4-Methoxythiophenol: Properties, Uses & Lab Guide

4-Methoxythiophenol is a para-substituted aromatic thiol widely employed in organic synthesis as a versatile sulfur nucleophile, protecting-group reagent, and building block for pharmaceuticals, materials science, and chemical biology research.

Chemical Name

4-Methoxythiophenol (p-Methoxythiophenol)

CAS Number

696-63-9

Molecular Formula

C7H8OS

Molecular Weight

140.20 g/mol

Appearance

Colorless to pale yellow liquid with characteristic thiol odor

Key Functional Groups

Thiol (–SH) and methoxy (–OCH3) on para-substituted benzene ring

SKU

M101780 (Aladdin Scientific)

Chemical Properties and Structure

4-Methoxythiophenol (CAS 696-63-9) features a benzene ring bearing a thiol group (–SH) at the para position relative to a methoxy group (–OCH₃). The electron-donating methoxy substituent increases electron density on the aromatic ring and modestly raises the nucleophilicity of the sulfur atom compared with unsubstituted thiophenol. The compound has a molecular formula of C₇H₈OS and a molecular weight of approximately 140.20 g/mol. It is a low-viscosity liquid at room temperature, miscible with common organic solvents such as ethanol, dichloromethane, and dimethylformamide, but only sparingly soluble in water. The thiol pKa is estimated to be around 10–11, consistent with typical aromatic thiols, making it a good sulfur nucleophile under mildly basic conditions.

Synthetic Applications and Research Uses

4-Methoxythiophenol serves multiple roles in synthetic and research chemistry:

• Nucleophilic thiol-ene and thiol-yne reactions: The reactive –SH group participates efficiently in radical or base-catalyzed thiol-ene coupling with alkenes and alkynes, enabling rapid construction of thioether-containing frameworks relevant to polymer science and drug discovery.
• S-Alkylation and S-arylation: Under basic conditions, the thiolate anion formed from 4-methoxythiophenol reacts with alkyl halides, epoxides, and activated aryl halides (including via Pd-catalyzed cross-coupling) to produce diaryl or alkyl aryl sulfides of pharmaceutical interest.
• Protecting group chemistry: The 4-methoxyphenylthio group can function as a protecting group for aldehydes and carboxylic acid derivatives, removable under oxidative or Lewis acid conditions.
• Surface functionalization: As an aromatic thiol, it forms self-assembled monolayers (SAMs) on gold surfaces, providing a methoxy-terminated interface useful in biosensing and surface science studies.
• Chemical biology probes: The compound is used as a building block for disulfide-containing prodrugs and glutathione-responsive drug delivery systems, exploiting thiol–disulfide exchange chemistry.

Handling, Storage, and Safety

4-Methoxythiophenol is a malodorous, flammable liquid that requires careful handling in a well-ventilated fume hood at all times. Key safety considerations include:

• Personal protective equipment (PPE): Chemical-resistant gloves (nitrile or neoprene), safety goggles, and a laboratory coat are mandatory. A respirator is recommended for extended handling due to the potent thiol odor.
• Ventilation: All operations must be performed in an efficient fume hood to prevent inhalation of vapors.
• Flammability: Keep away from open flames, sparks, and high-heat sources. Store in a cool, dry, well-ventilated area away from oxidizers.
• Storage: Store under inert atmosphere (nitrogen or argon) in a tightly sealed amber glass container at 2–8 °C or room temperature as indicated by the supplier. Exposure to air promotes oxidation to the corresponding disulfide.
• Waste disposal: Dispose of residues and contaminated materials as hazardous chemical waste in accordance with local environmental regulations.
• First aid: In case of skin or eye contact, flush immediately with copious water for at least 15 minutes and seek medical attention. If inhaled, move to fresh air immediately.

Consult the Safety Data Sheet (SDS) provided by Aladdin Scientific (SKU M101780) for complete hazard information before use.

Preparation of Working Solutions and Common Protocols

The following guidance covers typical laboratory preparation of 4-methoxythiophenol solutions:

1. Thiolate stock solution: Dissolve 4-methoxythiophenol in anhydrous DMF or acetonitrile to a concentration of 0.1–1.0 M under inert atmosphere. Add 1.0 equivalent of a base (e.g., triethylamine or K₂CO₃) to generate the thiolate in situ prior to nucleophilic reactions. Prepare fresh or store at –20 °C under nitrogen.
2. Gold SAM preparation: Prepare a 1–5 mM solution of 4-methoxythiophenol in absolute ethanol. Immerse pre-cleaned gold substrates for 12–24 hours at room temperature under ambient conditions. Rinse thoroughly with ethanol and dry under a stream of nitrogen before characterization.
3. Thiol-ene coupling (radical): Combine substrate alkene with 1.0–1.2 equivalents of 4-methoxythiophenol and a radical initiator (e.g., AIBN, 5–10 mol%) in toluene or DMF. Heat to 60–80 °C or irradiate with UV light (365 nm) for 1–4 hours. Monitor conversion by TLC or NMR.
4. Aldehyde protection: Treat the target aldehyde with 2.0 equivalents of 4-methoxythiophenol in the presence of a Lewis acid catalyst (e.g., BF₃·Et₂O) in DCM at 0 °C to room temperature to form the dithioacetal. Deprotect under NBS/water or other oxidative conditions as needed.

Always verify reagent purity by GC or NMR before use in sensitive synthetic sequences.

Quality, Purity, and Procurement Considerations

For reproducible research results, reagent grade and purity of 4-methoxythiophenol are critical parameters. When procuring this compound, consider the following:

• Purity specifications: For synthetic applications, ≥97–98% purity (by GC) is typically required. Confirm the assay method (GC, HPLC, or titration) stated on the certificate of analysis (CoA).
• Appearance and odor check: Fresh material should be colorless to faint yellow. Discoloration to darker yellow or brown may indicate oxidation to the disulfide or other degradation products.
• Packaging and inert atmosphere: Request material packaged under nitrogen or argon to minimize air oxidation during storage and transit.
• Aladdin Scientific SKU M101780: Available from Aladdin Scientific; consult the product page for current lot-specific CoA, SDS, and pricing information.
• Regulatory considerations: Check local regulations regarding transport and storage of flammable thiol compounds before ordering.

Frequently asked questions

What is 4-Methoxythiophenol used for in organic synthesis?

4-Methoxythiophenol is primarily used as a sulfur nucleophile for S-alkylation, S-arylation, thiol-ene coupling, and protecting-group chemistry. Its electron-rich aromatic thiol group makes it an effective reagent for forming thioethers, dithioacetals, and self-assembled monolayers on gold surfaces.

How should I store 4-Methoxythiophenol to prevent degradation?

Store 4-Methoxythiophenol in a tightly sealed amber glass container under an inert atmosphere (nitrogen or argon) at 2–8 °C or at room temperature as specified by the supplier. Exposure to air and light promotes oxidation to the corresponding disulfide, leading to reduced purity and reactivity. Always check the appearance before use.

Is 4-Methoxythiophenol compatible with palladium-catalyzed cross-coupling reactions?

Yes, 4-methoxythiophenol can participate in palladium-catalyzed C–S cross-coupling reactions with aryl halides to give diaryl sulfides. However, thiols can act as catalyst poisons at high concentrations, so optimizing the Pd loading, ligand choice, and base is essential for successful reactions.

What precautions are needed when handling 4-Methoxythiophenol in the lab?

All handling must be performed in a well-ventilated fume hood due to the compound's intense and unpleasant thiol odor and flammable nature. Wear chemical-resistant gloves, safety goggles, and a lab coat at all times. Keep away from ignition sources, and consult the SDS for full hazard and first-aid information before working with this compound.

How can I confirm the purity of 4-Methoxythiophenol before use?

Purity can be confirmed by gas chromatography (GC), which is the most common method for thiol compounds, or by 1H NMR spectroscopy. Review the certificate of analysis (CoA) provided by Aladdin Scientific for lot-specific purity data. Discoloration or an unusually high disulfide content may indicate degradation and warrants re-testing.

References

1. March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure. Wiley-Interscience. General reference for nucleophilic substitution and thiol chemistry.
2. Hoyle, C. E.; Bowman, C. N. Thiol-Ene Click Chemistry. Angewandte Chemie International Edition, 2010. General reference for thiol-ene coupling methodology.
3. Love, J. C. et al. Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chemical Reviews, 2005. General reference for thiol SAM formation on gold surfaces.
4. Aladdin Scientific Product Page: 4-Methoxythiophenol, SKU M101780. Safety Data Sheet and Certificate of Analysis available on request.

Products are supplied for research and development use only. Not for use in humans, animals, diagnosis, or therapy.