3. A student familiar with the mechanism of the S_N2 and E2 reactions predicts that the stereoisomer 1 may produce 2 and 3 while stereoisomer 4 will produce only 5 under the conditions shown. Show her reasoning. [Note: You need to know how to draw cis-decalins.] [Note: You need to know how to draw cis-decalins. Look here (.ppt) or here (.html). ]

There are two conformations of cis-decalin 1 --namely, 1a and 1b. Conformation 1a is capable of effecting an SN2 displacement. Hydroxide reversibly deprotonates the alcohol, which, in turn, does an SN2 rear side displacement of chloride. Use the Jmol structures of 1a and 1b to compare 1a and 1b. Rotate the structures to get a good view. Click here. Conformation 1b has the alkoxide group remote from the C-Cl bond. The C-Cl bond is axial and may participate in an E2 elimination in only one direction. There is only one axial C-H bond anti to the C-Cl bond. Compound 4 is a diastereomer of 1, differing only at the hydroxyl center. Neither conformation 4a nor 4b can effect SN2 displacement. However, E2 elimination from conformation 4b (axial chlorine) is possible. It is likely that during E2 elimination the alkoxides 1b and 4b are protonated to form the alcohols.