General Aspects of electrophilic bromination a The rate of bromination is first order in alkene, but the order of bromine depends on how the reaction is run a Stereochemistry of addition to nonconjugated olefins is trans Conjugated olefins give a mixture of syn and anti products. The ratio of these products is highly dependent on alkene structure n Chlorination and iodination of olefins have similar kinetics and product stereochemistry to brominations. In synthesis, bromine and iodine have been used interchangebly Substituent Effects on bromination rates CH2=CH CH3CH=CH n-PrCH=cH 70 i-PrCH=CH2 t-BuCH=CH2 (CH3)2C=CH2 CiS-CH3 CH=CHCH3 (CH3)2C=C(cH3)2 1,800,000 The rate of addition is increased with increasing substitution on the double bond a Introduction of one bulky substituent(ie t-Bu) retards the rate slightly Schmid, G. H; Garratt, D.G. The Chemistry of Double Bonded Functional Groups, Wiley, Chichester 1977, pp. 765-789 Bouvier Chim. france 1968. 76 BA-Bromination 01 10/4/00 2: 40 PMSchmid, G. H.; Garratt, D.G. The Chemistry of Double Bonded Functional Groups, Wiley, Chichester 1977, pp. 765-789 Dubois, J.E.; Mouvier, G. Bull. Soc. Chim. France 1968, 765 The rate of addition is increased with increasing substitution on the double bond. Introduction of one bulky substituent (ie. t-Bu) retards the rate slightly. Cis olefins react faster the trans isomers . . . Alkene krel 1 61 70 57 27 5470 2620 1700 130,000 1,800,000 CH2=CH2 CH3CH=CH2 n-PrCH=CH2 i-PrCH=CH2 t-BuCH=CH2 (CH3)2C=CH2 cis-CH3CH=CHCH3 trans-CH3CH=CHCH3 (CH3)2C=CHCH3 (CH3)2C=C(CH3)2 Substituent Effects on Bromination Rates . . Chlorination and iodination of olefins have similar kinetics and product stereochemistry to brominations. In synthesis, bromine and iodine have been used interchangebly. Conjugated olefins give a mixture of syn and anti products. The ratio of these products is highly dependent on alkene structure. Stereochemistry of addition to nonconjugated olefins is trans . The rate of bromination is first order in alkene, but the order of bromine depends on how the reaction is run . General Aspects of Electrophilic Bromination 09A-Bromination 01 10/4/00 2:40 PM