Structure and stability of radicals

Another possibility is that we can get radical addition to an unsaturated molecule, e.g. an alkene. Writing in all the electron movement arrows, we have one of the double bond n electrons being used to make the new a bond with the original radical species, whilst the second n electron becomes located 

Note that if we choose not to put in all the curly arrows, we could write the mechanism in two ways either considering the radical as the attacking species or the double bond as the electron-rich species. The first version is perhaps more commonly used, but it is much more instmctive to compare the second one with an electrophilic addition mechanism (see Section 8.1). The rationalization for the regiochemistry of addition parallels that of carbocation stability (see Section 8.2). 

Most radicals have a planar or nearly planar structure. Carbon is sp hybridized in the methyl radical, giving three a C-H bonds, and the single electron is held in 

Although a radical is neutral, it is an electron-dehcient species that will be very reacdve as it attempts to pair off the odd electron. Because radicals are electron dehcient, electron-releasing groups such as alkyl groups tend to provide a stabilizing effect. The more electron-releasing groups there are, the more stable the radical. Thus, tertiary radicals are more stable than secondary radicals, which in turn are more stable than primary radicals. 

The order of stability is thus the same as with car-bocations, another electron-deficient species, and for the same reason. There is favourable delocalization of the unpaired electron through overlap of the a C-H (or C-C) bond into the singly occupied p orbital of 

We have already encountered both carbocations and car-banions, and spent some time discussing their structures (Section 2.4, p. 62). The methyl cation, and carbocations in general, are flat, r/ -hybridized molecules with an empty If orbital extending above and below the plane of the three substituents on carbon (Fig. 11.15). By contrast, the methyl anion is pyramidal. One might well guess that the methyl radical, with a single nonbonding electron, would have a structure intermediate between that of the two ions. 

Both theory and experiment agree that the structure of simple radicals is hard to determine However, it is clear that these molecules are not very far from planar. If the molecules are not planar, the pyramid is very shallow and the two possible forms are rapidly inverting (Fig. 11.16). 

FIGURE 11.15 Radicals are intermediate in structure between the planar carbocations and the pyramidal catbanions. 

FIGURE 11.16 A%1 radicals are either flat or very shallow, rapidly interconverting pyramids. 

TABLE 11.1 Bond Dissociation Energies for Some Hydrocarbons 

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The stereoselectivity of anti-Markovnikov adducts (161) and (162) produced through photo-induced electron-transfer reaction of (160) with MeOH in MeCN depends on the optimum structures and stabilities of the corresponding radical and carbanion intermediates (163) and (164). In PhH, steric hindrance in an exciplex, comprising an excited singlet sensitizer and (160), forced cis addition of MeOH to (160) to give trans-isomer (161) as the major addition product. 

The major carbon centered reaction intermediates in multistep reactions are carboca-tions (carbenium ions), carbanions, free radicals, and carbenes. Formation of most of these from common reactants is an endothermic process and is often rate determining. By the Hammond principle, the transition state for such a process should resemble the reactive intermediate. Thus, although it is usually difficult to assess the bonding in transition states, factors which affect the structure and stability of reactive intermediates will also be operative to a parallel extent in transition states. We examine the effect of substituents of the three kinds discussed above on the four different reactive intermediates, taking as our reference the parent systems [ ]+, [ ]-, [ ], and [ CI I21-... 

As expected, fluorine substitution has some consequences on structure and stability of the radicals, which are different from the hydrocarbon counterparts. a-F radicals prefer the pyramidal structure because of minimizing 1 repulsion. The trifluoromethyl radical F3C is essentially tetrahedral and has a significant barrier to inversion of about 25 kcal mol - .39 In contrast, the methyl radical H3C itself is planar. Fluorine /J to the radical site is of minor structural consequence. Thus, the pcrfluoro-/er/-butyl radical exhibits a more planar geometry. 

The electronic structure of donor and acceptor components is more or less predictable and the general relationship between, for example, structure and donor (acceptor) strength or structure and stability of ion radicals, is quite familiar to organic chemists. The design of new electronic structures is not only quite possible but represents the main trend of current research activities in the field. On the other hand, only preliminary steps have been made toward predicting and designing of crystal structure [15-17]. 

Hyperconjugation by a C-Sn o bond (and indeed by most carbon-metal a bonds) is much more effective than C-H hyperconjugation, and it is an important factor in determining the structure and stability of not only radicals and cations, but also of compounds with filled n systems such as allyl-, benzyl-, and cyclopentadienyl-stannanes. The importance of vinyl-, allyl-, and aryl-stannanes in organic synthesis owes much to the stabilisation of radical and cation intermediates by a stannyl substituent, and under suitable conditions this can accelerate a reaction by a factor of more than 1014. 

It can now be predicted with confidence that machine calculations will lead gradually toward a really fundamental quantitative understanding of the rules of valence and the exceptions to these toward a real understanding of the dimensions and detailed structures, force constants, dipole moments, ionization potentitils, and other properties of stable molecules and equally unstable radicals, anions, and cations, and chemical reaction intermediates toward a basic understanding of activated states in chemical reactions, and of triplet and other excited states which are important in combustion and explosion processes and in photochemistry and in radiation chemistry and also of intermolecular forces further, of the structure and stability of metals and other solids of those parts of molecular wave functions which are important in nuclear magnetic resonance, nuclear quadrupole coupling, and other interaction involving electrons and nuclei and of very many other aspects of the structure of matter which are now understood only qualitatively or semi-empirically. 

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