Oxygen assumes only two oxidation states in its organic compounds (–1 in peroxides and –2 in other compounds). Remarkably, sulfoxides (equation # 2), sulfinate salts (# 3) and sulfite anion (# 4) also alkylate on sulfur, despite the partial negative formal charge on oxygen and partial positive charge on sulfur. Although equivalent oxonium salts of ethers are known, they are only prepared under extreme conditions, and are exceptionally reactive. Sulfides, for example, react with alkyl halides to give ternary sulfonium salts (equation # 1) in the same manner that 3º-amines are alkylated to quaternary ammonium salts. R–S (–) Na (+) + (CH 3) 2CH–Br (CH 3) 2CH–S–R + Na (+) Br (–)Īlthough the basicity of ethers is roughly a hundred times greater than that of equivalent sulfides, the nucleophilicity of sulfur is much greater than that of oxygen, leading to a number of interesting and useful electrophilic substitutions of sulfur that are not normally observed for oxygen. Thiolate conjugate bases are easily formed, and have proven to be excellent nucleophiles in S N2 reactions of alkyl halides and tosylates. Since hydrogen sulfide (H 2S) is a much stronger acid than water (by more than ten million fold), we expect, and find, thiols to be stronger acids than equivalent alcohols and phenols. The chemical behavior of thiols and sulfides contrasts with that of alcohols and ethers in some important ways. Sulfur analogs of alcohols are called thiols or mercaptans, and ether analogs are called sulfides.
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