1559Tch0455-6910/22/0520:19Page62 EQA 62.Chapter 4 CYCLOALKANES 28.Both rings can flip.so there are four possible combinations.two of which are identical: Most stable:each ri 2.e ( rhospieleepineahdiearetpeodpoionandeuamineeaeh Methyl is pse CH CH,LH Methyl is pseudoaxial cnergy.One a 30.Only a boat-related conformation permits both bulky groups to avoid axial positions.This molecule will adopt a shape in which both groups are"pseudoequatorialIt will be based on the twist-boat of28. Both rings can flip, so there are four possible combinations, two of which are identical: 29. Notice how some positions around a boat conformation of cyclohexane are axial-like (“pseudoaxial”) and some are equatorial-like (“pseudoequatorial”): If you draw conformations placing the methyl group in each different type of position and examine each conformation for strain, you will see the following: Of the two with the methyl in an equatorial-like position, the one on the left has the bond to the CH3 staggered with respect to neighboring COH bonds. The other possibility is higher in energy as a result of the eclipsing interaction shown. The two conformations with pseudoaxial CH3 are both quite high in energy. One actually has three diaxial interactions involving the methyl group (only one is shown; make a model to see the rest!), and the worst of them all has a serious transannular interaction due to the close approach between the CH3 and the H shown. 30. Only a boat-related conformation permits both bulky groups to avoid axial positions. This molecule will adopt a shape in which both groups are “pseudoequatorial.” It will be based on the twist-boat of Diaxial interactions Worst conformation: transannular interaction CH3 H CH3 H Methyl is pseudoaxial CH3 CH3 H Best conformation Methyl is pseudoequatorial Eclipsed a a a a e e e e a pseudoaxial e pseudoequatorial H H Most stable: each ring is attached by an equatorial bond to the other Least stable: each ring is attached by an axial bond to the other These two are identical H H H H H H 62 • Chapter 4 CYCLOALKANES 1559T_ch04_55-69 10/22/05 20:19 Page 62