7-54 In E2, the two groups to be eliminated must be coplanar. In confonnationally mobile systems like acyclic molecules, or in cyclohexanes, anti-coplanar is the preferred orientation where the H and leaving group are 180° apart. In rigid systems like norbornanes, however, SYN-coplanar (angle 0°) is the only possible orientation and E2 will occur, although at a slower rate than anti-coplanar. The structure having the H and the Cl syn-coplanar is the trans, which undergoes the E2 elimination. (It is possible that the other Hand Cl eliminate from the trans isomer; the results from this reaction cannot distinguish between these two possibilities.)
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