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1559T_ch06_099-11210/22/0520:19Pa9e111 EQA Solutions to Problems.111 52.(BrCHa,H4一c一cH+Br (b)BrCH.CH:CH.CH. +BC,cH,a,a,ag2测 →H2O BTCH-CH-CHaCH-CH20:Br-+ (BrCHCH.CH:CHC+H,-Br H +BICH CHCH.CHCH NH NH BrCH.CH-CH-CH-CH2NH 53.In an acyelic( oncyclic)hal ndthe bond the hree noreacting groups around this carbon have expanded to120 本图 22pe-1w Nu bondat a hypothetical S2 reaction on a halocyclopropane (below).The substrate.with ring ccause its ca 4ge hond angle comp However.bond ng sta would beve higher activation 52. (a) (b) (c) 53. In an acyclic (noncyclic) haloalkane the hybridization around the halogen-bearing carbon is sp3 , and the bond angles are about 109°. In the transition state for an SN2 reaction this carbon has rehybridized to sp2 , and the bond angles involving the three nonreacting groups around this carbon have expanded to 120°: Now take a look at a hypothetical SN2 reaction on a halocyclopropane (below). The substrate, with ring bond angles of 60°, is already very strained because its carbons would of course prefer to have bond angles of 109°, corresponding to sp3 hybridization. The amount of strain in the substrate is related to this 109 60 49° bond angle compression. However, bond angle compression in the SN2 transition state would be even worse: It wants its bond angles to open up even more, to 120°! With a bond angle compression of 120 60 60°, the SN2 transition state for a halocyclopropane is much more strained than the halocyclopropane itself, leading to a much higher activation barrier (Ea). A similar analysis fits for halocyclobutanes. X H X H Nu Substrate strain: 109 60 49 angle compression Transition state strain: 120 60 60 angle compression Nu higher activation energy ‡ X R R R X R R R Nu SN2 transition state: RCR s open to 120 Substrate: RCR s 109.5 Nu ‡ NH4 NH3 CH2CH2CH2CH2CH2NH2 Br N N H H H NH4 NH3 H BrCH2CH2CH2CH2CH2NH2 Br BrCH2CH2CH2CH2CH2 Br NH3 CH2CH2CH2CH2CH2O O Br Br BrCH2CH2CH2CH2CH2O H H2O OH Br BrCH2CH2CH2CH2CH2 Br OH CH2CH2S Br S BrCH2CH2S H Br OH CH2 CH2 Solutions to Problems • 111 1559T_ch06_099-112 10/22/05 20:19 Page 111