Electrophilic Additions to Conjugated Dienes Allylic Carbocations

Rank a conjugated diene, a nonconjugated diene, and an allene in order of stability. 

2 i Electrophilic Additions to Conjugated Dienes Allylic Carbocations 

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. 

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

How can we account for the formation of 1,4-addition products The answer is that allylic carbocations are involved as intermediates. When 1,3-butadiene is protonated, two carbocation intermediates are possible a primary carbo-cation and a secondary allylic cation (recall that allylic means next to a double bond ). Since an allylic cation is stabilized by resonance between two forms (Section 11.9), it is more stable and forms faster than a nonallylic carbocation. 

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Electrophilic additions to conjugated dienes usually involve allylic cations as intermediates. Unlike simple carbocations, an allylic cation can react with a nucleophile at either of its positive centers. Let s consider the addition of HBr to buta-1,3-diene, an electrophilic addition that produces a mixture of two constitutional isomers. One product, 3-bromobut-l-ene, results from Markovnikov addition across one of the double bonds. In the other product, 1 -bromobut-2-ene, the double bond shifts to the C2—C3 position. 

As with alkenes, the regioselectivity of electrophilic addition to conjugated dienes is governed by the stability of the resulting carbocation. Protonation of a conjugated diene always occurs at the end of the diene unit because an allylic carbocation results. 

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. 

Electrophilic addition of HCJ to a conjugated diene involves the formation of allylic carbocation intermediates. Thus, the first step is to protonate the two ends of the diene and draw the resonance forms of the two allylic carbocations that result. Then... 

The mechanism of 1, 4-addition starts off in the same way as a normal electrophilic addition. We shall consider the reaction of a conjugated diene with hydrogen bromide as an example (Following fig.). One of the alkene units of the diene uses its n electrons to form a bond to the electrophilic hydrogen of HBr. The H-Br bond breaks at the same time to produce a bromide ion. The intermediate carbocation produced has a double bond next to the carbocation centre and is called an allylic carbocation. 

Like the electrophilic addition of HX to an alkene, the addition of HBr to a conjugated diene forms the more stable carbocation in Step [1], the rate-determining step. In this case, however, the carbocation is both 2° and allylic, and thus two Lewis structures can be drawn for it. In the second step, nucleophilic attack of Br can then occur at two different electrophilic sites, forming two different products. 

If the conjugated diene is not symmetrical, the major products of the reaction are those obtained by adding the electrophile to whichever terminal sp carbon results in formation of the more stable carbocation. For example, in the reaction of 2-methyl-l,3-butadiene with HBr, the proton adds preferentially to C-1 because the positive charge on the resulting carbocation is shared by a tertiary allylic and a primary allylic carbon. Adding the proton to C-4 would form a carbocation with the positive charge shared by a secondary allylic and a primary allylic carbon. Because addition to C-1 forms the more stable carbocation, 3-bromo-3-methyl-l-butene and l-bromo-3-methyl-2-butene are the major products of the reaction. 

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