1.3-Butadiene electrophilic 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... 

Addition Reactions. 1,3-Butadiene reacts readily via 1,2- and 1,4-free radical or electrophilic addition reactions (31) to produce 1-butene or 2-butene substituted products, respectively. 

Conjugated dienes also undergo electrophilic addition reactions readily, but mixtures of products are invariably obtained. Addition of HBr to 1,3-butadiene, for instance, yields a mixture of two products (not counting cis-trans isomers). 3-Bromo-l-butene is the typical Markovnikov product of 1,2-addition to a double bond, but l-bromo-2-butene appears unusual. The double bond in this product has moved to a position between carbons 2 and 3, and HBr has added to carbons 1 and 4, a result described as 1,4-addition. 

Electrophilic addition to a conjugated diene at or below Toom temperature normally leads to a mixture of products in which the 1,2 adduct predominates over the 1,4 adduct. When the same reaction is carried out at higher temperatures, though, the product ratio often changes and the 1,4 adduct predominates. For example, addition of HBr to 1,3-butadiene at 0°C yields a 71 29 mixture of 1,2 and 1,4 adducts, but the same reaction carried out at 40 °C yields a 15 85 mixture. Furthermore, when the product mixture formed at 0 °C is heated to 40 °C in the presence of HBr, the ratio of adducts slowly changes from 71 29 to 15 85. Why ... 

The electrophilic addition of HBr to 1,3-butadiene is a good example of how a change in experimental conditions can change the product of a reaction. The concept of thermodynamic control versus kinetic control is a useful one that we can sometimes take advantage of in the laboratory. 

Figure 14.6 Energy diagram for the electrophilic addition of HBr to 1,3-butadiene. The 1.2 adduct is the kinetic product because it forms faster, but the 1,4 adduct is the thermodynamic product because it is more stable.

Electrophilic addition of Br2 to isoprene (2-methyl-l,3-butadiene) yields the following product mixture ... 

Addition reactions, 20 243. See also Electrophilic addition reactions aldehydes, 2 63-64 allyl alcohol, 2 234-239 butadiene, 4 368—370 carboxylic acids, 5 44-45 ethylene, 10 597—598 quinoline, 21 184 quinone, 21 246-261 toluene, 25 165... 

Diels-Alder adduct from cyclopentadiene, 8 222t Diels-Alder reactions of, 25 488-489 economic aspects of, 25 507-509 electrophilic addition of, 25 490 in ene reactions, 25 490 esterification of, 25 491 free-radical reactions of, 25 491 from butadiene, 4 371 Grignard-type reactions of, 25 491 halogenation of, 15 491—492 health and safety factors related to, 25 510-511... 

The electrophilic addition of nitrosyl chloride with 3-methyl-l,2-butadiene at 0°C afforded 2-nitroso-3-chloro-3-methyl-l-butene, in which the nitrosyl group was connected to the central carbon atom [10]. 

The electrophilic addition of Br2 or Cl2 to 2,3-butadiene nitrile afforded 3,4-dihalo-2-butenenitriles [261]. 

The industrial use of 1,3-dienes and of their electrophilic reactions has strongly stimulated the field in recent years. Because of the low cost of butadiene, abundantly available from the naphtha cracking process, very large scale applications in the synthesis of polymers, solvents and fine chemicals have been developed, leading to many basic raw materials of the modem chemical industry. For example, the primary steps in the syntheses of acrylonitrile and adiponitrile have been the electrophilic addition of hydrocyanic acid to butadiene24. Chlorination of butadiene was the basis of chloroprene synthesis25. 

This is comparabie to the eiectrophiiic addition of HBr to butadiene (see Section 8.2), though the addition is in the reverse sense overaii, in that Br adds before H in the radicai reaction, whereas H adds before Br in the ionic mechanism. As with the electrophilic addition, we shall nsnally obtain a mixtnre of the two prodncts. 

Elliott and Shepherd (70) have investigated the effect of Os(II) coordination on dienes and report that the metal acts as a protecting group for electrophilic addition of bromine. Thus, the action of bromine on [Os(NH3)5 T 2-l,2-(l,3-butadiene) ]2+ is thought to result in the 3,4-dibromo-1-butene analog. 

In addition to protection, a change of diene reactivity is effected by coordination to carbonyl. Butadiene forms the very stable complex 56 and its reactions are different from those of free butadiene. Electrophiles attack C(l) or C(4) of the complexed dienes, and reactions that are impossible with uncomplexed dienes now become possible. 

The composition of the product mixture formed upon electrophilic addition to conjugated dienes often changes with the temperature at which the reaction is conducted. For example, when HBr is added to 1,3-butadiene at — 80°C, the major product is formed by 1,2-addition. In contrast, when the reaction is run at 45°C, the major product is the one resulting from 1,4-addition ... 

The mechanism of electrophilic addition of HX involves two steps addition of H (from HX) to form a resonance-stabilized carbocation, followed by nucleophilic attack of X at either electrophilic end of the carbocation to form two products. Mechanism 16.1 illustrates the reaction of 1,3-butadiene with HBr. 

In many of the reactions we have learned thus far, the more stable product is formed faster—that is, the kinetic and thermodynamic products are the same. The electrophilic addition of HBr to 1,3-butadiene is different, in that the more stable product is formed more slowly— that is, the kinetic and thermodynamic products are different. Why is the more stable product formed more slowly ... 

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