Comparison with Alkenes Cyclohexene adds Br2,AH =-121 kJ Addition to benzene is endothermic,not normally seen. Substitution of Br for H retains aromaticity,AH =-45 kJ. Formation of sigma complex is rate- limiting. > Chapter 17 5Chapter 17 5 Comparison with Alkenes • Cyclohexene adds Br2 , H = -121 kJ • Addition to benzene is endothermic, not normally seen. • Substitution of Br for H retains aromaticity, H = -45 kJ. • Formation of sigma complex is rate￾limiting. =>