Butoxyl

Some details of the chain-initiation step have been elucidated. With an oxygen radical-initiator such as the /-butoxyl radical, both double bond addition and hydrogen abstraction are observed. Hydrogen abstraction is observed at the ester alkyl group of methyl acrylate. Double bond addition occurs in both a head-to-head and a head-to-tail manner (80). 

Some details of the chain-initiation step have been elucidated. With an oxygen radical—initiator such as the /-butoxyl radical, both double bond... 

Butanediol, 28 2-Butanone, 29 2-Butanone peroxide, 29 n-Butoxyethanol, 29 Butoxyl, 29... 

Consider the reaction of re/7-butoxyl radical with hydrocarbons such as 1, 7-octadiene, RH 7... 

A fast, unimolecular reaction can be used to excellent advantage. The rm-butoxyl radical offers the advantage that /3-scission occurs with a known rate constant. For Eq. (5-31), ki = 1.4 X 106 s-1 in water.8 In the presence of a hydrogen donor, AH, the competition is... 

The most common way to generate sulfonyl radicals for spectroscopic studies has been the photolysis of solutions containing di-t-butyl peroxide, triethylsilane and the corresponding sulfonyl chloride in a variety of solvents (equations 4-6). The slowest step in this sequence is the reaction between t-butoxyl radicals and triethylsilane (ks = 5.3 x 106m 1s-1)26 since that for chlorine abstraction (equation 6) is extremely efficient (cf. Table 4). 

The relative reactivities of alkyltin compounds towards tert-butoxyl radicals, ketone triplets, and succinimidyl radicals are dominated by the steric effect of the alkyl ligands (R" > R"), but that towards bromine atoms follows the reverse sequence (R" < R ). 

If the initiator is a peroxide compound capable of generating very active but nonselective alkoxyl radicals, the concentration of the inhibitor must be sufficiently high to provide the above conditions. For instance, the ratio krJk2i=50 (408 K) for the reaction of the tert-butoxyl radical with ionol and cyclohexane [46]. In this case, alkoxyl radicals will react with InH more readily than with RH, provided [InH] > 0.2 mol L-1. 

Alkoxyl radicals react with phenols extremely rapidly. The rate constants of the reactions of tert-butoxyl radicals with some phenols in benzene are given below (Y is the /(-substituent in the phenol molecule) [43,44],... 

FIGURE 2. Relative reactivities toward the ferf-butoxyl radical of individual positions (per hydrogen atom) relative to a hydrogen atom in a methyl group in pentane. Reprinted with permission from Reference 23. Copyright (1985) American Chemical Society... 

Aryl radicals are much more reactive than alkyl radicals (k ca. 2 x 106M-1 s 1), and rather more reactive than cyclopropyl radicals (ca. 8 x 107) or vinyl radicals (ca. 3 x 10s). Fluoroalkyl radicals are also about 100 times more reactive than alkyl radicals, because of contributions from structures Rf 6 - H -4+SnR3 to the transition state. Singlet-excited acetone is twice as reactive as triplet-excited acetone, which shows a similar reactivity to that of the /-butoxyl radical (k ca. 2 x 108M-1 s-1). 

The kinetics of reactions involving the tributylstannyl radical have been refined by laser flash time-resolved UV spectroscopy. The measured extinction coefficient of the BujSn- radical in benzene was 1620 40 M 1cm 1 at 400 nm, the rate constant of the reaction of the /-butoxyl radical with Bu3SnH was (3.5 0.3) x 108M 1 s 1, and the rate constant for the self-reaction of the Bu3Sn radical was (3.6 0.3) x 109 M s1. S29... 

The disappearance of nitroxides may also occur by unimolecular reactions. A good example is found with t-butoxy t-butyl nitroxide, known to be a relatively short-lived species, for which three fragmentation pathways are possible (1 la,b,c). Loss of butoxyl radicals (path a) was shown to be reversible (Perkins and Roberts, 1974), and, in the absence of any substrate which... 

This simple expedient appears to have been employed first by Lunazzi et al. (1972), who compared the reactivities of ethylbenzene and a methylthiophene towards hydrogen abstraction by t-butoxyl radicals using MNP as trap (Scheme 3). Features in the spectra of the spin adducts of thienylmethyl and... 

Suitable low-temperature sources of butoxyl radicals for these experiments are di-t-butyl peroxyoxalate [24], and di-t-butyl hyponitrite [25].10... 

A simple extension of the competition technique is to the comparison of scavenger efficiencies. Thus pairs of spin traps have been allowed to compete for a variety of radicals, including t-butoxyl, phenyl, and primary alkyl. Much more revealing, however, is the type of experiment in which the bimolecular trapping of a radical is allowed to compete with some other reaction of that radical whose absolute rate constant is known. In this way, the rate constant for the trapping reaction itself is accessible. 

When t-butoxy t-butyl nitroxide was generated in the presence of cumene as a hydrogen donor, its disappearance was much more rapid that in the absence of cumene. This was attributed to rapid fragmentation to t-butoxyl radicals and nitrosobutane (11a) which, in the absence of a hydrogen donor, is reversible. 

It was possible to show that the fragmentation to give t-butoxyl radicals is more rapid than that which gives t-butyl by a factor of 100 (at 40°). From an estimate of the rate of reaction of t-butoxyl radicals with cumene, it was also possible to deduce a rate constant for scavenging of butoxyl radicals by MNP. 

In one of the earliest investigations of spin trapping, olefin polymerization was employed to demonstrate the utility of the method as a qualitative probe for free-radical reactions (Chalfont et al., 1968). The polymerization of styrene, initiated by t-butoxyl radicals, proved to be an excellent system with which to obtain spectra attributable to spin adducts with MNP (a) of the initiator radical... 

Fig. 7 Esr spectra obtained the polymerization of styrene by t-butoxyl radicals in the presence of MNP (Chalfont et al., 1968). Traces (a) and (b) were recorded on different scales using high scavenger concentration (c) represents an intermediate, and (d) a low scavenger concentration. The shoulder arrowed in (b) reveals a trace of growing-chain radical [29], whilst the lines marked x in (a) are due to [27]...

---