Ethers, Epoxides, Thiols, and Sulfides: Structure, Properties, and Reactions

Chapter 13: Ethers and Epoxides; Thiols and Sulfides

13.2 Naming Ethers

Ethers are organic compounds containing an oxygen atom connected to two alkyl or aryl groups. Their nomenclature follows specific conventions to ensure clarity in structure identification.

• Common Naming: Name each alkyl group attached to the oxygen, followed by the word 'ether'. Example: Ethyl methyl ether.

• IUPAC Naming: The larger group is considered the parent hydrocarbon, and the smaller group attached to oxygen is named as an alkoxy substituent. Example: Ethoxy pentane.

13.3 Structure and Properties of Ethers

Ethers exhibit unique physical properties due to their inability to form hydrogen bonds among themselves, which affects their boiling points and solubility.

• Hydrogen Bonding: Ethers cannot hydrogen bond with themselves, resulting in lower boiling points compared to alcohols of similar molecular weight.

• Boiling Point Comparison:

  Compound	Boiling Point (°C)
  Ethanol	78
  Dimethyl ether	-25
  Propane	-42

• Effect of Size: Larger ethers have higher boiling points due to increased London dispersion forces.

  Ether	Boiling Point (°C)
  Dimethyl ether	-25
  Diethyl ether	35
  Dipropyl ether	91

13.4 Crown Ethers

Crown ethers are cyclic compounds that contain several ether groups and are known for their ability to solvate metal ions, especially in organic solvents.

• Metal Ion Solvation: Crown ethers stabilize metal atoms with a full or partial positive charge, enhancing their solubility and reactivity in organic media.

• Grignard Reactions: Ethers are used as solvents in Grignard reactions because they stabilize the Mg atom.

• Structure: The number in the crown ether's name refers to the total number of atoms and the number of oxygens.

  Crown Ether	Solvated Ion
  12-Crown-4	Li+
  15-Crown-5	Na+
  18-Crown-6	K+

13.5 Preparation of Ethers

Ethers can be synthesized through several methods, with the Williamson ether synthesis being the most versatile for asymmetrical ethers.

• Williamson Ether Synthesis: Involves the reaction of an alkoxide ion with a primary alkyl halide via an SN2 mechanism. $R-OH + NaH \rightarrow R-O^- + Na^+ + H_2$ $R-O^- + R'-X \rightarrow R-O-R' + X^-$

• Alkoxymercuration-Demercuration: Another method for ether synthesis, which proceeds with Markovnikov regioselectivity. $R_2C=CR_2 + Hg(OAc)_2, ROH \rightarrow R_2C(OR)-CR_2(HgOAc) \xrightarrow{NaBH_4} R_2C(OR)-CR_2H$

13.6 Reactions of Ethers

Ethers are generally inert but can undergo acid-promoted cleavage to yield alkyl halides and alcohols.

• Acidic Cleavage: Ethers react with strong acids (HBr, HI) to produce alkyl halides. The mechanism depends on the nature of the R group:

  • If R is tertiary, cleavage occurs via SN1.

  • If R is aryl or vinyl, substitution does not occur.

• General Reaction: $R-O-R' + HX \rightarrow R-X + R'-OH$

13.8 Preparation of Epoxides

Epoxides are three-membered cyclic ethers that can be synthesized from alkenes using peroxy acids or from halohydrins.

• Peroxy Acid Method: Alkenes react with peroxy acids (e.g., MCPBA, peroxyacetic acid) to form epoxides. $RCH=CH_2 + RCO_3H \rightarrow RCH(O)CH_2$

• Stereospecificity: The reaction preserves the stereochemistry of the alkene (cis/trans).

• Halohydrin Method: Halohydrins, formed from alkenes, can be converted to epoxides via base-induced intramolecular SN2 reaction. $RCH(OH)CH_2Br + NaOH \rightarrow RCH(O)CH_2$

13.10 Ring-opening Reactions of Epoxides

Epoxides are highly reactive due to ring strain and can be opened by nucleophiles, leading to useful synthetic transformations.

• Nucleophilic Ring Opening: Strong nucleophiles attack the less hindered carbon in an SN2 process, followed by protonation. $R_2C(O)CR_2 + Nu^- \rightarrow Nu-CR_2-CR_2-OH$

• Regioselectivity: Nucleophilic attack occurs at the least hindered carbon.

• Stereoselectivity: Inversion of configuration is observed at the attacked carbon.

• Acid-catalyzed Opening: Under acidic conditions, weak nucleophiles (e.g., water, alcohols) can open the epoxide after protonation.

13.11 Thiols and Sulfides

Thiols and sulfides are sulfur analogs of alcohols and ethers, respectively, and exhibit distinct chemical properties due to the presence of sulfur.

• Thiols: Compounds containing an –SH group. Named by replacing 'ol' with 'thiol'.

• Preparation: Thiols are prepared by nucleophilic substitution of alkyl halides with thiolate ions. $R-Br + NaSH \rightarrow R-SH + NaBr$

• Sulfides: Sulfur analogs of ethers, generally prepared by nucleophilic attack of a thiolate on an alkyl halide. $R-S^- + R'-Br \rightarrow R-S-R' + Br^-$

• Disulfide Formation: Oxidation of thiols leads to disulfides. $2 R-SH + Br_2 \rightarrow R-S-S-R + 2 HBr$

Additional info: Sulfides can be further oxidized to sulfoxides and sulfones, which are important in organic synthesis.