Diene, conjugated electrophilic addition reactions

One of the most striking differences between conjugated dienes and typical alkenes is in their electrophilic addition reactions. To review briefly, the addition of an electrophile to a carbon-carbon double bond is a general reaction of alkenes (Section 6.7). Markovnikov regiochemistry is found because the more stable carbo-cation is formed as an intermediate. Thus, addition of HC1 to 2-methylpropene yields 2-chloro-2-methylpropane rather than l-chloro-2-methylpropane, and addition of 2 mol equiv of HC1 to the nonconjugated diene 1,4-pentadiene yields 2,4-dichloropentane. 

Interactive to use a web-based palette to predict products from electrophilic addition reactions to conjugated dienes. 

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. 

Predicting the Product of sn Electrophilic Addition Reaction of a Conjugated Diene... 

Conjugated dienes undergo several reactions not observed for nonconjugated dienes. One is the 1,4-addition of electrophiles. When a conjugated diene is treated with an electrophile such as HCl, 1,2- and 1,4-addition products are formed. Both are formed from the same resonance-stabilized allylic carbocation intermediate and are produced in varying amounts depending on the reaction conditions. The L,2 adduct is usually formed faster and is said to be the product of kinetic control. The 1,4 adduct is usually more stable and is said to be the product of thermodynamic control. 

The amount of 1,2- and 1,4-addition products formed in the electrophilic addition reactions of conjugated dienes depends greatly on the reaction conditions. 

Section 8.6 Electrophilic Addition Reactions of Conjugated Dienes... 

When a conjugated diene undergoes an electrophilic addition reaction, two factors— the temperature at which the reaction is carried out and the structure of the reactant—determine whether the 1,2-addition product or the 1,4-addition product will be the major product of the reaction. 

Electrophilic addition is the characteristic reaction of alkenes, and conjugated dienes undergo addition with the same electrophiles that react with alkenes, and by similar mechanisms. Hydrogen chloride, for example, adds to the diene unit of 1,3-cyclopentadiene to give 3-chlorocyclopentene. Mechanism 10.3 is analogous to the electrophilic addition of HCl to alkenes. 

Conjugated dienes undergo two-step electrophilic addition reactions just like simple alkenes (Section 6.3). However, certain features are unique to the reactions of conjugated dienes. 

However, in contrast to compounds with either isolated or cumulative double bonds, electrophilic addition reactions with alkenes that contain conjugated n-systems routinely produce products resulting from the interaction of both double bonds. While hydroboration appears to be an exception, with each double bond reacting separately (and the second more rapidly than the first), conjugated dienes, such as 1,3-butadiene (CH2=CH-CH=CH2), suffer addition across each double bond ( 1,2-addition ) as well as across the entire conjugated system ( 1,4-addition ). Commonly, the products ( 1,2-addition and 1,4-addition ) are formed concurrently. 

Besides the addition of halides and hydrogen-halide adds to alkenes or alkynes, other industrially relevant electrophilic addition reactions involve hydratization reactions (addition of water to alkenes and alkynes, forming alcohols), cationic polymerization (addition of carbocation to an alkene), hydrogenation (addition of hydrogen to alkenes to form alkanes), and Diels-Alder reactions (addition of an alkene to a conjugated diene to form complex, unsaturated hydrocarbon structures). 

---