2- Methylpentane bromination

Using Table III in Ref. 68 (p. 692), calculate the expected product composition from the gas-phase photochemical chlorination and bromination of 3-methylpentane under conditions (excess hydrocarbon) in which only monohalogenation would occur. 

The bromination of propane C3H8 proceeds via a free radical chain reaction. The dominant major product is HBr, with 2-bromopropane produced in substantial amounts. 1-bromopropane, though produced in much smaller amounts, can still be classified as a major product. 2 3-dimethylbutane is a minor product, and hexane and 2-methylpentane are trace products. 

Because of the greater selectivity of the bromine atom, radical brominations can be useful in synthesis as long as the compound to be brominated has one hydrogen that is considerably more reactive than the others. The reaction of 2-methylpentane with bromine, shown previously, gives predominantly a single product because there is only one tertiary hydrogen. Because allylic and benzylic radicals are stabilized by resonance, bromination at these positions can also be successfully accomplished. An example is provided by the following equation ... 

In a stereochemical study, it was shown that the free radical ring cleavage of all-m-1,2,3-trimethylcyclopropane (10) by bromine proceeded with inversion of configuration at one center and by nonstereospecific reaction at the other center. Equal amounts of S)-meso-2,4-di-bromo-3-methylpentane and tf/-2,4-dibromo-3-methylpentane were obtained by this reaction. The reaction of the corresponding trans derivative was much slower than that of the all-cis compound. 

Bromine 2-Bromo-2-methylpentane Hydrogen (76% isolated yield) bromide... 

Scheme 11.4 illustrates some representative halogenation reactions. The reaction in Entry 1 was conducted by slow addition of bromine to excess 2-methylpentane at 60°C, with irradiation from a tungsten light bulb. The reaction in Entry 2 is a typical benzylic bromination, carried out at 125°C with irradiation from a sun lamp. Entries 3 and 4 are examples of NBS bromination using benzoyl peroxide as the initiator. Entry 3 is interesting in that none of the allylic isomer 2-bromo-3-heptene is found. Entries 5 and 6 are examples of chlorination by f-butyl hypochlorite in which the f-butoxy radical is the chain carrier. Note that in Entry 6, both the primary and secondary allylic products are formed. The reaction in Entry 7 uses sulfuryl chloride as the halogenation reagent. Note that in contrast to chlorination with CI2 (see p. 1021), the reaction shows selectivity for the benzylic position. 

Methyl-2-pentene added with stirring under Ng to slightly more than the equivalent amount of 9-borabicyclo[3.3.1]nonane in tetrahydrofuran, refluxed 1 hr., tetrahydrofuran replaced by methylene chloride, cooled to 0, bromine added during 1 min. in the dark by syringe through a septum inlet, stirred 0.5 hr. at 0° and 1 hr. at 25°, all in the dark 2-bromo-4-methylpentane. Y 74%. -Similarly 2-Methylbut-2-ene 2-bromo-3-methylbutane. Y 88% by GLC. F. e. s. C. F. Lane and H. C. Brown, J. Organometal. Chem. 2(5, C 51 (1971). 

Bargamov and Bargamova reported on new polyflnorinated l-amino-1,2,3-triazoles 60 which were obtained by oxidation of the bis-hydrazones of aliphatic polyfluorinated a-dicarbonyl componnds with snlfnryl chloride, bromine, or selenium dioxide in an aprotic solvent [64, 65]. In a later article these anthors described an oxidation of a dihydrazone of l,l,l,5,5,5-hexafluoro-4-triflnoro-methylpentane-2,3-dione in the presence of the H2SO4 - P2O5 mixtnre (molar ratio 3 1) to 4-(l,l,l,3,3,3-hexaflnoroisopropyl)-5-triflnoromethyl-2/f-l,2,3-tri-azole 61 (no yield was given) [66]. 

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