Ethers, Sulfides, Epoxides, and Sulfur Functional Groups

Description

Ethers are organic molecules consisting of an ${\rm {R{-}O{-}R}}$. Ethers are used as anesthetics; diethyl ether was the first anesthetic but has since been replaced by less flammable and volatile compounds. Ethers, such as diethyl ether, are prepared industrially by dehydration of alcohols. The Williamson synthesis of ethers is the main method of creating ethers in the laboratory. However, ethers can also be prepared from alkenes via alkoxymercuration-demercuration. Epoxides are cyclic ethers made of two carbons and an oxygen. Epoxides are synthesized from alkenes using peroxy acids. Epoxides can also be synthesized from a halohydrin with a strong base.

Thiols (RSH) are sulfur analogs of alcohols (ROH). Sulfides (RSR), or thioethers, are sulfur analogs of ethers (ROR). Disulfides (RSSR) are sulfur analogs of peroxides (ROOR). Disulfides are biologically important molecules. The three-dimensional structure of many proteins is partially due to the presence of disulfide bonds between cysteine residues on the same or neighboring protein molecules.

Thiols are prepared in the laboratory by a substitution reaction with the addition of NaSH to RX. Sulfides are prepared in a reaction similar to the Williamson ether synthesis, with NaOH followed by RX. Because thiols are more acidic than their alcohol analogs, NaOH can be used instead of NaH in the Williamson synthesis. Thiols may be oxidized with NaOH, H₂O, or Br₂ to make disulfides. Sulfides easily oxidize to sulfoxides, which are further oxidized to sulfones if an excess of oxidizing agent is used.

At A Glance

• Ethers are organic molecules consisting of an ${\rm {R{-}O{-}R}}$. In common nomenclature, simple ethers are named by naming the alkyl groups on either side of the ether. In IUPAC nomenclature, ethers are named as a prefix with a number indicating the position of the shorter carbon chain and given the name alkyloxy.
• Ethers are prepared industrially via dehydration of an alcohol. In the laboratory, the Williamson synthesis and alkoxymercuration-demercuration are the main methods of preparing ethers.
• Ethers are nonreactive to a wide range of reagents, which makes them very useful in organic synthesis.
• Epoxides are prepared from alkenes by using peroxy acids. Epoxides are also prepared from a halohydrin with a strong base.
• Epoxides undergo a ring-opening reaction with either an acid or a nucleophile to give an anti-product. When nucleophiles are used in ring-opening reactions, the product will always give the anti-product, with the OH going to the more-substituted carbon.
• Sulfur functional groups are analogous to their oxygen counterparts. A thiol (SH) is analogous to an alcohol, a sulfide (RSR) is analogous to an ether, a disulfide (RSSR) is analogous to a peroxide, and a sulfoxide (RS(O)R) is analogous to a ketone.
• Preparation of compounds containing sulfur functional groups involves simple substitution reactions, a reaction similar to the Williamson ether synthesis, and oxidation reactions.