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1559reh06_099-11210/22/0520:19Page111 Solutions o Problems111 52.BrCH,cH24t一B7 cn.cu-一cfaH+Br (b)BrCHzCH:+-:H 一Br +BICH.CH.CH.CH.CH. →H20 BrTCH:CH:CH-CH:CH:O:--Br-+ 0 @BCH,CHCH,Ccr+一Br H BICH-CH CH-CH-CHSNH*NH NH+ /NI +BT CH.CH.CH-CH-CHNH 53.In an acyclic(noncyclic)haloalkane the hybridization around the halogen-bearing carbon is spand has rehy reacung groups nded to 120 109,corresponding to s rse:It wants its bond angles to open up even more.to a bond angle compression of propane i 蕊文一制 higher activation er52. (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