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64 Chapter 4 CYCLOALKANES 25. CH₂CH₃ Br CH₃CH₂ CH₂CH₃ (a) (b) (c) CI Cl CH₃CH₂ CH₂CH₃ F F CH₃ CH₃ (d) (e) (f) Br CH₃ 26. (a) The very low relative radical chlorination reactivity of cyclopropane implies abnormally strong C-H bonds and an abnormally unstable cyclopropyl radical. (b) Radicals prefer sp² hybridization, with 120° bond angles. So in the cyclopropyl radical, the bond angle strain at the radical carbon is greater (120° - 60° = 60° bond angle compression) than at a carbon in cyclopropane itself (109.5° - 60° = 49.5° bond angle compression). Forming the radical therefore increases ring strain and is more difficult in cyclopropane than in a molecule lacking bond angle distortion to begin with. 27. Initiation: Br-Br heat or light (hv) Propagation: : Br H HBr + H H Br Br Br H : + H Termination: : Br Br: Br₂ Br : H H H H H H