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b) In this compound there are two hydrogens (circled) that can have a trans-diaxial geometry with the bromine. This results in the formation of two isomeric alkenes. H H H a Br H a H e + H e a CH₂CH₃ CH₂CH₃ CH₃CH₂ H Br 9.6 The bulky t-Bu group must be equatorial and locks the ring in a single conformation. For elimination to occur the leaving group, OTs, and a hydrogen on an adjacent carbon must be in a trans-diaxial arrangement. The isomer on the left has the OTs group axial when the t-butyl group is equatorial, so it can readily undergo E2 elimination. The isomer on the right has the OTs group equatorial when the t-butyl group is equatorial, so it cannot readily react. 9.7 For an elimination reaction that can produce structural isomers which have the double bond in different positions, the major product can be predicted using Zaitsev's rule: The major product is the more highly substituted alkene. a) + b) + c) + cis + trans major major major 9.8 (E)-2-Butene is the major product because it is more stable and the conformation leading to it is more stable. 133