Answers to the Exercises "SN/E Competition"
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Due to the stabilization of the partially formed double bond in the transition state by alkyl substituents, E2 eliminations proceed most rapidly with tertiary heteroalkyl compounds and most slowly with primary heteroalkyl compounds. The polar, aprotic solvent DMSO supports a bimolecular mechanism. However in order to favour an E2 elimination with a primary heteroalkyl compound, strong bases with bulky, sterically-demanding substituents must be applied. The bulky substituents lead to very strong steric interactions that destabilize the trigonal bipyramidal transition state of the competing reaction. However, the cyanide anion is not a strong base, nor does it contain bulky substituents. On the other hand, it is a good nucleophile.