stable than a 1° alkyl radical. Allyl and benzyl radicals are more stable than alkyl radicals, because their unpaired electrons are delocalized. Electron delocalization increases the stability of a molecule. The more stable a radical, the faster it can be formed. Therefore, a hydrogen atom, bonded to either an allylic carbon or a benzylic carbon, is substituted more selectively in the halogenation reaction. The percentage substitution at allylic and benzyhc carbons is greater in the case of bromination than in the case of chlorination, because a bromine radical is more selective. 

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