Isobutyl free radical

Reactions of the Isobutyl Free Radical with Oxygen... 

The work showed clearly that the photolysis of l,l -azoisobutane was an excellent source of isobutyl radicals. In view of these findings we have used l,l -azoisobutane—oxygen mixture photolysis to study the reactions of the isobutyl free radical with oxygen. Some new kinetic data concerning the low temperature oxidation of this interesting radical have been found, and these are reported. 

Rate Constants for the Primary Reactions of the Isobutyl Free Radical with Oxygen. The major primary reaction of the simple alkyl free radicals near room temperature is the association with oxygen, Reaction 7a. 

However, the isobutyl radical represents a special case where the occurrence of Reaction 8a at room temperature is more favorable. The enthalpy changes for Reactions 8a involving ferf-butyl, ethyl, and isobutyl free radicals are —5.8, —8.8, and —12.6 kcal./mole, respectively. Insofar as the over-all enthalpy change is reflected in the minimum potential energy at the transition state involved in Reaction 8a, we would expect k8a for isobutyl to be the largest for the free radicals considered. This... 

Slater, D. H., and J. G. Calvert (1968). The photo-oxidation of l,l -azoisobutane. Reactions of the isobutyl free radical with oxygen. Adv. Chem. Ser. 76, 58-68. 

VEs can also copolymerize by free-radical initiation with a variety of comonomers. According to the and rvalues of 0.023 and —1.77 (isobutyl vinyl ether), VEs are expected to form ideal copolymers with monomers of similar and e values or alternating copolymers with monomers such as maleic anhydride (MAN) that have high values of opposite sign (Q = 0.23 e = 2.25). 

As will be seen in a later section, substituted benzenes rearrange photochemically. Thus o-xylene isomerizes to m-xylene and 1,3,5-tri-isobutyl benzene isomerizes to the 1,2,4- and the 1,2,3-triisobutyl benzenes.410 Such isomerizations conceivably could proceed through free-radical intermediates but Wilzbach and Kaplan and their coworkers have shown that the ring carbon to which the moving substituent is attached also changes position with the substituent. These authors offer the very reasonable explanation that the formation of benzvalene followed by rupture of bonds other than the new ones just formed could lead to rearrangements of the type in question. It should be noted that both benzvalene and prismane could serve as intermediates in this way but that Dewar benzene could not. 

Primary chlorides reacted predominantly by a direct mechanism (Sn2 or a multicentered process). Isobutyl or neopentyl halides led to contributions from electron transfer (free radicals) and halogen-metal exchange (anionoid) mechanisms. 

The evidence in the case of styrene, where both modes of radiation-induced polymerization can be conveniently studied, is quite convincing that reduction of the concentration of water changes the predominating mode of propagation from purely free radical to essentially ionic. Evidence for an ionic propagation initiated by radiation has also been obtained in pure a-methylstyrene (3, 24), isobutylene (12, 32), cyclopenta-diene (5), / -pinene (2), 1,2-cyclohexene oxide (II), isobutyl vinyl ether (6), and nitroethylene (38), although the radical process in these monomers is extremely difficult, if not impossible, to study. 

Use of triphenylmethyl and cycloheptatrienyl cations as initiators for cationic polymerization provides a convenient method for estimating the absolute reactivity of free ions and ion pairs as propagating intermediates. Mechanisms for the polymerization of vinyl alkyl ethers, N-vinylcarbazole, and tetrahydrofuran, initiated by these reagents, are discussed in detail. Free ions are shown to be much more reactive than ion pairs in most cases, but for hydride abstraction from THF, triphenylmethyl cation is less reactive than its ion pair with hexachlorantimonate ion. Propagation rate coefficients (kP/) for free ion polymerization of isobutyl vinyl ether and N-vinylcarbazole have been determined in CH2Cl2, and for the latter monomer the value of kp is 10s times greater than that for the corresponding free radical polymerization. 

There seems little doubt that in radiation induced polymerizations the reactive entity is a free cation (vinyl ethers are not susceptible to free radical or anionic polymerization). The dielectric constant of bulk isobutyl vinyl ether is low (<4) and very little solvation of cations is likely. Under these circumstances, therefore, the charge density of the active centre is likely to be a maximum and hence, also, the bimolecular rate coefficient for reaction with monomer. These data can, therefore, be regarded as a measure of the reactivity of a non-solvated or naked free ion and bear out the high reactivity predicted some years ago [110, 111]. The experimental results from initiation by stable carbonium ion salts are approximately one order of magnitude lower than those from 7-ray studies, but nevertheless still represent extremely high reactivity. In the latter work the dielectric constant of the solvent is much higher (CHjClj, e 10, 0°C) and considerable solvation of the active centre must be anticipated. As a result the charge density of the free cation will be reduced, and hence the lower value of fep represents the reactivity of a solvated free ion rather than a naked one. Confirmation of the apparent free ion nature of these polymerizations is afforded by the data on the ion pair dissociation constant,, of the salts used for initiation, and, more importantly, the invariance, within experimental error, of ftp with the counter-ion used (SbCl or BF4). Overall effects of solvent polarity will be considered shortly in more detail. 

Application of these arguments to those data which are unequivocally free ion values does seem to work, however. For example the radiation induced polymerizations produce pre-exponential factors of 10 —10 1 mole sec , slightly larger than the accepted range for the free radical polymerization of styrene. Similarly the work on iV-vfnylcarbazole [29] produces Ap — 10 1 mole" sec", somewhat larger than the free radical value [106], and presumably affected by solvation contributions. The data for isobutyl vinyl ether [31] indicate a value for Ap of 10 —10 , consistent with the idea that free cations are the main contributors. 

Our interpretation of orientation (See. 3.21) was based on an assumption that we have not yet justified that the relative amounts of isomeric halides we find in the product reflect the relative rates at which various free radicals were formed from the alkane. From isobiitane, for example, we obtain twice as much isobutyl chloride as /c/7-butyl chloride, and wc assume from this that, by abstraction of hydrogen, isobutyl radicals arc formed twice as fast as /err-butyl radicals. 

H. C. Brown (of Purdue University) and Glen Russell (now of Iowa State University) decided to test the possibility that free radicals, like carbonium ions, might rearrange, and chose the chlorination of isobutane as a good test case, because of the large dilVerence in stability between rm-butyl and isobutyl radicals. If rearrangement of alkyl radicals can indeed take place, it should certainly happen here. 

Free-radical chlorination of either w-propyl or isopropyl bromide gives 1-bromo-2-chloropropane, and of either isobutyl or / r/-butyl bromide gives l-bromo-2-chloro-2-methylpropane. What appears to be happening Is there any pattern to this behavior ... 

Free radical copolymerization is initiated upon UV irradiation of mixtures of isobutyl vinyl ether and acrylonitrile (7), presumably as a result of photoexcitation of the comonomer charge transfer complexes. The excited complexes dissociate into ion-radicals which initiate radical propagated copolymerization. 

Initiation reactions are usually started by an active free radical such as peroxide (-0-0-), e.g. benzoyl peroxide is a good inititator for the free radical addition polymerisation of styrene to produce polystyrene AICI3 is an initiator for the cationic addition polymerisation of isobutylene to form isobutyl synthetic rubber or azobisiso-butyronitrile compounds (-N=N-) (abbreviated to AIBN). Propagation reactions are the continuing process and, eventually, lead to the termination stage that occurs by combination or disproportionation. This usually occurs when the free radicals combine with themselves and signals the end of the polymerisation process. All polymers formed by this process are thermoplastics. Table 4.1 is a list of common polymers prepared by the addition process. 

M.U. Kahveci, M.A. Tasdelen, and Y. Yagci, Photochemically initiated free radical promoted living cationic polymerization of isobutyl vinyl ether. Polymer 2007, 48(8), 2199-2202. 

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