Epoxide
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Section 10.10 Ring Opening of Epoxides
Protonation of an oxygen of an ether generates a leaving group.
Using an acid to protonate an ether results in low yields
An epoxide is a three-membered cyclic ether. Like cyclopropane, epoxides have a large amount of ring strain and much more reactive than normal ethers, because of this the O-C bond can be broken in a nucleophilic substation reaction.
Both carbons of an epoxide ring are electrophilic, so either might react.
In basic or neutral conditions reactions with epoxides usually follow a Sn2 mechanism and react at the less hindered secondary carbon with inversion of configuration.
Nucleophilic ring opening can also be accomplished in acid solution, The oxygen is firstprotonated thus making it a better leaving group, which is usually an SN1 rxn, but in this case it follows in between and Sn1 and Sn2 with stereochemistry of nucleophile approaching from the side opposite the leaving oxygen: the regiochemistry is that predicted by Sn1 mechanism with substitution occurring at the more stable carbocation.. This acidic condition often results in a different C-O bond breaking than that which occurs in basic or neutral condition.
Border line Sn2 mechanism-mechanism when rxn has features of both Sn2(stereochemistry) and Sn1(regiochemistry) mechanism.
Section 10.11 Elimination of Hydrogen Halide (Dehyrahalogenation)
Elimination rxns are useful for preparation of akenes.
Steric hinderance can be used to slow down the competition of competing Sn2 rxns.
Tertiary and secondary substrates give good yields of elimination when treated with strong bases.