organic chem

Topics: Chemical bond, Carbon-carbon bond, Covalent bond Pages: 8 (3526 words) Published: October 29, 2014
Electrophilic Addition to Conjugated Dienes Conjugated dienes undergo two-step electrophilic addition reactios just as do simple alkenes. However, certain features are unique to the reactions of conjugated dienes Addition of one equivalent of HBr to 1,3-butadiene at -78C gives a mixture of two constitutional isomers 3-bromo-1-butene and 1-bromo-2-butene. The designations 1,2- and 1,4- used here to describe additions to conjugated dienes do not refer to IUPAC nomenclature. Rather, they refer to the four-atom system of two conjugated double bonds and indicate that addition takes place at either carbons 1 and 2 or 1 and 4 of the four-atom system. The bromobutenes formed by addition of 1 mole of HBr to butadiene can in turn undergo addition of a second mole of HBr to give a mixture of dibromobutenes. Our concern at this point is only with the products of a single addition of HBr. Addition of one equivalent of Br2 at -15C also gives a mixture of 1,2-addition and 1,4-addition products. We can account for the formation of isomeric products in the addition of HBr in the following way. Electrophilic addition is initiated by reaction of a terminal carbon of one of the double bonds with HBr to form an allylic carbocation intermediate best represented as a hybrid of two contributing resonance structures. The addition is completed by rapid reaction of the allylic cation with bromide ion. Reaction at one carbon bearing partial positive charge gives the 1,2-addition product reaction at the other gives the 1,4-addition product. Mechanism Kinetic versus Thermodynaic Control of Electrophilic Addition Electrophilic Addition to conjugated dienes gives a mixture of 1,2-addition and 1,4-addition products. Additional experimental observations about the products of electrophilic additions to 1,3-butadiene. For addition of HBr at -78C and addition of Br2 at -15C, the 1,2-addition product predominates over the 1,4-addition product. Generally at lower temperatures, the 1,2-addition products predominate over 1,4-addition products. For addition of HBr and Br2 at higher temperatures (generally 40-60C), the 1,4-addition products predominate. If the products of low temperature addition are allowed to remain in solution and be warmed to a higher temperature, the composition of the products changes over time and becomes identical to the products obtained when the reaction is carried out at higher temperatures. The same result can be accomplished at the higher temperature in a far shorter time by adding a Lewis acid catalyst, such as FeCl3 or ZnCl2, to the mixture of low temperature addition products. Thus, under these higher temperature conditions, an equilibrium is established between the 1,2- and 1,4-addition products in which the 1,4-addition product predominates. If either the pure 1,2- or pure 1,4- addition product is dissolved in an inert solvent at higher temperature and a Lewis acid catalyst added, an equilibrium mixture of 1,2- and 1,4-addition product forms. The same equilibrium is obtained regardless of which isomer is used as the starting material. Chemists interpret these results using the twin concepts of kinetic control and equilibrium control of reactions. For reactions under kinetic (rate) control, the distribution of products is determined by the relative rates of formation of each. We see the operation of kinetic control in the following way. At lower temperatures, the reaction is essentially irreversible and no equilibrium is established between 1,2- and 1,4-addition products. The 1,2-addition product predominates under these conditions because the rate of 1,2-addition is greater than the rate of 1,4-addition. For reactions under thermodynamic (equilibrium) control, the distribution of products is determined by the relative stability of each. At higher temperatures, the reaction is reversible and an equilibrium is established between the 1,2- and 1,4-addition products. The percentage of each product present at equilibrium is in...
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