Radical Secondary

Because the starting material (propane) and one of the products (H ) are the same m both processes the difference m bond dissociation energies is equal to the energy dif ference between an n propyl radical (primary) and an isopropyl radical (secondary) As depicted m Figure 4 20 the secondary radical is 13 kJ/mol (3 kcal/mol) more stable than the primary radical... 

FIGURE 4 20 The bond dis sociation energies of methy lene and methyl C—H bonds in propane reveal difference in stabilities between two isomeric free radicals The secondary radical is more stable than the primary... 

The propensity of nitriles to release cyanide subsequent to metaboHsm is the basis of their acute toxicity. Nitriles that form tertiary radicals at their alpha carbon atoms (eg, isobutyronitrile, 2-methylbutyronitrile) are substantially more acutely lethal than nitriles that form secondary radicals at their alpha carbons (eg, butyronitrile, propionitnle). Cyanohydrins are acutely toxic because they are unstable and release cyanide quickly. Alpha-aminonitriles are also acutely toxic, presumably by analogy with cyanohydrins. 

Radiation Effects. Polytetrafluoroethylene is attacked by radiation. In the absence of oxygen, stable secondary radicals are produced. An increase in stiffness in material irradiated in vacuum indicates cross-linking (84). Degradation is due to random scission of the chain the relative stabiUty of the radicals in vacuum protects the materials from rapid deterioration. Reactions take place in air or oxygen and accelerated scission and rapid degradation occur. 

In the mass spectrum (Figure 8) of the corresponding ketal of 5-deoxy-D-xt/Zo-hexose, 5-deoxy-l,2-0-isopropylidene-D- rt/Zo-hexofuranose (11), the peak from C-4-C-5 cleavage, m/e 159, is of minor relative intensity. Since the ions at m/e 159 are the same from both isomers, 10 and 11, the intensity difference must be attributable to the lower stability of the primary radical formed from C-5 of 11 compared with the secondary radical from 10 ... 

When an unsymmetrically substituted vinyl monomer such as propylene or styrene is polymerized, the radical addition steps can take place at either end of the double bond to yield either a primary radical intermediate (RCH2-) or a secondary radical (R2CH-). Just as in electrophilic addition reactions, however, we find that only the more highly substituted, secondary radical is formed. 

What are the reasons for the observed reactivity order of alkane hydrogens toward radical chlorination A look at the bond dissociation energies given previously in Table 5.3 on page 156 hints at the answer. The data in Table 5.3 indicate that a tertiary C—H bond (390 kj/mol 93 kcal/mol) is weaker than a secondary C-H bond (401 kj/mol 96 kcal/mol), which is in turn weaker than a primary C H bond (420 kj/mol 100 kcal/mol). Since less energy is needed to break a tertiary C-H bond than to break a primary or secondary C-H bond, the resultant tertiary radical is more stable than a primary or secondary radical. 

The success of intramolecular conjugate additions of carbon-centered radicals in multifunctional contexts is noteworthy. Compound 57 (see Scheme 10), prepared by an interesting sequence starting from meto-toluic acid (54) (see 54 > 55 > 56 > 57), can be converted to the highly functionalized perhydroindane 58 through an intramolecular conjugate addition of a hindered secondary radical.21-22 This radical cyclization actually furnishes a 6 1 mixture of perhydroindane diastereoisomers, epimeric at C-7, in favor of 58 (96 % total yield). It should be noted that a substantially less strained cis-fused bicyclo[4.3.0] substructure is formed in this cyclization. 

The simple initiation process depicted in many standard texts is the exception rather than the rule. The yield of primary radicals produced on thermolysis or photolysis of the initiator is usually not 100%. The conversion of primary radicals to initiating radicals is dependent on many factors and typically is not quantitative. The primary radicals may undergo rearrangement or fragmentation to afford new radical species (secondary radicals) or they may interact with solvent or other species rather than monomer. 

The radicals formed by imimolecular rearrangement or fragmentation of the primary radicals arc often termed secondary radicals. Often the absolute rate constants for secondary radical formation are known or can be accurately determined. These reactions may then be used as radical clocks",R2° lo calibrate the absolute rate constants for the bimolecular reactions of the primary radicals (e.g. addition to monomers - see 3.4). However, care must be taken since the rate constants of some clock reactions (e.g. f-butoxy [3-scission21) are medium dependent (see 3.4.2.1.1). 

For the case of initiators that produce both primary and secondary radicals (e.g. BPO) use of a doubly labeled initiator allows the different types of end groups to be distinguished [e.g. 117 and 118 - Scheme 3.98J and the reactivities of... 

In the stepwise decomposition of azo-compounds such as 4, products can arise from reactions within the primary diazenyl-alkyl radical pair or from the secondary radical pair produced by loss of nitrogen from the... 

The observation (Porter ef a ., 1972) that added BrCCla almost completely suppresses the polarization of the olefin, while leaving the polarization of trans-4 unalfected, points to the secondary radical pair as the principal immediate precursor of a-methylstyrene. A rate constant for the decomposition of thediazenyl radical of 10 -10 sec has been estimated. Cage collapse and free-radical formation are also thought to occur and appropriately polarized products have been identified (see above). 

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... 

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. 

In both mechanisms, the regiochemistry is determined by a preference for forming the most stable intermediate possible. For example, in the ionic mechanism, adds to produce a tertiary carbocation, rather than a secondary carbocation. Similarly, in the radical mechanism, Br adds to produce a tertiary radical, rather than a secondary radical, hi this respect, the two reactions are very similar. But take special notice of the fundamental difference. In the ionic mechanism, the proton comes on first. However, in the radical mechanism, the bromine comes on first. This critical difference explains why an ionic mechanism gives a Markovnikov addition while a radical mechanism gives an anti-Markovnikov addition. 

The reactions can also be effected by phenyliodonium diacetate.377 A mechanistic prototype can be found in the conversion of pentanol to 2-methyltetrahydrofuran. The secondary radical is most likely captured by iodine or oxidized to the carbocation prior to cyclization.378... 

The formation of so-called secondary radicals, for example, ozone and H02, starts as soon as the discharge pulse is finished. They contribute to NO oxidation to N02 by the following reactions ... 

The rate of Au(ffl) reduction should have a correlation with the cavitation efficiency at these frequencies. Therefore, the result of Fig. 5.8 suggests that maximum amounts of reductants are sonochemically formed at 213 kHz in the presence of 1-propanol. The existence of an optimum frequency in the sonochemical reduction efficiency would be explained as follows. As the frequency is increased, the number of cavitation bubbles can be expected to increase. This would result in an increase in the amount of primary and secondary radicals generated and an increase in the rate of Au(HI) reduction. On the other hand, at higher frequencies there may not be enough time for the accumulation of 1-propanol at the bubble/solution interface and for the evaporation of water and 1 -propanol molecules to occur during the expansion cycle of the bubble. This would result in a decrease in the amount of active radicals. Furthermore, the size of the bubbles also decreases with increasing frequency. These multiple effects would result in a very complex frequency effect. 

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