Dibromo-l-butene

Many7 other electrophiles besides HBr add to conjugated dienes, and mixtures of products are usually formed. For example, Br2 adds to 1,3-butadiene to give a mixture of l,4-dibromo-2-butene and 3,4-dibromo-l-butene. 

From the advent of organic chemistry, dienes (and polyenes) have played a very important role in both the theoretical and synthetic aspects. For example, 1,4-addition of bromine to 1,3-butadiene to form l,4-dibromo-2-butene rather than 3,4-dibromo-l-butene as the major product was a challenging problem for theoretical chemists, who interpreted the phenomenon in terms of resonance or delocalization of jr-electrons1. 

When 1,3-butadiene is treated with bromine under similar conditions, there is obtained not only the expected 3,4-dibromo-l-butene, but also l,4-dibromo-2-butene. Treatment with HCI yields not only 3-chloro-l-butene, but also 1-chlorot 2-butene. Hydrogenation yields not only 1-butene but also 2-butene,... 

Allylic bromides are much more labile than the corresponding chlorides, and isomerize to equilibrium mixtures unless special precautions are taken. The isomerization reactions are catalyzed by salts, Lewis acids and free radicals, and may occur in the absence of catalysts. Hatch et found that isomerization of 3,4-dibromo-l-butene to /ra/75-I,4-dibromo-2-butene in /7-decane, viz. 

Another common reaction of alkenes uses diatomic halogens such as bromine (Br2) to form 1,2-dibromides (see Chapter 10, Section 10.4.1). In this reaction, the alkene reacts as a Lewis base with the bromine atom to form a bromonium ion. When 1,3-butadiene (3) reacts with bromine, both 1,2 and 1,4 addition products are formed, just as with the HBr reaction. The products are 3,4-dibromo-l-butene (32) and a mixture otE- and Z-l,4-dibromo-2-butene (33 and 34). Initial reaction with bromine gives bromonium ion 29 however, when this reacts with bromide ion, there are two sites for reaction. If bromide attacks the less stericaUy hindered carbon atom, the product is 32, but the bromine ion may also attack the C=C unit to give products 33 + 34. Nucleophilic attack of this type is called an Sj reaction (nudeophilic substitution at an allylic carbon with displacement of the leaving group). 

Answer The allylic carbocation is resonance stabilized, so it is more stable than a cyclic bromonium ion. The products are 3,4-dibromo-l-butene and 1,4-dibromo-2-butene... 

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