1-Chloro-4-ethynylbenzene: Properties, Handling & Lab Use

1-Chloro-4-ethynylbenzene is a bifunctional aromatic building block bearing both a terminal alkyne and an aryl chloride, making it a versatile reagent for cross-coupling reactions, click chemistry, and the construction of complex organic architectures in modern synthetic research.

IUPAC Name

1-Chloro-4-ethynylbenzene

CAS Number

873-73-4

Molecular Formula

C₈H₅Cl

Molecular Weight

136.58 g/mol

Key Functional Groups

Terminal alkyne (C≡C–H) and aryl chloride (Ar–Cl)

Appearance

Colorless to pale yellow liquid or low-melting solid

Storage Conditions

Store in a cool, dry, well-ventilated area away from light and heat; refrigerator storage recommended

SKU

C115544

Chemical Identity and Physical Properties

1-Chloro-4-ethynylbenzene (CAS 873-73-4; also known as 4-chlorophenylacetylene or 1-ethynyl-4-chlorobenzene) is a para-substituted benzene derivative with the molecular formula C₈H₅Cl and a molecular weight of approximately 136.58 g/mol. The molecule possesses two synthetically useful functional groups: a terminal alkyne (–C≡CH) at the 4-position and an aryl chloride (Ar–Cl) at the 1-position.

It typically presents as a colorless to pale yellow liquid or low-melting solid at ambient temperature. Its terminal alkyne proton is mildly acidic (pKa ~25 in DMSO), enabling deprotonation with strong bases such as n-BuLi or NaH to generate reactive acetylide nucleophiles. The aryl chloride moiety participates in transition-metal-catalyzed cross-coupling reactions, though it is generally less reactive than the corresponding aryl bromide or iodide under standard conditions.

Common synonyms include 4-chlorophenylacetylene and p-chlorophenylacetylene, and researchers should use these interchangeably when searching chemical databases such as SciFinder or Reaxys.

Key Synthetic Applications

Sonogashira Cross-Coupling: The terminal alkyne of 1-Chloro-4-ethynylbenzene is well-suited for Sonogashira coupling with aryl or vinyl halides in the presence of a palladium/copper catalyst system and an amine base. This enables the construction of diarylacetylenes, enediynes, and conjugated oligomers relevant to materials science and medicinal chemistry. Notably, the aryl chloride group can also serve as an electrophilic coupling partner under optimized palladium catalysis conditions, providing orthogonal reactivity when used with the appropriate substrate and catalyst.

CuAAC Click Chemistry: The terminal alkyne group undergoes copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) with organic azides to yield 1,2,3-triazole adducts regioselectively. This reaction is widely exploited in bioconjugation, drug discovery, and polymer functionalization workflows. The electron-withdrawing chloro substituent modulates the electronic density of the alkyne, which can influence reaction kinetics.

Nucleophilic Acetylide Addition: Deprotonation with strong bases produces a para-chlorophenyl acetylide anion capable of adding to aldehydes, ketones, imines, and epoxides to afford propargylic alcohols and amines — key intermediates in natural product synthesis and pharmaceutical development.

Heterocycle Synthesis: Aryl-terminal alkynes are established precursors for transition-metal- or acid-mediated cyclization reactions to form indoles, isoquinolines, benzofurans, and other heterocyclic scaffolds of pharmaceutical relevance.

Safe Handling, Storage, and Waste Disposal

Hazard Overview: 1-Chloro-4-ethynylbenzene is an irritant and should be treated as a potentially harmful chemical. It may cause skin and eye irritation and is harmful if inhaled or ingested. Terminal alkynes can be mildly shock-sensitive in pure form under certain conditions; avoid exposing the neat compound to strong mechanical shock, open flames, or oxidizing agents. Always consult the current Safety Data Sheet (SDS) before use.

Personal Protective Equipment (PPE): Wear nitrile or neoprene gloves, safety glasses or a face shield, and a laboratory coat. Handle in a well-ventilated fume hood. Where aerosol generation is possible, use respiratory protection appropriate to the exposure level.

Storage: Store in a tightly sealed container in a cool (2–8 °C refrigerator recommended), dry location away from direct light, heat sources, ignition sources, and incompatible materials (strong oxidizers, strong bases, reactive metals). Ensure containers are clearly labeled. Check periodically for discoloration or polymerization, both of which may indicate degradation.

Waste Disposal: Dispose of waste material and contaminated items in accordance with local, regional, and national regulations governing halogenated organic solvents and waste. Do not pour down the drain. Use approved waste containers and engage a licensed hazardous waste disposal contractor.

Quality, Purity, and Analytical Verification

Purity Assessment: Research-grade 1-Chloro-4-ethynylbenzene is typically supplied at ≥97% purity (GC). Lot-specific purity data and analytical certificates (CoA) should be requested from the supplier and reviewed before critical synthetic steps.

NMR Characterization: In ¹H NMR (CDCl₃), the terminal alkyne proton appears as a singlet near δ 3.1–3.2 ppm. The aromatic protons of the para-substituted ring appear as two doublets in the region δ 7.2–7.4 ppm. In ¹³C NMR, the alkyne carbons resonate at approximately δ 77–83 ppm.

GC-MS Verification: GC-MS analysis should show a dominant molecular ion at m/z 136/138 (in approximately 3:1 ratio reflecting the ³⁵Cl/³⁷Cl isotope pattern), confirming the presence of a single chlorine atom.

IR Spectroscopy: A strong, sharp absorption near 3300 cm⁻¹ (≡C–H stretch) and a characteristic triple bond stretch near 2100–2140 cm⁻¹ confirm the terminal alkyne functionality.

Stability Indicators: Discoloration to brown or yellow and increased viscosity may indicate oligomerization or oxidative degradation. Such material should not be used in sensitive reactions without repurification.

Frequently asked questions

What is the difference between using 1-Chloro-4-ethynylbenzene as a coupling partner versus a nucleophile?

As a coupling partner in Sonogashira reactions, the terminal alkyne reacts directly with an aryl or vinyl halide under Pd/Cu catalysis without requiring deprotonation. As a nucleophile, the terminal alkyne must first be deprotonated with a strong base (e.g., n-BuLi, NaH) to form a metal acetylide, which then adds to electrophilic substrates such as carbonyl compounds or epoxides. The choice of pathway depends on the target molecule and compatibility with other functional groups present.

Can the aryl chloride group in 1-Chloro-4-ethynylbenzene participate in cross-coupling reactions?

Yes, aryl chlorides can participate in palladium-catalyzed cross-coupling reactions, but they generally require more active catalyst systems compared to aryl bromides or iodides. Bulky, electron-rich phosphine ligands (e.g., SPhos, RuPhos) or N-heterocyclic carbene (NHC) ligands are typically needed to achieve efficient oxidative addition of the C–Cl bond to palladium. The terminal alkyne group must be protected or reacted first if chemoselectivity between the two functional groups is required.

How should I store 1-Chloro-4-ethynylbenzene to maximize shelf life?

Store the compound in a tightly sealed amber or opaque glass vial at 2–8 °C, away from light, heat, and moisture. Exposure to air and light can promote slow oxidative degradation or oligomerization of the terminal alkyne. Under appropriate storage conditions, the reagent should remain stable for an extended period; however, always verify purity by GC or NMR before use in critical experiments.

Is 1-Chloro-4-ethynylbenzene suitable for CuAAC click chemistry applications?

Yes, the terminal alkyne of 1-Chloro-4-ethynylbenzene makes it a suitable substrate for CuAAC click chemistry with organic azides, yielding 1,4-disubstituted 1,2,3-triazoles under standard copper(I) catalysis conditions. The para-chloro substituent provides an electron-withdrawing effect that can modulate reaction rate and product electronics. This compound is useful for preparing triazole-based ligands, bioconjugates, and functional materials.

References

1. Sonogashira, K.; Tohda, Y.; Hagihara, N. A convenient synthesis of acetylenes: catalytic substitutions of acetylenic hydrogen with bromoalkenes, iodoarenes and bromopyridines. Tetrahedron Letters, 1975, 16(50), 4467–4470. doi:10.1016/S0040-4039(00)91094-3
2. Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B. A stepwise Huisgen cycloaddition process: copper(I)-catalyzed regioselective 'ligation' of azides and terminal alkynes. Angewandte Chemie International Edition, 2002, 41(14), 2596–2599. doi:10.1002/1521-3773(20020715)41:14<2596::AID-ANIE2596>3.0.CO;2-4
3. Wolfe, J. P.; Buchwald, S. L. Scope and limitations of the Pd/BINAP-catalyzed amination of aryl bromides. Journal of Organic Chemistry, 2000, 65(4), 1144–1157. doi:10.1021/jo991699y

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