Preparing Alkenes:A Preview of Elimination Reactions In biological pathways dehydrations normally take place on substrates in which-OH is positioned two carbons away from a carbonyl group H 0 H3C ACP H30 ACP H20 H B-Hydroxybutyryl ACP trans-Crotonyl ACP
In biological pathways dehydrations normally take place on substrates in which –OH is positioned two carbons away from a carbonyl group Preparing Alkenes: A Preview of Elimination Reactions
8-2 Halogenation of Alkenes Halogenation -Addition reaction of alkenes Addition of Br,and Cl,to alkenes to yield 1,2-dihalides CI CI + Cl2 H一C一C一H H H Ethylene 1,2-Dichloroethane (ethylene dichloride)
Halogenation ▪ Addition reaction of alkenes ▪ Addition of Br2 and Cl2 to alkenes to yield 1,2-dihalides 8-2 Halogenation of Alkenes
Halogenation of Alkenes Halogenation of cycloalkenes Only trans-stereoisomer of dihalide product is formed Reaction occurs with anti stereochemistry -the two halogen atoms come from opposite faces of double- bond,one from top face and one form bottom face Br-Br Br Br Br Cyclopentene trans-1,2-Dibromo- cis-1,2-Dibromo- cyclopentane cyclopentane (sole product) (Not formed)
Halogenation of cycloalkenes ▪ Only trans-stereoisomer of dihalide product is formed ▪ Reaction occurs with anti stereochemistry – the two halogen atoms come from opposite faces of doublebond, one from top face and one form bottom face Halogenation of Alkenes
Halogenation of Alkenes Reaction occurs through an intermediate bromonium ion (R2Br+),formed by interaction of the alkene with Br2 and simultaneous loss of Br Br: :Br: CC ---C- :Br: An alkene A bromonium ion
Reaction occurs through an intermediate bromonium ion (R2Br+), formed by interaction of the alkene with Br2 and simultaneous loss of BrHalogenation of Alkenes
Halogenation of Alkenes Bromonium ion shields one side of molecule so that reaction with Br ion occurs only from opposite side Top side open to attack .e :Br :Br: B Bottom side shielded from attack Cyclopentene Bromonium ion trans-1,2-Dibromo- intermediate cyclopentane
Bromonium ion shields one side of molecule so that reaction with Brion occurs only from opposite side Halogenation of Alkenes