Methyl-butyl-2 radical

Of the two extremes experimental studies indicate that the planar sp model describes the bonding m alkyl radicals better than the pyramidal sp model Methyl rad ical IS planar and more highly substituted radicals such as tert butyl radical are flattened pyramids closer m shape to that expected for sp hybridized carbon than for sp ... 

If every collision of a chlorine atom with a butane molecule resulted in hydrogen abstraction, the n-butyl/5ec-butyl radical ratio and, therefore, the 1-chloro/2-chlorobutane ratio, would be given by the relative numbers of hydrogens in the two equivalent methyl groups of CH3CH2CH2CH3 (six) compared with those in the two equivalent methylene groups (four). The product distribution expected on a statistical basis would be 60% 1-chloro-butane and 40% 2-chlorobutane. The experimentally observed product distribution, however, is 28% 1-chlorobutane and 72% 2-chlorobutane. 5ec-Butyl radical is therefore formed in greater anounts, and n-butyl radical in lesser anounts, than expected statistically. 

There are some indications that the situation described above has been realized, at least partially, in the system styrene-methyl methacrylate polymerized by metallic lithium.29 29b It is known51 that in a 50-50 mixture of styrene and methyl methacrylate radical polymerization yields a product of approximately the same composition as the feed. On the other hand, a product containing only a few per cent of styrene is formed in a polymerization proceeding by an anionic mechanism. Since the polymer obtained in the 50-50 mixture of styrene and methyl methacrylate polymerized with metallic lithium had apparently an intermediate composition, it has been suggested that this is a block polymer obtained in a reaction discussed above. Further evidence favoring this mechanism is provided by the fact that under identical conditions only pure poly-methyl methacrylate is formed if the polymerization is initiated by butyl lithium and not by lithium dispersion. This proves that incorporation of styrene is due to a different initiation and not propagation. 

Giese and Kretzschmar7j found the rate of addition of hexenyl radicals to methyl acrylate increased 2-fold between aqueous tetrahydrofuran and aqueous ethanol, Salikhov and Fischer74 reported that the rate constant for /-butyl radical addition to acrylonitrile increased 3.6-fold between tetradecane and acetonitrile. Bednarek et al75 found that the relative reactivity of S vs MMA towards phenyl radicals was ca 20% greater in ketone solvents than it was in aromatic solvents. 

Figure 1.13 Temperature dependence of A U /Alc values for /-butyl radicals with dodecane (—) or 3-methyl-3-pentanol (---------------) as solvent.

In one study, Ingold and coworkers166 measured the rate constants for the reactions of several alkyl radicals with tributyltin hydride using a laser flash photolytic technique and direct observation of the tributyltin radical. They also used this technique with tributyltin deuteride to determine the primary hydrogen-deuterium kinetic isotope effects for three of these reactions. The isotope effects were 1.9 for reaction of the ethyl radical, and 2.3 for reaction of the methyl and n -butyl radicals with tributyltin hydride at 300 K. 

The cumulative effects of multiple substituents have been studied at length in search of particularly stable radicals. It is generally found that the repetitive addition of identical substituents leads to a stepwise decrease in RSE values. This is well illustrated by the comparison of the methyl, ethyl, isopropyl, and ferf-butyl radicals with RSE values of 0.0, - 13.8, - 23.3, and - 28.3 kj/mol. Thus, while the stability of the alkyl radicals clearly increases with the number of alkyl substituents attached to the radical center, the substituent ef-... 

Hindered di-t-alkylamines RNHBu1 (R = t-Bu, t-octyl or 1-adamantyl) have been synthesized from t-alkylamines as follows. Reaction with peracetic acid gave the nitrosoalkanes RNO, which were treated with t-butyl radicals, generated from t-butylhydrazine and lead(IV) oxide, to yield t-butyloxyhydroxylamines. Reduction with sodium naphthalide in THF gave the products (equation 12). The di-t-alkyl-amines are inert to methyl iodide and dimethyl sulphate but can be alkylated by methyl fluorosulphonate42. 

The photochemistry of di-terf-butyl nitroxide was studied149. When di-tert-butylnitroxide (DTBN) is excited at 254 nm to the rnt state in pentane solution, it is cleaved to terf-butyl radical and 2-methyl-2-nitrosopropane (with quantum yield of 0.21). The tert-butyl radical is scavenged by DTBN to give di-ferf-butyl-terf-butoxyamine150 (equation 129). 

Esters of aromatic acids in aprotic solvents form radical-anions detected by cyclic voltammetry on a short time scale [144]. Ethyl benzoate has E° = -2,19 V V5. see [145], Follow-up reactions of radical-anions from methyl and ethyl benzoate result from protonation by extraneous water. rm-Butyl benzoate radical-anion undergoes very rapid cleavage of the alkyl-oxygen bond to give benzoate ion and rerf-butyl radical. 

Interconversions of acychc carbon-centered radicals between n and a types are low-energy processes. The methyl radical is planar, but increasing alkyl substitution at the radical center results in an increasing preference for pyramidalization. The ferf-butyl radical is pyramidalized with the methyl groups 10° from planarity (the deviation from planarity for a tetrahedral atom is 19°) and a barrier to inversion of 0.5 kcal/mol. When a radical center is in a carbocycle, a planar radical is favored for all cases except the cyclopropyl radical, and the barrier for inversion in cyclopropyl is only 3 kcal/mol. ... 

The utility of ESR spectra for determining the structures of radicals is demonstrated by considering some examples. Methyl group substitution for hydrogen in the methyl radical ultimately results in slight deviation from planarity with a low inversion barrier. The a values for methyl, ethyl, isopropyl, and terf-butyl are 38.3, 39.1, 41.3, and 45.2 G, respectively. The ferf-butyl radical is indicated to have 10° deviation from planarity, which is confirmed by infrared (IR) and Raman spectroscopy. ... 

As expected, fluorine substitution has some consequences on structure and stability of the radicals, which are different from the hydrocarbon counterparts. a-F radicals prefer the pyramidal structure because of minimizing 1 repulsion. The trifluoromethyl radical F3C is essentially tetrahedral and has a significant barrier to inversion of about 25 kcal mol - .39 In contrast, the methyl radical H3C itself is planar. Fluorine /J to the radical site is of minor structural consequence. Thus, the pcrfluoro-/er/-butyl radical exhibits a more planar geometry. 

Very recently Bartlett has found that irradiation of azo-bis-3-methyl-2-phenyl-2-butane, 45, in frozen benzene at 77°K produces a triplet species with an amazingly long lifetime, on the order of weeks.464 The ESR spectrum of the triplet species indicates a very weakly interacting pair of electrons about 7 A apart. If the solution is warmed, the triplet ESR spectrum is replaced by that of a monoradical. The triplet species most likely is the pair of 3-methyl-2-phenyl-2-butyl radicals held apart by a molecule of nitrogen but held rigid by the frozen medium. 

Optically active acrylic, chloro-acrylic and methacrylic esters of sec. butyl alcohol, 2-methyl-butyl alcohol, 1.3-dimethyl-butyl alcohol, 1-methyl-benzyl alcohol, bomeol and menthol have been polymerized mostly by radical mechanism (Tables 16, 17, 18). 

Lenhardt et al. [30] conducted the first direct study of the reaction of butyl radicals with 02, reporting room-temperature rate coefficients for n-butyl, s-butyl, t-butyl, and 3-hydroxy s-butyl, where the radicals were prepared by broadband flash photolysis of the iodides. The bimolecular rate coefficients were independent of pressure over the range 1 to 4 torr, showing that these association reactions are in the high-pressure limit. The rate coefficients increased in the order n-butyl < s-butyl < f-butyl < 5-hydroxy s-butyl. On the other hand, the CH3 + 02 association has been shown to be well into the unimolecular falloff at pressures from 0.5 to 6 torr at room temperature [62]. Falloff behavior is not unexpected for the smaller CH3 radical, in contrast with C4H9 radicals. Methyl was generated by 193-nm photolysis of nitromethane. 

Another interesting example belonging to the same general principle was described by Graham (56). On one hand he prepared an amine terminated polystyrene (sodium amide initiation in liquid ammonia) and showed that it contained only one terminal primary amine group per polymer chain. On the other hand copolymers were prepared by free-radical initiated solution copolymerization of small amounts of /S-iso-cyanatoethyl methacrylate with several other monomers as methyl, butyl and lauryl methacrylates, acrylonitrile and styrene. 

---