1559T_ch04_55-6910/22/0520:19Pa9e57 ⊕ EQA 5 ouions o Problems·57 18.(a)lodocyclopropane (b)trans-1-Methyl-3-(1-methylethylcyclopentane (c)cis-1.2-Dichlorocyclobutane (d)cis-1-Cyclohexyl-5-methylcyclodecane (e)To tell whether this is cis or trans,draw in the hydrogens on the substituted carbons: Groups on bottom of ring One Br on top.one on bottom,.frans-1,3-dibromocyclohexane (f)Similarly. On top一⑧ ⊕ cis-1.2-dibromocyclohexane (a) (b) (e) -C d cal.the bon carbon in cyclopropane itself (15645 bond angle comp ssion).Forming the radical value to begi h is the DH for the C-C bond between CHa (2)groups the t 18. (a) Iodocyclopropane (b) trans-1-Methyl-3-(1-methylethyl)cyclopentane (c) cis-1,2-Dichlorocyclobutane (d) cis-1-Cyclohexyl-5-methylcyclodecane (e) To tell whether this is cis or trans, draw in the hydrogens on the substituted carbons: One Br on top, one on bottom, trans-1,3-dibromocyclohexane. (f ) Similarly, cis-1,2-dibromocyclohexane 19. (a) (b) (c) (d) (e) (f ) 20. (a) The very low relative radical chlorination reactivity of cyclopropane implies abnormally strong COH 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. 21. In all cases the reference value to begin with is the DH° for the COC bond between CH2 (2°) groups, i.e., DH° for CH3CH2OCH2CH3, 88 kcal mol
1 (Table 3-2). (a) Cleavage of a COC bond in cyclopropane requires a smaller net energy input because ring strain is relieved in the process. Breaking a “normal” COC bond would require 88 kcal mol
1 input, but because 28 kcal mol
1 is recovered as a result of strain relief in opening the three-membered CH3 Cl F F CH3 CH3 Cl CH3CH2 CH2CH3 Cl Br Cl CH3CH2 CH2CH3 Cl Br Cl CH2CH3 Cl Br H On top On bottom H Br Br Br H H Groups on top of ring Groups on bottom of ring Solutions to Problems • 57 1559T_ch04_55-69 10/22/05 20:19 Page 57