Tertiary carbon radicals

The carbon is electrically neutral, but it does have a greater electronegativity than hydrogen, and so it attracts slightly the electrons within the carbon/ hydrogen bonds. Now suggest the relative order of stability of the primary, secondary and tertiary carbon radicals. tertiary>secondary>primary... 

In the step above, Br attacked the alkene at the less substituted carbon, in order to form the more substituted carbon radical (C ). Tertiary radicals are more stable than secondary radicals, for the same reason that tertiary carbocations are more stable than secondary carbocations. Just as alkyl groups donate electron density to... 

Figure 4-3 shows that a partial radical character is on the first and third carbon atoms. Both of the carbon-carbon bonds are equal and intermediate in length between a single and a double bond. The allylic radical is more stable than a tertiary carbon radical. 

The oxidation of other rubbers has been studied by FT-IR including polychloro-prenes >. These results suggest that the thermal oxidation of polychloroprenes involves the 1,2 and 3,4-structural irregularities in the initial stage. In particular, it is felt that the initial step is the abstraction of a tertiary allylic chlorine or hydrogen from the 1,2 or 3,4 units yielding a tertiary carbon radical. 

It was found that radicals are not involved in ketone formation in Gif oxidation.164,165 As a minor route, in contrast, tertiary alcohols are formed through carbon radicals. Key intermediates164 are the Fe(V)=0 species 11, formed from a p-oxoiron dimer that inserts into the C—H bond, and the alkoxy-iron species 12 ... 

Additional examples of alkyl substituents and their names are listed in Table 3-2. These are further classified according to whether they are primary, secondary, or tertiary. An alkyl group is described as primary if the carbon at the point of attachment is bonded to only one other carbon, as secondary if bonded to two other carbons, and tertiary if bonded to three other carbons. Thus, if R is any hydrocarbon radical, the different kinds of alkyl groups are... 

From C-H bond-dissociation energies of alkanes (see Table 4-6), the ease of formation and stabilities of the carbon radicals is seen to follow the sequence tertiary > secondary > primary. By analogy, the secondary l-bromo-2-propyl radical, 5, is expected to be more stable and more easily formed than the primary 2-bromo-1-propyl radical, 6. The product of radical addition should be, and indeed is, 1-bromopropane ... 

How the enthalpy AH of the substrate/reagent pair R—H/Cl changes when R and H—Cl are produced from it is plotted for four radical chlorinations in Figure 1.23 (left). These afford carbon radicals, the methyl, a primary, a secondary, and a tertiary radical. The reaction enthalpies A77 for all four reactions are known and are plotted in the figure. Only the methyl radical is formed slightly endothermically (AH = +2.0 kcal/mol). The primary radical, which is more stable by 4.3 0.7 kcal/mol (cf. Table 1.2), is formed exothermically with A77 = -2.3 kcal/mol. The formation of the more stable secondary and tertiary radicals are more exothermic by -4.7 and -7.6 kcal/mol, respectively. 

Dehydrodimerization. On excitation with a mercury vapor lamp, mercury is converted to an excited state, Hg, which can convert a C—H bond into a carbon radical and a hydrogen atom. This process can result in dehydrodimerization, which has been known for some time, but which has not been synthetically useful because of low yields when carried out in solution. Brown and Crabtree1 have shown that this reaction can be synthetically useful when carried out in the vapor phase, in which the reaction is much faster than in a liquid phase, and in which very high selectivities are attainable. Secondary C—H bonds are cleaved more readily than primary ones, and tertiary C—H bonds are cleaved the most readily. Isobutane is dimerized exclusively to 2,2,3,3-tetramethylbutane. This dehydrodimerization is also applicable to alcohols, ethers, and silanes. Cross-dehydrodimerization is also possible, and is a useful synthetic reaction. 

The photoreactions of aliphatic amines with aromatic hydrocarbons have also been reported by several groups. With tertiary amines, deprotonation occurs from the radical cations of amines at the a-carbon to generate carbon radicals which react with the radical anion of aromatic hydrocarbons. With secondary amines, deprotonation from the radical cations of amines occurs both at the a-carbon and at the nitrogen atom, so that the reaction becomes complicated [64-65]. 

Carbon radicals are classified as primary (1°), secondary (2°), or tertiary (3°) by the number of R groups bonded to the carbon with the unpaired electron. A carbon radical is sp hybridized and trigonal planar, like sp hybridized carbocations. The unhybridized p orbital contains the unpaired electron and extends above and below the trigonal planar carbon. 

The high preference for C-T attack of the a-silyl carbon radical derived from 24 could be due to the formation of an incipient tertiary C-2 radical A, although steric hindrance of the 2 -methyl group cannot be ruled out. 

In the case of methane, there was only one type of hydrogen that the chlorine radical could attack, but in a larger alkane there is often a choice. One of the principal factors that determines which hydrogen will be abstracted is the stability of the resultant carbon radical. Suggest what will be the order of stability of primary, secondary and tertiary carbon radicals. 

Irradiation of a methanolic solution of l-methyltricyclo[4.1.0.0 ]heptane (22) in the presence of naphthalene-l-carbonitrile gave 6-methoxy-7-methylbicyclo[3.1.1]heptane (23a, 93Vo, isolated yield 56%). In an aqueous system, the corresponding hydroxy derivative 23b was isolated in 70% yield. The orientation of addition was anti-Markovnikov and the substituents were located in the less hindered positions. The same bond was cleaved when the 2-/er/-butyl derivative 24 was submitted to this reaction. With two methyl substituents, i.e. 26, a mixture of two stereoisomers 27A,B and a dehydrogenated product 28 was obtained whose formation could be explained by the occurence of a tertiary carbon radical. ... 

Loss of formaldehyde from radicals RCH2O occurs when R is relatively stable it has been implicated as a consequence of O-N fission of the 6-nitrates of nitrocellulose, where the product radical is essentially anomeric. " " Thermolysis of 2,2-dimethyl-1,3-propanediol dinitrate yields 2,2-dimethyloxirane by N-O homolysis, loss of CH2O and displacement of NO2 from oxygen by the tertiary carbon radical (Figure 6.44). A similar mechanism also appears to take place during PETN thermolysis, whose products are NO2, formaldehyde and a condensation polymer of isolactic acid. 

Recent reinterpretation by Ayscough and Munari [73] assigned the spectrum M to radical (III). Its change with temperature [71] would be due to hindered oscillations of the methyl and methylene groups on the tertiary carbon radical. 

Motherwell and Potier have been interested in the reactivity of thionitrite esters as a potential surrogate of nitric oxide toward carbon-centered radicals. Tertiary thionitrite esters react with Barton esters to give after decarboxylation the corresponding oximes or the nitroso-dimers in moderate yield [57]. 

Since these molecular fragments are produced in a hydrogen-rich environment, they promptly stabilize by hydrogen abstraction from the surrounding hydrocarbon polymer. This process produces dodecane and acid metal sulfide species, in addition to polystyryl radicals. Hydrogen atoms should be principally removed from tertiary carbons in the polymer chains, because in this case resonant-stabilized carbon radicals result ... 

A radical mechanism is proposed for this process, because the C-H bond at the allylic carbon, benzylic carbon, or tertiary carbon is relatively weaker than other C-H bonds, which can homolytically cleave and form radicals, as shown below. 

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