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Solutions to Problems 77 reasons. First of all, the rigid fragment of the cyclopropane ring is replaced by two freely spinning CH₃ groups. Secondly, the original 60° angle between the bonds to these carbon atoms widens to the full tetrahedral value of 109.5° upon ring-opening. The increase in steric size of the substituent is considerable and not at all communicated adequately by line drawings. Now that we know what to look for, let's examine conformations of B comparable to those of A drawn above. The diaxial form is just about impossible, there being no available conformation lacking prohibitive steric compression between one of the CH₃'s of the tert-butyl group and one or both axial H's on the same ring face. However, as indicated by the intersecting arcs, the diequatorial conformer suffers from a similar interaction between methyl groups, which is unavoidably present in all staggered conformations of the substituent. Basically, hydrogen atoms on these two CH₃'s are being forced to try to occupy the same volume of space: CH₃ H O CH₃ H CH₃ CH₃ Diaxial CH₃ CH₃ O Diequatorial CH₃ The molecule escapes this dilemma by adopting a shape based upon the boat shown below, but twisted to relieve eclipsing interactions. This unorthodox conformation places the methyl group at C3 in a pseudoaxial position, out of the immediate proximity of the tert-butyl group. (The situation is not terribly different from that described in Problem 30.) CH₃ H CH₃ CH₃ CH₃ H 54. (a) 55. (d) 56. (d) 57. (a) Smallest = most stable (diequatorial)