Radicals isobutyl

It was shown that no rearrangement of isobutyl radical to tert-butyl radical (which would involve the formation of a more stable radical by a hydrogen shift) took place during the chlorination of isobutane. 

Comparison of the tert-butyl radical (a 3° radical) and the isobutyl radical (a 1° radical) relative to isobutene ... 

Figure 10.1 (a) Comparison of the potential energies of the propyl radical (+H ) and the isopropyl radical (+H ) relative to propane. The isopropyl radical — a 2° radical — is more stable than the 1° radical by 15 kJ mole-1, (b) Comparison of the potential energies of the tert-butyl radical (+H ) and the isobutyl radical (+H ) relative to isobutane. The 3° radical is more stable than the 1° radical by 29 kJ mole-1. 

Talbutal differs from butabarbital in that a sec-butyl radical is used in talbutal, whereas in butabarbital, an isobutyl radical can be used as a substituent on C,. Talbutal is used as a sedative, soporific drug for the same indications as butabital. Synonyms for this drug are profundol, lotusate, and others. 

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. 

If Reaction 6 occurred in our system, the products observed would be those resulting from the singlet oxygen-azoisobutane interaction, and the photo-oxidation results would not be relevant to reactions of the isobutyl radical with oxygen. 

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... 

Where X might be an alkoxyl radical RO. Previous work (20) on the reactions of isobutyl radicals indicate that Reaction 4 only becomes an important source of isobutene at. temperatures above 150°C. Thus, experimental evidence suggests convincingly that Reactions 1 and 4 would not contribute significantly to the isobutene yield. 

In another series of runs (Table II) the high intensity, full arc photolysis of azoisobutane-oxygen mixtures was studied to estimate the rate constant sum, k7 + kg. For the conditions of light intensity used, the 2,5-dimethylhexane product of the isobutyl combination reaction was formed in small but accurately measurable amounts for runs using moderate pressures of oxygen. If octane is formed only in Reaction 2, it can provide a useful monitor of the isobutyl radical concentration and allow k7 + ks to be estimated from these rate data. However, if this technique is to be employed, one must evaluate carefully the importance of the possible alternative source of octane through the primary process C. 

This might be the case since the full mercury arc was used in this series of experiments. Vibrationally nonequilibrated ethyl radicals were observed in azoethane photolysis at the shorter wavelength (5). It is expected that the rate constant for the reaction of a thermally equilibrated isobutyl radical with oxygen will be nearly equal to that for the ethyl radical. Certainly one should accept the present estimate of k7 + k as a lower limit of the value for thermally equilibrated isobutyl radicals. We plan to test the order of the reaction more extensively and redetermine k7 + kg using the flash photolysis technique employed in the methyl (21) and ethyl (5) radical studies with oxygen to shed further light on this problem. 

Palmer and Lossing (73) obtained analogous results with isobutane. Thus for iso-CiHio, the relative primary yields of terl-butyl to isobutyl radicals were found to be in the ratio, 7 1. However, when the tertiary hydrogen atom in the molecule was replaced by deuterium this ratio dropped to about 1 2. 

It will now be instructive to examine the n-butane reaction (76). In this case the reaction follows almost exclusively a single path leading to the formation of sec-butyl radicals. The percentage of the quenching done by the two methylene groups is very nearly the same as that for the tertiary C-H bond in isobutane (i.e. >90%). However, the primary yield of w-butyl radicals ( 2%) from w-butane is decidedly less than that for isobutyl radicals ( " 14%) from isobutane. This behavior can be readily interpreted on the basis of a cyclic transition-state structure, but not with an open-chain transition state. For the two reaction sequences, we may write ... 

An attack of methyl radical on propene produces predominantly butyl (90%), but also the isobutyl radical [127]. In additions to higher alkenes, neither of the two C atoms of the double bond is preferred by the methyl radical, which lacks electrophilic character. The relative reactivity of methyl with respect to ethylene, propene, 1-butene, and 2-methylpropene is roughly equal [128],... 

The second chain is again initiated by H or CH3 but goes through the isobutyl radical, —CH2—CH(CH3)2, which can only stabilize itself in a simple way by splitting off CH3 (reactions 5, 8, and 6 ). Its stoichiometry is represented by isobutane —> CH4 + C3TI6. From the preponderance of H2 and C4H8 as products of the reaction it would seem that the principal mode of attack of H or CH3 on isobutane is to abstract the tertiary H atom and produce the -butyl radical (reactions 5, 8). ... 

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. 

Yet how do we know, in this case, that every isobutyl radical that is formed ultimately yields a molecule of isobutyl chloride Suppose some isobutyl radicals were to change- by rearrangement of atoms—into /erZ-butyl radicals, which then react with chlorine to yield /cr/-butyl chloride. This supposition is not so far-... 

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. 

Problem 3.18 (a) What results would have been obtained if some isobutyl radicals had rearranged to rerr-butyl radicals (b) Suppose that, instead of rearranging, isobutyl radicals were, in effect, converted into tert-b xiy radicals by the reaction... 

All three include the C4H9 radical and can decompose spontaneously (or collision-ally) to eliminate either the primary (-CH2-CH(CH3)2) or secondary (-CH(CH3)-CH2-CH3) isobutyl radical. Consequently, several differences are observed in the MIKE/CAD spectra. However, the greatest differences appear in the decomposition spectra of m/z 103 ion (Table 22), which is consistently present when the polypeptide has an He or Leu residue in terminal position. 

The difference in behavior of ethylene and acetylene with triisobutylalane is shown by the fact that, with ethylene, the isobutyl radical is removed very easily as isobutene. 

The results for n-butyl, sec.-butyl and isobutyl radicals have been recalculated from the original data on the assumption that log(fc/l mole sec ) = 8.6 for the combination reactions of these radicals (see text). 

Coming to the reaction of methyl radicals with propene, there are two sites for attack, producing secondary butyl or isobutyl radicals. Miyoshi and Brinton [164] analysed their products and estimated that about 90% of the attack is on the terminal carbon. The reported rate coefficients in Table 48 are composites of terminal and non-terminal attack, but the relatively small amount of the latter means the rates are very close to those for addition to the terminal carbon atom. In this case also, Kerr and... 

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