4-Ethynylpiperidine Hydrochloride

This compound is typically obtained as a white to off-white crystalline powder. Its molecular formula is C7H12ClN, corresponding to a molecular weight of 145.63. The melting point generally falls within the range of 160–165 °C, often with decomposition evidenced by darkening. The calculated density is approximately 1.09 g/cm³ under ambient conditions. It is freely soluble in water and polar organic solvents such as methanol and ethanol due to the hydrochloride salt form, while showing limited solubility in aprotic solvents like acetonitrile and negligible solubility in nonpolar solvents such as dichloromethane and hexane. The molecule contains a piperidine ring with an ethynyl group at the 4position, presented as the hydrochloride salt. The terminal alkyne is susceptible to Sonogashira coupling, click chemistry, and other transformations, while the piperidine nitrogen provides basicity and nucleophilic reactivity. Storage in tightly sealed containers protected from light and moisture at ambient temperature is generally adequate, though desiccated conditions are recommended for prolonged storage. Contact with strong oxidizing agents and heavy metal salts should be avoided.

4-Ethynylpiperidine hydrochloride combines a saturated nitrogen heterocycle with a terminal alkyne functionality within a compact molecular framework. The piperidine ring provides a conformationally flexible, basic amine capable of protonation and hydrogen bonding, a structural motif widely exploited in medicinal chemistry to modulate solubility and receptor interactions. The ethynyl group at the 4-position introduces a rigid, linear handle for further functionalization through copper-catalyzed azide-alkyne cycloaddition (click chemistry), Sonogashira cross-coupling, or metal coordination. The hydrochloride salt form enhances aqueous solubility and crystalline stability compared to the free base. This combination of a basic amine and a reactive alkyne on a saturated heterocyclic scaffold makes the compound a versatile building block for constructing diverse molecular libraries in pharmaceutical research, chemical biology, and materials science, where the ethynyl group enables rapid and selective conjugation while the piperidine provides solubility and target engagement.

Click Chemistry Building Block
This ethynylpiperidine derivative is extensively employed in copper-catalyzed azide-alkyne cycloaddition reactions for bioconjugation, drug discovery, and materials science. The terminal alkyne reacts efficiently with azide-functionalized molecules to form stable 1,2,3-triazole linkages, enabling the construction of antibody-drug conjugates, targeted therapeutics, and molecular probes under mild, bioorthogonal conditions.

Pharmaceutical Intermediate
In medicinal chemistry, the compound serves as a key building block for synthesizing kinase inhibitors, G-protein coupled receptor modulators, and other therapeutic agents. The piperidine nitrogen can be acylated or alkylated to introduce pharmacophoric groups, while the ethynyl handle allows attachment of diverse aryl, heteroaryl, or other functional moieties through Sonogashira couplings. This dual reactivity enables rapid exploration of structure-activity relationships in drug discovery programs targeting neurological disorders, oncology, and infectious diseases.

Ligand for Metal Complexes
The ethynyl group can coordinate to transition metals or serve as a handle for attaching metal-binding moieties. Piperidine-based ligands derived from this scaffold are investigated for their catalytic activity in cross-coupling and hydrogenation reactions. The basic nitrogen can also participate in metal coordination, enabling the design of bidentate ligand systems with well-defined geometries.

Materials Science Applications
The rigid, linear alkyne functionality makes this compound valuable for constructing conjugated polymers and metal-organic frameworks. Incorporation into polymeric backbones through cross-coupling polymerization yields materials with tunable optoelectronic properties for applications in organic electronics and sensing. The piperidine ring can impart solubility and processability while providing sites for post-synthetic modification.