Saved Bookmarks
| 1. |
Give the products and mechanism for each of the following reactions. Explain why nucleophilic attack on the epoxide occurs at different sites in the two reactions. |
Answer» Solution :Attack by a strong nucleophile suchas `CH_3 O^(-)` (methoxide ION) on an epoxide occurs at the least hindered carbon, similar to an `S_N 2` reaction. Since methoxide is regenerated in the last step , this mechanism suggests that only a catalytic AMONT of `CH_3 O^(-)` is required .  Attack by a weak nucleophile such as methanol `(CH_3 OH)` can occur only when the epoxide has been protonated so that a better leaving GROUP is formed . Under these conditions, the epoxide C-O bonds are weakened because the leaving group has been improved. THe protonated epoxide can be viewed as a resonance hybrid of the following structures :  A weak nucleophile attacks the resonance hybrid carbon with the largest `delta^+` charge. The magnitude of the `delta^+` is determined by the relative importance of the resonance contributors. Resonance contributor (a) is the most important because all the atoms have a full octet. Contributor (b) is a secondary carbocation with benzene ring resonance whereas contributor(c) is a primary carbocation , so contribution (b) is more important than (c) .These relative contribiutions in the resonance hybrid reveal that the benzylic carbon has more `delta+` than the primary carbon, so the nucleophile attacks at the benzylic carbon. (The same reasoning applies to any three -MEMBERED ring with two carbons and a third atom bearing a positive charge, such as a protonated epoxide ,a halonium ion or a mercurinium ion )  Since the proton source is regenerated in the last step, this mechanism suggests that only a catalyst amount of acid is needed. |
|