Isobutene, reaction with hydrogen bromid

With ethyl bromide as solvent a brief, exploratory study [77] showed that the rates and DPs were more irreproducible than with other alkyl halides, and this was ascribed, at least partly, to the relatively high dissociation constant of ethyl bromide to ethylene and hydrogen bromide. No evidence was obtained whether ethyl bromide itself is a co-catalyst, and the putative co-catalyst in the reaction was residual water and possibly traces of t-BuBr formed from HBr and isobutene. Experiments with [C4H8] = 0.11-0.92 mole/1, [TiCl4] (2.7-11.2) x 103 mole/1, T = -38° to -103° showed that EDP = 5.5 0.5 kcal/ mole that below about -60° the DP is almost independent of monomer concentration and that kjkp = 2 x 10 4 at -63° and 8 x 10"5 at -71°. 

In the polar reaction, a proton in HBr first adds to the terminal sp2 carbon in isobutene to produce a stable tert-butyl cation (8), and then it reacts with the counter bromide anion to form tert-butyl bromide. Thus, the proton in HBr adds to the less substituted sp2 carbon in alkene to produce a more stable carbocation. This is based on the Markovnikov rule. In radical reactions, the hydrogen atom of HBr is abstracted first by the initiator, PhCO (or Ph ) derived from (PhC02)2, and the formed bromine atom then adds to the terminal sp2 carbon in isobutene to form the stable (3-bromo tert-butyl radical (9), and then it reacts with HBr to produce /so-butyl bromide and a bromine atom. This bromine atom again... 

Free-radical Reactions.—Publications have appeared dealing with the bidirectional addition of trifluoroiodomethane and hydrogen bromide across the C=C bond in the olefin CF3 CH CHMe, peroxide-initiated addition of 1,2-dibromotetrafluoroethane to ethylene, propene, and isobutene, the addition of pentafluoroiodoethane to 3,3,4,4-tetrafluorohexa-l,5-diene (see p. 29), peroxide-initiated cyclodimerization of 3,3,4,4-tetrafluoro-4-iodobut-l-ene (see p. 29), and telomers from tribromofluoromethane or tetrabromomethane and bromotrifluoroethylene as high-density fluids for gyroscope flotation. The telomerization of chloromethanes with tetra-fluoroethylene provides a measure of the relative reactivity for both chlorine and hydrogen abstraction by the CF2 CF2 radical. The chain-transfer... 

A ubiquitous co-catalyst is water. This can be effective in extremely small quantities, as was first shown by Evans and Meadows [18] for the polymerisation of isobutene by boron fluoride at low temperatures, although they could give no quantitative estimate of the amount of water required to co-catalyse this reaction. Later [11, 13] it was shown that in methylene dichloride solution at temperatures below about -60° a few micromoles of water are sufficient to polymerise completely some decimoles of isobutene in the presence of millimolar quantities of titanium tetrachloride. With stannic chloride at -78° the maximum reaction rate is obtained with quantities of water equivalent to that of stannic chloride [31]. As far as aluminium chloride is concerned, there is no rigorous proof that it does require a co-catalyst in order to polymerise isobutene. However, the need for a co-catalyst in isomerisations and alkylations catalysed by aluminium bromide (which is more active than the chloride) has been proved [34-37], so that there is little doubt that even the polymerisations carried out by Kennedy and Thomas with aluminium chloride (see Section 5, iii, (a)) under fairly rigorous conditions depended critically on the presence of a co-catalyst - though whether this was water, or hydrogen chloride, or some other substance, cannot be decided at present. 

The reaction is faster with electron-rich alkenes. The rate constants for addition of bromine to a series of alkenes were found to increase in the order ethene < propene < 2-butene isobutene < 2-methyl-2-butene. In other words, each methyl group that replaces a hydrogen atom on ethene increases reactivity. The addition of bromine to substituted ethenes in methanol with added sodium bromide can be correlated to the equation... 

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