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Solutions to Problems 75 The resulting molecule has these two new hydrogens pointing into the center of the ring, in prohibitively close contact with the carbon and hydrogens on the opposite side of the ring. The steric strain of this transannular interaction makes this a very high energy conformation. 49. (a) From left to right: chair cyclohexane, boat (or twist-boat) cyclohexane, chair cyclohexane, envelope cyclopentane. (b) All are trans. (c) α means below and ß means above. Therefore, 3α-OH, 4α-CH₃, 8α-CH₃, 11α-OH, OCCH₃. 0 (d) The boatlike cyclohexane ring is the most unusual feature; most steroids have only chair cyclohexanes. The boat shape is a result of the unusual cis relationship of the groups at positions 9 and 10, and also at positions 5 and 8. Note also the unusual number and location of methyl groups: at positions 4, 8, 10, and 14, instead of the more common pair of methyl groups at 10 and 13. OH (a) (b) 50. + O + HO· For (a), = +98.5 102.5 = -4.0 kcal mol⁻¹; for (b), = -96 kcal mol⁻¹. Overall, = -100 kcal mol⁻¹. 51. (a) Propagation: + RH HCI + Can happen initially ICl₂ RCI + i Main steps of chain reaction i + RH HCI + + I Termination (one possibility): R+ i Cl RCI + I (b) There are four tertiary hydrogens, but the one that is ß (up) is too hindered to be chlorinated, because of its 1,3-diaxial interactions with the two ß methyl groups. So the sites of chlorination are the three tertiary α (down) hydrogens: Too hindered CH₃ CH H H H H Major sites of chlorination