Solutions_Manual_for_Organic_Chemistry_6th_Ed 164

Solutions_Manual_for_Organic_Chemistry_6th_Ed 164 - 7-54 In...

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Unformatted text preview: 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 1 80°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 H and Cl eliminate from the trans isomer; the results from this reaction cannot distinguish between these two possibilities.) H Cl extremely slow to eliminate­ H and C\ not coplanar 7-55 It is interesting to note that even though three-membered rings are more strained than four membered rings, three-membered rings are far more common in nature than four-membered rings. Rearrangement from a four-membered ring to something else, especially a larger ring, will happen quickly. . H trans H Cy �oplanar l '/syn­ Cl Cl without rearrangement H� + � CH2 H20:.. y-- unstable 1 ° carbocation-­ short lifetime if it exists at all with rearrangement-hydride shift H H20: (P�H2 3° carbocation, but still in a strained 4-membered ring mmor • minor minor 157 ...
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