Answers to the Exercises "SN/E Competition"
Ethyl propyl ether is the main product of this reaction. The product ratio propylene : ethyl propyl ether amounts to 9 : 91. Due to considerable steric interactions in the trigonal bipyramidal transition state, reactions proceed most rapidly in association with primary heteroalkyl compounds and most slowly (or virtually never at all) with tertiary heteroalkyl compounds. With primary heteroalkyl compounds, the competing E2 elimination only occurs when strong, bulky bases are applied. The ethoxide anion is a relative strong base. Nevertheless, it is not bulky and sterically-demanding enough to favour the elimination with primary heteroalkyl compounds. In addition, ethoxide can attack ethyl bromide in an reaction, as it is a relative good nucleophile.
In this case, the mechanism is out of the question, because the activation energy of the formation of the intermediate high-energy primary carbenium ion is too large. With primary heteroalkyl compounds, eliminations as well as substitutions virtually never follow a unimolecular mechanism with an intermediate carbenium ion. Even the application of the polar and protic solvent ethanol does not enable the unimolecular mechanism to exceed the bimolecular mechanism.