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Introduction to Addition Reactions of Dienes

Isolated dienes' addition reactions proceed less or more as supposed from the behavior of simple alkenes. So, if 1,5-hexadiene's one molar equivalent is treated with one equivalent of bromine a mixture of the 5,6-dibromo-1-hexene, 1,2,5,6-tetrabromohexane and unreacted diene is obtained, with the dibromo compound being the important product (about 50%).

CH2=CH(CH2)2CH=CH2 + Br2  →  BrCH2CHBr(CH2)2CH=CH2 + BrCH2CHBr(CH2)2CHBrCH2Br + CH2=CH(CH2)2CH=CH2

                                                  5,6-dibromo-1-hexene        1,2,5,6-tetrabromohexane         1,5-hexadiene

Identical reactions of conjugated dienes, in contrast, frequently give products that are not expected. The addition of bromine to 1,3-butadiene is an illustration. As displayed below in the equation, a approximately 50:50 mixture of 3,4-dibromo-1-butene and 1,4-dibromo-2-butene (mainly the E-isomer) is obtained. The latter compound is notable in that the remaining double bond is found in a location where there was no double bond in the reactant. This interesting relocation needs an demonstration.

CH2=CH-CH=CH2 + Br2  →  BrCH2CHBr-CH=CH2 +   BrCH2CH=CHCH2Br                                                                                   3,4-dibromo-1-butene    1,4-dibromo-2-butene

The supposed addition product from reactions of this type is the result of 1,2-addition, that is bonding to the adjacent carbons of double bond. The not expected product comes from 1,4-addition, that is bonding at the terminal carbon atoms of a conjugated diene with a shift of the remaining double bond to the 2,3-location. These numbers denote to the four carbons of the conjugated diene and are not IUPAC nomenclature numbers. Product compositions are frequently temperature dependent, like the addition of HBr to 1,3-butadiene illustrates.

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Bonding of an electrophilic group or atom to one of the end carbon atoms of a conjugated diene generates an allyl cation intermediate. By charge delocalization, Such type of cations are stabilized and it is this delocalization that accounts for the 1,4-addition product generated in such type of addition reactions. The positive charge is distributed over carbons no.2 and no.4 therefore it is at these sites that the nucleophilic component bonds, as displayed in the picture. Note: the allyl cation's resonance stabilization of is greater than comparable stabilization of 1,3-butadiene, since in the former charge is delocalized but separated and created in the latter..

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A description for the temperature affects is displayed in the following energy picture for the addition of HBr to 1,3-butadiene. The primary step in which a proton bonds to carbon no.1 is the rate determining step, like indicated by the large activation energy (light gray arrow in the diagram). The second faster step is the product determining step and there are two reaction paths (colored blue for 1,2-addition and magenta for 1,4-addition in the diagram). The 1,2-addition has a lower activation energy than 1,4-addition, but then the 1,2-product the 1,4-product is more stable. At higher temperatures, equilibrium is established among the products and thermodynamically favored 1,4-product dominates. At low temperatures, the products are created irreversibly and reflect the relative rates of the two competing reactions. This is termed as the kinetic control.

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