Answers to the Exercises "SN/E Competition"
Octadec-1-ene is the main product of this E2 elimination. The product ratio octadec-1-ene : tert-butyl octadecyl ether amounts to 87 : 13. Due to considerable steric interactions in the trigonal bipyramidal transition state, reactions proceed most rapidly with primary heteroalkyl compounds and most slowly (or virtually not at all) with tertiary heteroalkyl compounds. With primary heteroalkyl compounds, the competing E2 elimination only occurs when strong, bulky bases are applied. Potassium tert-butoxide is a strong base with very bulky, sterically-demanding substituents.
In this case, the mechanism cannot occur 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 tert-butanol does not enable the unimolecular mechanism to exceed the bimolecular mechanism.