Tertiary radicals, stability

Another family of inhibitors, hydrogen-donating agents such as iso-propyl and 1,4-di-ivo-propyl benzenes, was investigated by Lamouroux in order to reduce the formation of TBP-TBP dimers, which exhibit a very high plutonium retention of TBP (47). The presence of at least one mobile hydrogen on the iso-propyl group could produce a benzylic tertiary radical stabilized by resonance. The addition of such compounds reduced the concentration of the TBP-TBP dimers by about 50%. 

Stabilize a neighboring, empty p-orbital, so too, alkyl groups can stabilize a neighboring, partially filled orbital. This preference for forming a tertiary radical (rather than a secondary radical) dictates that Br" will attack the less substituted carbon. This explains the observed anti-Markovnikov regiochemistry. 

The effects of substitutents on the y-carbon on the efficiency of the type II cleavage are presented in Table 3.15.<89) These data indicate that the rate constant of cleavage increases as the strength of the y C—H bond decreases, that is, from a primary to a secondary to a tertiary hydrogen atom. The substitution of groups capable of radical stabilization, such as — or... 

The calculations also suggested that 5 was favored over 6 by 2.4 kcal mol 1 as found experimentally. The explanation was based on the higher stability of a tertiary alkoxide compared to a primary alkoxide [15], which outweighed the opposite trend for radical stabilization. Epoxide opening was irreversible [16]. 

In the case of asymmetrical ketones, two different modes of a-cleavage can occur, with the major products being formed via the more stable pair of initially-formed radicals. For alkyl radicals, the stability of the radical increases as its complexity increases and radical stabilities are tertiary > secondary > primary. 

The efficiency of product formation in solution is also controlled by the stabilities of the radicals. Stable radicals such as tertiary alkyl radicals or benzyl radicals lead to efficient decarbonylation in solution. Because of steric factors involving bulky groups, tertiary radicals tend to preferentially undergo disproportionation rather than radical combination and so the quantum yield of the products formed by disproportionation exceeds that of the radical combination product. 

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 selectivity of radical bromination reactions depends, in part, on the increased stability of secondary or tertiary radical intermediates compared with primary radicals. In Section 9.2 we noted that allyl and benzyl radicals were especially... 

Stabilized by resonance delocalization indeed, they are even more stable than tertiary radicals. In the presence of a suitable initiator, bromine dissociates to bromine atoms that will selectively abstract an allylic or a benzylic hydrogen from a suitable substrate, generating the corresponding allyl and benzyl radicals. 

At the same time, delocalization of unpaired spin in the free-radical product appears to be important for the course of the substitution reaction. For example hydrogen shift in sabinene radical cation 39a leads to a conjugated system (40 ) nucleophilic attack on l-aryl-2-alkylcyclopropane radical cations 43 or 47 produces benzylic radicals nucleophilic attack on 39a generates an allylic species and attack on the tricyclane radical cations 55 or 56 forms tertiary radicals. Apparently, formation of delocalized or otherwise stabilized free radicals is preferred. 

The well known difference in reactivity in transfer reactions of primary, secondary and tertiary hydrogens is most probably neither due to steric acceleration nor to a difference in electronic stability of primary, secondary and tertiary radicals ... 

Tables 9.3 and 9.4 list selected bond dissociation energies and radical heats of formation. Note particularly that the decrease in energy required to remove hydrogen in the series methane, primary, secondary, tertiary, parallels increasing radical stability, and that aldehydic, allylic, and benzylic hydrogens have bond dissociation energies substantially lower than do alkyl hydrogens.

Table 1.2 illustrates an additional substituent effect on radical stability. Here the dissociation enthalpies of reactions that lead to (poly)alkylated radicals (alk)3-nHnC are listed ( alk stands for alkyl group). From these dissociation enthalpies it can be seen that alkyl substituents stabilize radicals. A primary radical is by 4 kcal/mol more stable, a secondary radical is by 7 kcal/mol more stable, and a tertiary radical is by 9 kcal/mol more stable than the methyl radical. 

The discrepancy from the experimental values is due to the fact that H atoms bound to different types of C atoms are replaced by chlorine at different rates. The substitution of Cfcrt— H takes place via a tertiary radical. The substitution of Csec—H takes place via the somewhat less stable secondary radical, and the substitution of Cprjm—H takes place via even less stable primary radicals (for the stability of radicals, see Table 1.2). According to Hammond s postulate, the rate of formation of these radicals should decrease as the radical s stability decreases. Hydrogen atoms bound to Ctert should thus be substituted more rapidly than H atoms bound to Csec, and these should in turn be substituted by Cl more rapidly than H atoms bound to Cprjm. As the analysis of the regioselectivity of the monochlorination of isopentane carried out by means of Table 1.4 shows, the relative chlorination rates of C —H, C —H, and C. —H are 4.4 33 1, in agreement with this expectation. 

In the other type of fragmentation, a bond is cleaved so that the positive charge remains with one fragment and the odd electron goes with the other. Only the positive fragment is detected and appears in the mass spectrum. The stability of the product cation and radical determine the favorableness of this type of cleavage. You are already quite familiar with the factors that affect the stability of cations, especially carbocations. Although radicals are inherently more stable than carbocations because they are less electron deficient, they are stabilized by the same factors that stabilize carbocations. Thus, tertiary radicals are more stable than secondary radicals, and secondary radicals are more stable than primary radicals. Resonance stabilization is also important. 

Radical reactions with acyl radicals sometimes involve decarbonylation as side-reactions, especially when stabilized secondary or tertiary radicals can be formed. These side-reactions can be suppressed using low-temperature reaction conditions together with different reducing agents such as tris(trimethylsilyl)silane.254... 

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