cla H—Cl H Cl H C=C +c-C C-C Alkene Carbocation Addition product intermediate c2004 Thomson - Brooks/Cole Less reactive toward electrophiles than alkenes substitution instead of addition
• Less reactive toward electrophiles than alkenes • substitution instead of addition
E + C—C △ Galk Alkene +E HE △Gt benzene Benzene +e Reaction progress c2004 Thomson.Brooks/Cole
Benzene's n electrons participate as a Lewis base in reactions with lewis acids The product is formed by loss of a proton, which is replaced by bromine FeBr is added as a catalyst to polarize the bromine reagent
• Benzene’s p electrons participate as a Lewis base in reactions with Lewis acids • The product is formed by loss of a proton, which is replaced by bromine • FeBr3 is added as a catalyst to polarize the bromine reagent
6+ 6+ Br—Br FeB Br3Fe…Br…Br Bromine Polarized bromine (a weak electrophile)(a strong electrophile) E2004 Thomson/Broo'cs cole Br Br B H H +Br-Br→→ H 02004 Thomson/Brooks Cole Br Br FeBr3 t HB Benzene Bromobenzene(80%) ●2004 NemsoBroos0
Formation of product from intermediate The cationic addition Br-Br intermediate transfers a proton to FeBra(from Br and feBr Br · This restores H FeBa aromaticity (in contrast Nonaromatic carbocation with addition in Fast alkenes ●a4 Thomson Books Co HBr FeBr
Formation of Product from Intermediate • The cationic addition intermediate transfers a proton to FeBr4 - (from Br- and FeBr3 ) • This restores aromaticity (in contrast with addition in alkenes)