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Unformatted text preview: Chapter 22 - Conjugated Dienes 1) Electrophilic Addition to Conjugated Dienes a) Conjugated dienes undergo two-step electrophilic addition reactios just as do simple alkenes. However, certain features are unique to the reactions of conjugated dienes b) Addition of one equivalent of HBr to 1,3-butadiene at -78°C 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 Br 2 at -15°C 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. CH 2 CH CH CH 2 HBr CH 2 CH CH CH 2 Br H CH 2 CH CH CH 2 H Br + +-78° C 1,3-Butadiene (90%) (1,2-addition) 3-Bromo-1-butene (10%) (1,4-addition) 1-Bromo-2-butene 1,3-Butadiene + Br 2 CH 2 CH CH CH 2-15° C CH 2 CH CH CH 2 Br Br CH 2 CH CH CH 2 Br Br + 3,4-Dirbromo-1-butene (54%) (1,2-addition) 1,4-Dibromo-2-butene (46%) (1,2-addition) i) Mechanism: c) Kinetic versus Thermodynaic Control of Electrophilic Addition i) 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. (1) For addition of HBr at -78°C and addition of Br 2 at -15°C, 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. (2) For addition of HBr and Br 2 at higher temperatures (generally 40°-60°C), the 1,4-addition products predominate. 2 CH 2 CH CH CH 2 + Br H CH 2 CH CH CH 2 H CH 2 CH CH CH 2 H CH 2 CH CH CH 2 Br H CH 2 CH CH CH 2 H Br Br- Br- (1,2-addition) (1,4-addition) A resonance stabilized allylic carbocation CH 2 CH CH CH 2 HBr CH 2 CH CH CH 2 Br H CH 2 CH CH CH 2 H Br + +-78° C 1,3-Butadiene (90%) (1,2-addition) 3-Bromo-1-butene (10%) (1,4-addition) 1-Bromo-2-butene 1,3-Butadiene + Br 2 CH 2 CH CH CH 2-15° C CH 2 CH CH CH 2 Br Br CH 2 CH CH CH 2 Br Br...
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- Chemical bond, CH CH CH2