Sec 1 PREPARATION AND PHYSICAL PROPERTIES PREPARATION .Halogenation(Allylic Halogenation) ●With HX ●From alcohol .Diazonium coupling (for aryl halide only) ■Use
■1.Halogenation H +Br2 hv anhy.AICl. Highly selective:bromination of 3C CH3 CH3 CH3C-H+Br2 CH3—C-Br+HB, CH3 CH3 90%
Sec 1 Preparation of RX Free radical halogenation produces mixtures,not good lab synthesis ounless:all H's are equivalent,or halogenation is highly selective. Free radical allylic halogenation produces alkyl halide with double bond on the neighboring carbon
Allylic Halogenation Allylic radical is resonance stabilized. Bromination occurs with good yield at the allylic position (sp3 C next to C=C). Avoid a large excess of Br,by using N-bromosuccinimide (NBS)to generate Br2 as product HBr is formed. 0 Br
Reaction Mechanism Free radical chain reaction initiation,propagation,termination. hy 2Br. H、（H Br· Br Br-