Olefins, hydrobromination

Commercial Olefin Reactions. Some of the more common transformations involving a-olefins ia iadustrial processes iaclude the oxo reaction (hydroformylation), oligomerization and polymerization, alkylation reactions, hydrobromination, sulfation and sulfonation, and oxidation. 

A considerable amount of hydrobromic acid is consumed in the manufacture of inorganic bromides, as well as in the synthesis of alkyl bromides from alcohols. The acid can also be used to hydrobrominate olefins (qv). The addition can take place by an ionic mechanism, usually in a polar solvent, according to Markownikoff s rule to yield a secondary alkyl bromide. Under the influence of a free-radical catalyst, in aprotic, nonpolar solvents, dry hydrogen bromide reacts with an a-olefin to produce a primary alkyl bromide as the predominant product. Primary alkyl bromides are useful in synthesizing other compounds and are 40—60 times as reactive as the corresponding chlorides (6). 

Examples of the first type are to be found in the determination of the heats of formation of olefins. For example. Prosen and Rossini give the heats of formation of a number of olefines in the ideal gas state, calculated from the known heats of combustion of the paraffins and the known heats of hydrogenation of the olefins 3. 27 q It is possible to compare some of these results with heats of combustion measured directly. For instance, Coops et al. give for - A//R( z-heptene-l 1.) 1,111-5 kcal (corrected to the more recent benzoic acid result) whereas Prosen and Rossini s value is 1,113 6 kcal (corrected to the liquid state using the value given for the latent heat of vaporization by Bent et al. ) a difference of 2 kcal, or 0-2 per cent. The differences for the pentenes are less than this. On the other hand, the differences between the heats of formation of isomers are known better than this. The accuracy of these differences obtained by hydrogenation calorimetry has been confirmed by some recent hydrobromination calorimetry Lacher et have measured the vapour phase heats of hydro-... 

Since the bromine adds to the least-substituted carbon atom of the bond, thus generating the more-substituted radical intermediate, the regiospecificity of radical-chain hydrobromination of olefins is opposite that of ionic addition. The early work on the mechanism of the reaction was undertaken to understand why Markow-nikoff s rule was violated under certain circumstances. Anti-Markownikoff additions were eventually traced to reaction conditions under which peroxides or light were causing initiation of the radical-chain process. The radical chain addition of hydrogen bromide to olefins is a synthetically useful reaction, as illustrated by entries 1 and 2 in Scheme 12.4. 

A novel way of studying a partially reacted polydiene is to deconnpose it in a metathesis reaction with a small olefin then to separate and identify the low molecular weight products by standard methods such as gas chromatography-mass spectrometry. This method has been applied to polybuta-1,4-diene partially aralkylated or hydrobrominated, to confirm the structure and position of the substituents. " The small olefin was oct-4-ene and the metathesis was promoted by a WCU/Sn(CH3)4 catalyst. The products of such a metathesis breakdown are, of course, dependent also on the nature and distribution of double bonds within the parent polymer. ... 

Brown, H. C., Lane, C. F. 1970. Base-induced reaction of organoboranes with bromine—K convenient procedure for anti-Markovnikov hydrobromination of terminal olefins via hydroboration-bromination. J. Am. Chem. Soc. 92 6660-6661. 

Photochemical bromination and hydrobromination and nickel-catalysed c/j-hydrogenation of the symmetrical acetylene (44) to give olefinic derivatives presents no difficulties. ... 

---