Answers to the Exercises "SN/E Competition"
Due to the 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 if strong, bulky bases are applied. However, the cyanide ion is merely a weak and non-bulky base. It is a good nucleophile though.
In this case, an E1 mechanism cannot occur because the activation energy for the formation of the intermediate high-energy primary carbenium ion is too large. Eliminations and substitutions with primary heteroalkyl compounds hardly ever show a unimolecular mechanism with an intermediate carbenium ion. In addition, the polar, aprotic solvent DMSO supports a bimolecular mechanism.