Polar radicals. 9. Kinetics and mechanism of the vapor-phase bromination of 1-bromobutane

Abstract

The vapor-phase photobromination of 1-bromobutane with molecular bromine (5:1) yields 11 products resulting from direct substitution or the elimination of a bromine atom from the 1 -(bromomethyl)propyl radical. The 1 -butene resulting from elimination undergoes addition, and allylic substitution and addition to yield polyhalogenated materials. The major portion of the bromine (65% ) produced 1,2,3-tribromobutane and 1,2,3,4-tetrabromobutane. Bromination with bromine-81 produced isotopically enriched products which were consistent with the proposed mechanism and showed that 33% of the 1,2-dibromobutane formed arose from the elimination-readdition process. Vapor phase photobromination carried out to partial completion with high concentrations of bromine and hydrogen bromide resulted in only minimal amounts of 0 scission of the 1 -(bromomethyl)propyl radicals. Under these conditions the bromination of perdeuterio-1-bromobutane allowed the determination of the relative rates of transfer, with bromine and hydrogen bromide, of the radicals formed in the reaction and further allowed the determination of the relative rates of abstraction at each position in the molecule. The relative rates of transfer and abstraction indicate that the radical formed 0 to the electronegative bromine atom is stabilized (i.e., more easily formed and less reactive with hydrogen bromide) compared to those in the other positions in the molecule.

A reinvestigation of the vapor phase bromination of 2-bromobutane

“…The conclusions from these observations are identical with those reached for the brornination of 1 -bromobutane under comparable conditions (20). It can be concluded that the a-bromoalkyl radical is stable under these conditions.…”

“…Such an allylic bromination pathway could account for the 1,2,3-tribromobutane, the unsaturated bromides, and the saturated polyhalogenated materials. A similar mechanism was proposed for the vapor phase bromination of 1-bromobutane (20). The fate of the p-bromoalkyl radical can be affected by either the temperature of the system or the concentration of the transfer agents.…”

“…There are numerous precedents for elimination of a bromine atom from p-bromoalkyl radicals (3,(8)(9)(10)(11)(12)(13)(14)(15). This elimination is responsible for the rapid vapor phase isomerization of transand cis-alkenes during photoinitiated bromination (16)(17)(18)(19)(20). It is also well documented that allylic bromides are formed in high yields in the bromination of alkenes when the bromine concentration is kept low (17).…”

“…First, the a and p radicals can both competitively transfer with bromine and hydrogen bromide to yield dibromide and starting material, the ratio depending upon the relative ratio of their rate constants and the concentration of the two transfer reagents. By analogy with the bromination of 1-bromobutane (20), where transfer with hydrogen bromide affects the a radical more strongly than it does the P radical, it is predicted that the presence of added HBr in the reaction mixture would lower the ratio of a/@ products. However, the second process which regenerates starting material from 2-butene (a product from the P radical) with hydrogen bromide would cause an apparent increase in the ratio of a/p products.…”

The relative rates of bromination of cyclohexane and cyclopentane with molecular bromine. Comparison of the reactions in solution and in vapor phase

“…4 Studies on the bromination of ~h b r o b u t a n e (5), and 1-br~mobutane (6) in the several model compounds, cyclohexane (I), 1-vapor phase, where neither the cage reaction nor the transfer reaction with hydrogen tribromide are im- 'Author to whom correspondence may be addressed.…”