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Solutions to Problems 171 apply in a clear-cut way: considerations determine which bond breaks, but the direction of approach of the nucleophile (back-side attack) is characteristic of an SN2 process. Strained cyclic ethers also react with basic nucleophiles. This is an process that displaces an alkoxide, which is a very bad leaving group. The nucleophile has to be a good one because the leaving group (a negatively charged alkoxide ion) is a terrible one. The reaction follows the reactivity order of 1° » 2° » 3°. Normally alkoxides cannot be displaced in reactions. In oxacyclopropanes, however, ring strain raises the energy content such that suitably reactive nucleophiles can displace negatively charged oxygen leaving groups (see graph, below). The only reason this reaction occurs at all is that the displacement reaction breaks open a small, strained ring. Please note that this is a reaction unique to small-ring ethers. Unstrained ethers are unreactive toward basic nucleophiles. Small 0 + CH₂ CH₂ Large E + CH₃ 0 CH₃ Products or Reaction coordinate 9-10. Sulfur Analogs of Alcohols and Ethers This short section expands on the obvious parallels between oxygen and sulfur that arise as a result of their relationship in the periodic table. As you saw earlier, the larger sized atoms are more nucleophilic, but less ba- sic. Thus comparisons of the chemical properties of pairs of species like HS⁻ HO⁻, H₂S H₂O, and CH₃SH CH₃OH are straightforward. Larger atoms are also more readily oxidized, and sulfur chemistry in- cludes a variety of oxidized species. Common examples are SO₂ and H₂SO₄. New systems introduced here in- clude sulfonic acids (RSO₃H), sulfoxides (RSOR'), and sulfones (RSO₂R'). Solutions to Problems 28. Equilibrium always lies on the side with the weaker acid-base pair. (a) Left; (b) left; (c) right; (d) right. 29. (a) CH₃CH₂CH₂I (b) (c) I (d)