1.3- Butadiene hydrogen bromide addition

Both resonance forms of the allylic carbocation from 1 3 cyclopentadiene are equivalent and so attack at either of the carbons that share the positive charge gives the same product 3 chlorocyclopentene This is not the case with 1 3 butadiene and so hydrogen halides add to 1 3 butadiene to give a mixture of two regioisomeric allylic halides For the case of electrophilic addition of hydrogen bromide at -80°C... 

When the major product of a reaction is the one that is formed at the fastest rate we say that the reaction is governed by kinetic control Most organic reactions fall into this category and the electrophilic addition of hydrogen bromide to 1 3 butadiene at low temperature is a kmetically controlled reaction... 

When however the ionic addition of hydrogen bromide to 1 3 butadiene is car ried out at room temperature the ratio of isomeric allylic bromides observed is differ ent from that which is formed at — 80°C At room temperature the 1 4 addition product predominates... 

FIGURE 10.8 Energy diagram showing relationship of kinetic control to thermodynamic control in addition of hydrogen bromide to 1,3-butadiene. 

Conjugated dienes also undergo addition reactions by radical-chain mechanisms. Here, the addition product almost always is the 1,4 adduct. Thus radical addition of hydrogen bromide to 1,3-butadiene gives l-bromo-2-butene, presumably by the following mechanism ... 

In one of the earliest investigations of regioselectivity in radical addition reactions to polyenes, the addition of hydrogen bromide to 1,3-butadiene was observed to yield mainly the 1,4-addition product in the presence of peroxides. The preference for attack at the Cl position of 1,3-butadiene has subsequently been observed for a large number of radicalsOnly for the addition of the methyl radical has the ratio of addition to the Cl vs C2 actually been measured. A value of Cl C2 = 1.0 0.01 has been found . For all other cases, products arising from attack at C2 have not been reported. This is also true for radical addition to 2,3-dimethyl-l,3-butadiene . Additions to 1,3-pentadiene occur predominantly at the Cl position due to the steric effect exerted by the terminal methyl This is a reflection of the reduced... 

Allyl radicals substituted at only one of the terminal carbon centers usually react predominantly at the unsubstituted terminus in reactions with nonradicals. This has been shown in reactions of simple dienes such as butadiene, which react with hydrogen bromide, tetrachloromethane or bromotrichloromethane to yield overall 1,4-addition products . The reaction of allyl radicals with hydrogen donors such as thiols or tin hydrides has been investigated and reviewed repeatedly. In most cases, the thermodynamically more favorable product is formed predominantly. This accords with formation of either the higher substituted alkene or the formation of conjugated tt-systems. Not in all cases, however, is the formation of the thermodynamically more favorable product identical to overall 1,4-addition to the diene. In those cases in which allyl radicals are formed through reaction of dienes with tin hydrides or thiols, the... 

Conjugate addition. See also Michael reaction of bromine to 1,3-butadiene, 382 of hydrogen bromide to 1,3-butadiene, to 0 ,p-unsaturated aldehydes and ketones, 722-725, 728-729, 846-847, 852 Conjugation... 

The energy diagram of Figure 10.9 illustrates kinetic and thermodynamic control in the addition of hydrogen bromide to 1,3-butadiene. At low temperature, addition takes place irreversibly. Isomerization is slow because insufficient thermal energy is available to permit the products to surmount the energy barrier for ionization. At higher temperatures isomerization is possible, and the more stable product predominates. 

Butadiene shows 1,4-addition reactions with electrophilic reagents other than hydrogen chloride. Two examples are shown here, the addition of hydrogen bromide (in the absence of peroxides) and the addition of bromine ... 

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