Alkyl radicals secondary

The free radicals that we usually see in carbon chemistry are much less stable than these Simple alkyl radicals for example require special procedures for their isolation and study We will encounter them here only as reactive intermediates formed m one step of a reaction mechanism and consumed m the next Alkyl radicals are classified as primary secondary or tertiary according to the number of carbon atoms directly attached to the carbon that bears the unpaired electron... 

Addition of a bromine atom to C 1 gives a secondary alkyl radical... 

A secondary alkyl radical is more stable than a primary radical Bromine therefore adds to C 1 of 1 butene faster than it adds to C 2 Once the bromine atom has added to the double bond the regioselectivity of addition is set The alkyl radical then abstracts a hydrogen atom from hydrogen bromide to give the alkyl bromide product as shown m... 

There are three sorts of C-H bonds in cyclohexene, and Table 5.3 gives an estimate of their relative strengths. Although a typical secondary alkyl C-H bond has a strength of about 400 kj/mol (96 kcal/mol) and a typical vinylic C-H bond has a strength of 445 kj/mol (106 kcal/mol), ail allylic C-H bond has a strength of only about 360 kj/mol (87 kcal/mol). An allylic radical is therefore more stable than a typical alkyl radical with the same substitution by about 40 kj/mol (9 kcal/mol). 

However, the situation is not as clear-cut as it might at first seem since a variety of other factors may also contribute to the above-mentioned trend. Abuin et a/.141 pointed out that the transition state for addition is sterically more demanding than that for hydrogen-atom abstraction. Within a given series (alkyl or alkoxy), the more nucleophilic radicals are generally the more bulky (i.e. steric factors favor the same trends). It can also be seen from Tabic 1.6 that, for alkyl radicals, the values of D decrease in the series primary>secondary>tertiary (i.e. relative bond strengths favor the same trend). 

Rate constants tor reactions of carbon-centered radicals tor the period through 1982 have been compiled by Lorand340 and Asmus and Bonifacio- 50 and for 1982-1992 by Roduner and Crocket.3 1 The recent review of Fischer and Radom should also be consulted.j41 Absolute rate constants for reaction with most monomers lie in the range 105-106 M"1 s"1. Rate data for reaction of representative primary, secondary, and tertiary alkyl radicals with various monomers are summarized in Table 3.6. 

The primary alkyl radical, H, is anticipated to be more reactive and may show different specificity to the secondary or tertiary radical, Tv In VAc and VC polymerizations the radical H appears more prone to undertake intermolecular (Sections 4.3.1.1 and 4.3.1.2) or intramolecular (4.4.3.2) atom transfer reactions. 

Diallyl monomers find significant use in cyclopolymerization (Section 4.4.1). Transfer to monomer is of greater importance in polymerizations of allyl than it is in diallyl monomers.184 This might, in part, reflect differences in the nature of the propagating species [e.g. a secondary alkyl (115) v.v a primary alkyl radical (116)]. Electronic factors may also play a role,185... 

Consider now a series of compounds A, that react with two reagents, Bi and B2. A good example is the reactions of alkyl radicals (the A, s are R") with BrCCl3 (B ) and CCI4 (B2). The radicals considered are planar, tt radicals that are primary, secondary, and tertiary, but not bridgehead.21 The scheme is... 

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... 

Table 1 shows the kinetic data available for the (TMSjsSiH, which was chosen because the majority of radical reactions using silanes in organic synthesis deal with this particular silane (see Sections III and IV). Furthermore, the monohydride terminal surface of H-Si(lll) resembles (TMSjsSiH and shows similar reactivity for the organic modification of silicon surfaces (see Section V). Rate constants for the reaction of primary, secondary, and tertiary alkyl radicals with (TMSIsSiH are very similar in the range of temperatures that are useful for chemical transformations in the liquid phase. This is due to compensation of entropic and enthalpic effects through this series of alkyl radicals. Phenyl and fluorinated alkyl radicals show rate constants two to three orders of magnitude... 

Nitrogen- and oxygen-centered radicals offer some specific features. The rate constant of the reaction of dialkylaminyl radicals with (TMSlsSiFl is close to the analogous reaction of secondary alkyl radicals. In the reactions of... 

N-Alkoxylamines 88 are a class of initiators in "living" radical polymerization (Scheme 14). A new methodology for their synthesis mediated by (TMSlsSiH has been developed. The method consists of the trapping of alkyl radicals generated in situ by stable nitroxide radicals. To accomplish this simple reaction sequence, an alkyl bromide or iodide 87 was treated with (TMSlsSiH in the presence of thermally generated f-BuO radicals. The reaction is not a radical chain process and stoichiometric quantities of the radical initiator are required. This method allows the generation of a variety of carbon-centered radicals such as primary, secondary, tertiary, benzylic, allylic, and a-carbonyl, which can be trapped with various nitroxides. 

These reactions exhibit excellent diastereoselectivity derived from the chiral oxazo-lidinone auxiliary. The Lewis acid forms a chelate with the oxazoline and presumably also serves to enhance reactivity. In addition to ethyl, other primary, secondary, and tertiary alkyl radicals, as well as acetyl and benzoyl radicals were used successfully in analogous reactions. 

Alkyl mercury halides participate in a photo-stimulated radical chain reaction of the anion of nitroalkanes (see Eq. 5.38) in which a 275-W sun lamp is used.56 c Primary, secondary, and tertiary alkyl radicals generated from alkyl mercury halides react with the anion of nitroalkanes to form new C-C bonds. 

The same iron (III) complexes also oxidize alkyl radicals, particularly those with secondary and tertiary centers, to the corresponding carbonium ions (7). 

The important role of reaction enthalpy in the free radical abstraction reactions is well known and was discussed in Chapters 6 and 7. The BDE of the O—H bonds of alkyl hydroperoxides depends slightly on the structure of the alkyl radical D0 H = 365.5 kJ mol 1 for all primary and secondary hydroperoxides and P0—h = 358.6 kJ mol 1 for tertiary hydroperoxides (see Chapter 2). Therefore, the enthalpy of the reaction RjOO + RjH depends on the BDE of the attacked C—H bond of the hydrocarbon. But a different situation is encountered during oxidation and co-oxidation of aldehydes. As proved earlier, the BDE of peracids formed from acylperoxyl radicals is much higher than the BDE of the O—H bond of alkyl hydroperoxides and depends on the structure of the acyl substituent. Therefore, the BDEs of both the attacked C—H and O—H of the formed peracid are important factors that influence the chain propagation reaction. This is demonstrated in Table 8.9 where the calculated values of the enthalpy of the reaction RjCV + RjH for different RjHs including aldehydes and different peroxyl radicals are presented. One can see that the value A//( R02 + RH) is much lower in the reactions of the same compound with acylperoxyl radicals. 

Nitroxyl radicals (AmO ) are known to react rapidly with alkyl radicals and efficiently retard the radical polymerization of hydrocarbons [7]. At the same time, only aromatic nitroxyls are capable of reacting with alkylperoxyl radicals [10,39] and in this case the chain termination in the oxidation of saturated hydrocarbons occurs stoichiometrically. However, in the processes of oxidation of alcohols, alkenes, and primary and secondary aliphatic amines in which the chain reaction involves the HOT, >C(0H)02 , and >C(NHR)02 radicals, possessing the... 

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