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2-Butyne

2-Butyne, with the IUPAC name but-2-yne, is a symmetrical internal featuring a carbon-carbon between the second and third carbon atoms in a four-carbon chain, and its molecular formula is C₄H₆. The is CH₃C≡CCH₃, and it is also known by synonyms such as dimethylacetylene and crotonylene. This compound appears as a clear, colorless to light yellow liquid that is highly volatile and flammable. Key physical properties of 2-butyne include a molecular weight of 54.09 g/, a melting point of -32 °C, and a boiling point of 27 °C. Its is 0.691 g/mL at 25 °C, and it has a of -25 °C, underscoring its extreme flammability. Chemically, as an , 2-butyne reacts with oxidizing agents and may undergo exothermic in the presence of catalysts, but it lacks the terminal that makes terminal alkynes acidic. In , 2-butyne serves as a valuable intermediate for producing alkylated hydroquinones, such as those used in the manufacture of , and in the preparation of pharmaceutical compounds. It is typically stored under refrigerated conditions (2-8 °C) to maintain stability, and handling requires precautions due to its hazards, including skin , eye , and respiratory issues from inhalation. Commercial availability often features purity greater than 97% by .

Structure

Molecular formula and nomenclature

2-Butyne has the molecular formula C_4H_6 and the \ce{CH3C#CCH3}, consisting of a four-carbon chain with a between the second and third carbon atoms. The IUPAC name for this is , derived from the parent chain "" with the suffix "-yne" indicating the presence of a , and the "2-" specifying the position of the to give it the lowest possible number in the chain. Common synonyms for but-2-yne include dimethylacetylene and crotonylene, reflecting its structure as an internal with two methyl groups attached to the triple-bonded carbons. In contrast, (but-1-yne) is a terminal alkyne with the structural formula \ce{CH3CH2C#CH}, where the triple bond is located at the end of the chain between carbons 1 and 2.

Bonding and geometry

The triple bond in 2-butyne is composed of one σ bond formed by the overlap of sp hybrid orbitals from the adjacent carbon atoms and two π bonds resulting from the sideways overlap of unhybridized p orbitals. The experimental C≡C is 1.214 ± 0.001 Å, as determined by gas-phase , reflecting the high electron density and strong bonding characteristic of s. The carbon atoms at positions 2 and 3 exhibit hybridization, with each possessing two hybrid orbitals and two p orbitals. This hybridization configuration leads to a linear around the triple bond, with bond angles of 180° for the C–C≡C–C framework, minimizing repulsion between the bonded groups. Due to the cylindrical symmetry of the , the two methyl groups experience a very low torsional barrier to internal , measured at 2.5 cm⁻¹ (approximately 30 J/mol), allowing nearly free rotation at . This barrier arises primarily from weak hyperconjugative interactions rather than steric hindrance. The equilibrium conformation of 2-butyne with respect to orientation remains undetermined experimentally, as the low torsional barrier blurs the distinction between the eclipsed (D_{3h}) and staggered (D_{3d}) forms; theoretical calculations favor the staggered D_{3d} structure by a small margin, but spectroscopic evidence supports a dynamic closer to free rotation.

Properties

Physical properties

2-Butyne appears as a clear, colorless with a pungent, petroleum-like . It is highly volatile, existing as a at standard but readily evaporating due to its low . The compound has a of 54.09 g/mol. Key physical measurements include a of 0.691 g/mL at 25 °C, a of -32 °C, and a of 27 °C. These properties reflect its behavior as a small-molecule , making it suitable for handling under controlled conditions to prevent rapid vaporization. Regarding , 2-butyne shows low solubility in , rendering it immiscible under standard conditions. In contrast, it is miscible with common organic solvents such as and , facilitating its use in non-aqueous chemical processes.

Thermodynamic properties

The (ΔH_f°) of 2-butyne in the gas phase is +145.1 ± 1.0 kJ/mol at 298 K. This value, determined from , reflects the endothermic nature of the molecule relative to its constituent elements in their standard states, highlighting the energetic cost of forming the carbon-carbon . An alternative measurement yields +148.0 ± 1.5 kJ/mol, consistent within experimental . Phase transitions of 2-butyne involve moderate energy changes. The (ΔH_fus) is 9.23 kJ/ at the of 241 . Calorimetric studies also identify solid-solid transitions in the low-temperature regime, with associated enthalpies contributing to anomalous capacities observed between 145 K and 160 K; these transitions arise from rotational disordering in the lattice. The (ΔH_vap) at the normal of 300 K is 26.7 kJ/, indicating relatively weak intermolecular forces in the liquid phase dominated by van der Waals interactions. Entropy and heat capacity data further elucidate the molecular degrees of freedom. The standard molar entropy (S°) of gaseous 2-butyne at 298 K is 283 J/mol·K, accounting for translational, rotational, and vibrational contributions, with the linear enhancing rotational compared to branched isomers. For the liquid phase, S° is approximately 195 J/mol·K near 291 K. The constant-pressure (C_p) of the gas increases with temperature, from 78.0 J/mol·K at 298 K to higher values at elevated temperatures due to progressive activation of vibrational modes; for instance, it is about 39 J/mol·K at 50 K. In the , C_p is around 124 J/mol·K at 290 K. These properties underscore 2-butyne's volatility and suitability for gas-phase applications.

Spectroscopic properties

The (IR) spectrum of 2-butyne exhibits a weak C≡C stretching absorption at approximately 2230 cm⁻¹, resulting from the molecule's high (D_{3h}), which makes the symmetric stretch nearly IR-inactive but observable as a weak band in the gas phase. The C-H stretching vibrations of the equivalent methyl groups occur in the characteristic range of 2900–3000 cm⁻¹. Raman spectroscopy complements IR by activating the symmetric C≡C stretching mode, observed as a strong band at ~2260 cm⁻¹ due to the change in polarizability. In nuclear magnetic resonance (NMR) spectroscopy, the ¹H NMR spectrum displays a single peak at 1.74 ppm (singlet, 6H) for the equivalent methyl protons, reflecting the molecule's symmetry. The ¹³C NMR spectrum shows two signals: one at 3.6 ppm for the methyl carbons and another at 74.5 ppm for the alkyne carbons, consistent with the distinct environments of sp³- and sp-hybridized carbons. Microwave spectroscopy reveals a very low torsional barrier to internal methyl in neutral 2-butyne, estimated at 2–6 cm⁻¹, indicating nearly free and a planar, symmetric conformation. Pulsed-field ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy of the cation confirms this low barrier, with evidence of nearly free internal and weak Jahn-Teller distortion leading to dynamic conformers of C_{2v} and C_{2h} .

Synthesis

From dihalides

2-Butyne is synthesized on a laboratory scale through the double dehydrohalogenation of vicinal dihalides, such as 2,3-dichlorobutane or 2,3-dibromobutane, employing a strong base like sodium amide (NaNH₂) in liquid ammonia. This process involves two sequential E2 elimination steps: the first removes one equivalent of hydrogen halide to form a vinyl halide intermediate, and the second eliminates another equivalent to yield the internal alkyne. The balanced reaction equation using 2,3-dibromobutane as the precursor is: \ce{CH3CHBrCHBrCH3 + 2 NaNH2 -> CH3C#CCH3 + 2 NaBr + 2 NH3} The reaction requires conditions and an inert atmosphere, such as , to minimize side reactions like over-deprotonation or of the product. Two equivalents of NaNH₂ are typically employed, and the procedure is conducted at low temperatures (around -33°C, the of liquid ) to ensure selectivity. Yields for this transformation are generally high, ranging from 70-85% for similar internal alkynes, depending on the purity of the dihalide starting material. This double dehydrohalogenation serves as a classical method for preparing internal alkynes and has been established since the early . The vicinal dihalides are readily obtained by of halogens to the corresponding alkenes, making this route versatile for accessing symmetrical internal alkynes like 2-butyne.

Isomerization of terminal alkynes

One method for synthesizing 2-butyne involves the base-catalyzed of the terminal alkyne (also known as ethylacetylene), which shifts the from the terminal position to the internal position. This rearrangement is typically carried out using ethanolic (KOH) or (NaNH₂) as the base, proceeding through a transient allene intermediate. The reaction can be represented as follows: \ce{CH3CH2C#CH ->[base] [CH2=C=CHCH3] ->[base] CH3C#CCH3} Under reflux conditions in with KOH, the equilibrium strongly favors the internal alkyne 2-butyne due to its greater thermodynamic stability compared to the terminal isomer, resulting in yields of approximately 90%. The mechanism involves sequential deprotonation at the propargylic position (the carbon adjacent to the ) by the , forming an allenyl anion , followed by reprotonation at the appropriate to migrate the triple bond inward. This process repeats until the more stable internal alkyne predominates, with the allene serving as a key transient species that facilitates the bond shift without net addition or loss of atoms.

Reactions

Electrophilic additions

2-Butyne, as a symmetrical internal , undergoes reactions where the electron-rich acts as a , attacking various s to form intermediates that can further react. These additions typically proceed via a mechanism involving the π electrons of the attacking the , generating a intermediate stabilized by the adjacent . The then reacts with a , often leading to or products that may tautomerize or add further. This reactivity is analogous to that of alkenes but often allows for double addition due to the remaining double bond in the initial product./Alkynes/Reactivity_of_Alkynes/Electrophilic_Addition_Reactions_of_Alkynes) In the of halides (HX, where X = Cl or Br), 2-butyne follows , with the adding to the carbon with more hydrogens, though symmetry makes irrelevant. The reaction yields 2-halo-2-butene as the primary product from the first , with the attaching to one of the triple-bonded carbons. Excess HX can lead to geminal dihalides, but controlled conditions typically stop at the . For example, treatment with HBr produces (E/Z)-2-bromobut-2-ene. The involves of the to form a vinylic at the internal carbon, followed by attack._Complete_and_Semesters_I_and_II/Map:Organic_Chemistry(Wade)/10:_Alkynes/10.03:Reactions_of_Alkynes-_Addition_of_HX_and_X) Hydration of 2-butyne, catalyzed by mercury(II) ions (Hg²⁺) in acidic conditions, adds across the to afford butan-2-one (methyl ethyl ketone) after keto-enol tautomerization. The Hg²⁺ catalyst coordinates to the , facilitating nucleophilic attack by and promoting Markovnikov orientation, resulting in an intermediate that tautomerizes to the . This reaction is particularly useful for synthesizing symmetrical s from internal alkynes, with the process typically conducted in dilute with HgSO₄. Unlike terminal alkynes, no issues arise due to symmetry. Halogenation of 2-butyne with halogens such as Br₂ or Cl₂ proceeds via electrophilic addition, often requiring excess reagent for complete saturation. The initial addition forms a vinyl dihalide, which further reacts to yield the geminal tetrahalide, 2,2,3,3-tetrabromobutane in the case of bromine or the analogous tetrachloride with chlorine. The mechanism mirrors that of alkenes, with the halogen forming a halonium-like intermediate or vinyl carbocation, leading to anti addition in each step. These reactions are typically carried out in inert solvents like CCl₄, and the tetrahalides serve as precursors for dehalogenation back to alkynes./Alkynes/Reactivity_of_Alkynes/Electrophilic_Addition_Reactions_of_Alkynes)

Reduction reactions

The reduction of 2-butyne primarily involves the addition of hydrogen to the , yielding either alkenes or alkanes depending on the conditions and reagents employed. Partial hydrogenation using Lindlar's selectively reduces the to a cis double bond, producing cis-2-butene. This consists of palladium on , poisoned with and (or derivatives thereof), which moderates the reaction to prevent over-reduction. The process proceeds via syn addition of hydrogen on the surface, ensuring stereoselectivity for the cis isomer under mild conditions, typically at and . The reaction is represented as: \ce{CH3C#CCH3 + H2 ->[Lindlar's][Pd/BaSO4][quinoline] cis-CH3CH=CHCH3} This method exploits the symmetric structure of 2-butyne to favor clean cis selectivity without isomerization complications. In contrast, dissolving metal reduction with sodium (or ) in liquid at low temperatures (around -33 °C) yields trans-2-butene through anti addition. The mechanism involves sequential to form a vinyl radical and anion, followed by protonation steps that favor the more stable trans-vinyl anion due to reduced steric hindrance. This stereoselective transformation is highly effective for internal alkynes like 2-butyne, proceeding without over-reduction under controlled conditions. The equation is: \ce{CH3C#CCH3 + 2[Na/NH3] -> trans-CH3CH=CHCH3 + 2NaNH2} Careful monitoring of reagent stoichiometry ensures the reaction halts at the alkene stage. Complete hydrogenation to the saturated , n-butane, requires two equivalents of and more active catalysts such as , , or , often supported on carbon or used as . These conditions drive the reaction through an unisolable intermediate to the fully reduced product, typically under elevated or for efficiency. The overall transformation is: \ce{CH3C#CCH3 + 2H2 ->[Pt][or][Pd][or][Ni] CH3CH2CH2CH3} This method lacks as the final product is achiral, but it provides quantitative yields for total saturation.

Applications

Organic synthesis

2-Butyne plays a key role as a building block in , particularly for constructing carbon frameworks in pharmaceuticals and natural products, leveraging its internal functionality for selective C-C bond formations. In the of , 2-butyne is utilized alongside to prepare alkylated s, which serve as critical intermediates for assembling the chroman ring system with the required substitution pattern. This alkylation strategy introduces the necessary methyl groups on the hydroquinone core, facilitating subsequent coupling with the phytyl to yield the target . 2-Butyne derivatives are employed as reagents in coupling reactions to generate extended chains incorporated into drug intermediates. For example, 2-butyne biscarbonate acts as a in a Rh(III)-catalyzed [4 + 2] cyclization with N-methoxybenzamides and maleimides, enabling the construction of heterocycles. This approach enhances synthetic efficiency by combining coupling and cyclization steps. The [2+2] of 2-butyne with s forms cyclobutenone derivatives, providing strained scaffolds valuable in pharmaceutical synthesis due to their potential for ring-opening or expansion to larger carbocycles and heterocycles. The reaction involves the concerted addition across the and the ketene C=C, yielding 3,4-dimethylcyclobutenone as the product from dimethylacetylene and ketene; computational studies confirm the preference for cyclobutenone over alternative oxete pathways for such internal alkynes, with activation barriers around 7-11 kcal/mol (30-45 kJ/mol) depending on substituents. These cyclobutenones are exploited in total syntheses of bioactive natural products and drug-like molecules, such as through Nazarov cyclization or photoinduced rearrangements. A representative application is the Diels-Alder reaction of 2-butyne as a dienophile with dienes to afford alkyne-containing heterocycles. For instance, in the of cycloiptycene frameworks from carbon nanobelts, 2-butyne undergoes [4+2] with a polycyclic , generating a bridged bicyclic with ΔG° = -15.6 kcal/mol, which can be further functionalized to fused heterocyclic systems relevant to pharmaceutical scaffolds like kinase inhibitors. This highlights 2-butyne's utility in creating rigid, alkyne-embedded structures for .

Industrial uses

2-Butyne is commercially available from major chemical suppliers such as , where it is provided in high purity (≥99%) for laboratory research and small-scale applications. In processes, 2-butyne serves as a key model compound for developing and testing catalysts in selective reactions, which are essential for removing impurities from olefin streams to produce high-purity and other alkenes without over-hydrogenation. For instance, it is routinely used to evaluate Lindlar's catalyst and similar Pd-based systems, achieving stereoselective reduction to cis-2-butene under mild conditions, mimicking industrial purification steps. In , 2-butyne is employed in the formation of π-complexes and acetylide-like intermediates for catalytic applications, including alkyne cyclotrimerization to derivatives, which supports the synthesis of aromatic building blocks for . 2-Butyne plays a significant role in research on polymerization, particularly as a symmetric for producing conjugated polyacetylenes using nickelocene-based s, which exhibit semiconducting and conductive properties suitable for electronic materials. These studies contribute to the of conductive polymers by exploring and optimizing efficiency for scalable production.

Safety

Hazards

2-Butyne is classified as a highly , falling under Flammable Liquids Category 1 according to GHS standards, due to its extremely low of -25°C (-13°F). This property makes it prone to ignition from common sources such as open flames, sparks, or hot surfaces, with vapors capable of forming explosive mixtures with air even at ambient temperatures. The lower explosive limit is reported as 1.4% by volume in air, indicating that concentrations above this threshold can lead to rapid or upon ignition. The compound's reactivity poses additional , particularly when in contact with strong oxidizing agents, where it can undergo violent reactions, potentially resulting in or explosions. Containers exposed to or may rupture due to buildup from vapor expansion. Furthermore, 2-butyne exhibits high , with a of 24-27°C (75-81°F), allowing it to readily evaporate and produce vapors heavier than air (vapor 1.86). These vapors can travel along the ground to distant ignition sources and flash back, exacerbating risks. In confined or poorly ventilated spaces, the accumulation of these dense vapors presents an asphyxiation hazard by displacing oxygen, potentially leading to oxygen-deficient atmospheres without prior warning. Proper and are essential to mitigate such risks during or use.

Toxicity and handling

2-Butyne is classified as a irritant (Category 2), causing redness and upon contact, and a serious eye irritant (Category 2A), potentially leading to redness, pain, and temporary vision impairment. of vapors can result in , manifesting as coughing, , , , , or , due to its specific target organ toxicity for the (single exposure, Category 3). No effects are specifically documented, and data, including LD50 values, are not well-established for oral, dermal, or routes, indicating it is not highly toxic but primarily acts as an irritant. Chronic effects from prolonged or repeated to 2-butyne are limited in available data, with no established evidence of carcinogenicity, mutagenicity, or ; however, repeated respiratory may contribute to ongoing or potential target organ effects, though specific studies are lacking. As a volatile , it may function as a simple asphyxiant in high concentrations by displacing oxygen in confined spaces, though this is not a primary classified . Safe handling of 2-butyne requires working in a well-ventilated area or to minimize vapor , with avoidance of ignition sources to prevent risks during manipulation. Storage should occur in a cool (2-8°C), dry, well-ventilated location with containers kept tightly closed to maintain integrity, though storage is not typically required. Appropriate includes chemical-resistant gloves, safety goggles or face shield, and protective clothing; a with appropriate filters (e.g., for vapors) is recommended when vapor levels may exceed safe thresholds.