18.7 Thiols and Sulfides

Thiols and sulfides are sulfur-containing organic compounds that share similarities with alcohols and ethers. These molecules play crucial roles in various chemical processes and biological systems, with unique properties stemming from the sulfur atom's size and reactivity.

Synthesis of thiols and sulfides often involves nucleophilic substitution reactions, while their reactivity is influenced by the sulfur atom's properties. Understanding these compounds is essential for grasping organic sulfur chemistry and its applications in fields like biochemistry and pharmaceuticals.

Thiols and Sulfides

Structure and properties of thiols

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• Thiols (R-SH) are sulfur analogs of alcohols (R-OH) with a sulfur atom bonded to an R group and a hydrogen atom
  • The hydrogen atom attached to the sulfur is called the thiol proton
• Named as "alkanethiols" by replacing the "-ol" suffix of the corresponding alcohol with "-thiol" (ethanethiol for CH3CH2SH)
• More acidic than alcohols due to greater charge stabilization by the larger sulfur atom, weaker S-H bond compared to O-H bond, and more stable thiolate anion (R-S⁻) than alkoxide anion (R-O⁻)
• Typical $pK_a$ values: thiols ~10-11, alcohols ~16-18
• The functional group in thiols is known as the sulfhydryl group (-SH)

Synthesis of thiols and sulfides

• Thiols synthesized from alkyl halides via SN2 reaction with hydrosulfide (HS⁻), a strong nucleophile (CH3CH2Br + HS⁻ → CH3CH2SH + Br⁻)
• Prepared by reducing sulfonic acids (R-SO3H) or sulfonyl chlorides (R-SO2Cl) with LiAlH4 to the corresponding thiol
• Sulfides (R-S-R') synthesized from thiols via SN2 reaction with alkyl halides
  1. Thiols deprotonated to form thiolate anion (R-S⁻), which acts as a nucleophile
  2. Thiolate anion reacts with alkyl halide (CH3CH2SH + NaOH → CH3CH2S⁻ + H2O; then CH3CH2S⁻ + R'-X → CH3CH2-S-R' + X⁻)
• Precautions: work with thiols in well-ventilated areas due to strong, unpleasant odors (similar to hydrogen sulfide); store thiols and sulfides under inert atmosphere to prevent oxidation

Sulfides vs ethers in reactions

• Sulfides (R-S-R') are sulfur analogs of ethers (R-O-R') with a sulfur atom bonded to two R groups
• Stronger nucleophiles than ethers due to the larger size and polarizability of sulfur, which is less electronegative than oxygen allowing for a more polarizable electron cloud
  • Sulfides react more readily with alkyl halides via SN2 reactions compared to ethers
• More easily oxidized than ethers
  • Sulfides oxidized to sulfoxides (R-SO-R') and sulfones (R-SO2-R') using oxidants (H2O2 or peroxyacids)
  • Ethers generally resistant to oxidation under similar conditions
• Oxidation of sulfides to sulfoxides is stereoselective
  • The lone pair on sulfur allows for two possible stereoisomers (R and S) when oxidized to a sulfoxide
  • Stereochemistry of the product depends on the oxidant and reaction conditions

Organosulfur compounds and their properties

• Organosulfur compounds are organic molecules containing sulfur atoms
• Sulfur can exist in various oxidation states, ranging from -2 to +6
• The nucleophilicity of sulfur-containing compounds is generally higher than their oxygen analogs due to sulfur's larger size and polarizability
• Examples of organosulfur compounds include thiols, sulfides, disulfides, and sulfoxides, each with distinct reactivity and properties