Isobutane bromination

Autoxidation of alkanes generally promotes the formation of alkyl hydroperoxides, but d4-tert-huty peroxide has been obtained in >30% yield by the bromine-catalyzed oxidation of isobutane (66). In the presence of iodine, styrene also has been oxidized to the corresponding peroxide (44). 

We have already seen that some radicals are much more selective than others (p. 944). The bromine atom is so selective that when only primary hydrogens are available, as in neopentane or ferf-butylbenzene, the reaction is slow or nonexistent and isobutane can be selectively brominated to give terf-butyl bromide in high yields. 

If the particular reaction studied is the unimolecular decomposition of a free radical, such as (3), then the use of a trap will enable the effective concentration of the radical to be measured. A radical trap will indicate the presence or absence of a free radical reaction and may sometimes provide evidence for a partly or entirely molecular reaction. Rate data for free radical reactions are derived assuming the occurrence of a steady state concentration of radicals. The time required to produce a steady state concentration of methyl radicals in the pyrolysis of AcH is shown for various temperatures in Fig. 1. Realistic values for rate coeflBcients may be obtained only if the time of product formation is long compared to the time to achieve the steady state concentrations of the radicals concerned. Thus deductions from the results from the bromination of isobutane , neopentane , and toluene have been criticised on the grounds that a steady state concentra-... 

ISOBUTANAL (78-84-2) Forms explosive mixture with air (flash point — 1°F/ — 18°C). Oxidizes slowly in air, forming isobutyric acid. Violent reaction with strong oxidizers, strong acids, bromines, ketones. Incompatible with caustics, ammonia, amines. 

In isobutane, for example, there are one 3 and nine I hydrogens. If we desire to halogenatc at the 3° center, the natural selectivity of bromine makes it the obvious halogen to choose. If we desire to halogenate a I carbon, a less selective, more reactive halogen would allow us to take best advantage of the statistical factor of nine possible I hydrogens available to be replaced in each molecule. Thus,... 

Two methods are used to determine bromine index of aromatic hydrocarbons which contain trace amounts of olefins and are substantially free of materials lighter than isobutane and have distillation end-point lower than 288 C. The meteods measure trace amounts of unsaturations in materials which have a bromine index below 500. 

The reaction is often initiated by photolysis of bromine. The hydrogen-abstraction step is rate-limiting, and the product composition is governed by the selectivity of the hydrogen abstraction. The enthalpy requirement for abstraction of hydrogen from methane, ethane (primary), propane (secondary), and isobutane (tertiary), by bromine atoms are -1-16.5, -1-10.5, -1-7.0, and -1-3.5 kcal/mol, respectively. These differences are reflected in the activation energies, and there is a substantial kinetic preference for hydrogen abstraction in the order tertiary > secondary > primary. Structural features that promote radical stability by delocalization, such as phenyl, vinyl, or carbonyl substituents, also lead to kinetic selectivity in radical brominations. 

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