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Vinyl halide
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In organic chemistry, a vinyl halide is a compound with the formula CH2=CHX (X = halide). The term vinyl is often used to describe any alkenyl group. For this reason, alkenyl halides with the formula RCH=CHX are sometimes called vinyl halides. From the perspective of applications, the dominant member of this class of compounds is vinyl chloride, which is produced on the scale of millions of tons per year as a precursor to polyvinyl chloride.[1] Polyvinyl fluoride is another commercial product. Related compounds include vinylidene chloride and vinylidene fluoride.
Synthesis
[edit]Vinyl chloride is produced by dehydrochlorination of 1,2-dichloroethane.[1]
Due to their high utility, many approaches to vinyl halides have been developed, such as:
- reactions of vinyl organometallic species with halogens
- Stork-Zhao olefination with, e.g., (Chloromethylene)triphenylphosphorane - a modification of the Wittig reaction
Reactions
[edit]Vinyl bromide and related alkenyl halides form the Grignard reagent and related organolithium reagents. Alkenyl halides undergo base elimination to give the corresponding alkyne. Most important is their use in cross-coupling reactions (e.g. Suzuki-Miyaura coupling, Stille coupling, Heck coupling, etc.).
See also
[edit]References
[edit]- ^ a b E.-L. Dreher; T. R. Torkelson; K. K. Beutel (2011). "Chlorethanes and Chloroethylenes". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.o06_o01. ISBN 978-3527306732.
- ^ Koh, Ming Joo; Nguyen, Thach T.; Zhang, Hanmo; Schrock, Richard R.; Hoveyda, Amir H. (2016). "Direct synthesis of Z-alkenyl halides through catalytic cross-metathesis". Nature. 531 (7595): 459–465. Bibcode:2016Natur.531..459K. doi:10.1038/nature17396. PMC 4858352. PMID 27008965.
- ^ Nguyen, Thach T.; Koh, Ming Joo; Shen, Xiao; Romiti, Filippo; Schrock, Richard R.; Hoveyda, Amir H. (2016-04-29). "Kinetically controlled E-selective catalytic olefin metathesis". Science. 352 (6285): 569–575. Bibcode:2016Sci...352..569N. doi:10.1126/science.aaf4622. PMC 5748243. PMID 27126041.
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Vinyl halide
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Fundamentals
Definition and Nomenclature
Vinyl halides are organic compounds featuring a halogen atom directly bonded to an sp²-hybridized carbon atom within a carbon-carbon double bond.[7] This structural motif distinguishes them as a subclass of alkenyl halides, where the halogen substitution occurs on the unsaturated carbon framework.[8] The simplest and most representative vinyl halides follow the general formula , where X denotes a halogen—fluorine (F), chlorine (Cl), bromine (Br), or iodine (I).[9] This class encompasses substituted derivatives, such as (with R as an alkyl or aryl substituent) and geminal dihalides termed vinylidene halides, exemplified by .[10] In nomenclature, "vinyl halide" strictly applies to the unsubstituted structure, while "alkenyl halide" denotes the extended family of such compounds.[11] Under IUPAC rules, these are named as haloalkenes, with the parent chain selected to include the double bond and halogen; for instance, is designated chloroethene, and a substituted example like becomes (E)- or (Z)-1-bromoprop-1-ene depending on stereochemistry.[12] The term "vinyl" traces its etymological roots to 19th-century German chemical nomenclature for the ethenyl group (), derived from the Latin vinum (wine), reflecting ethylene's historical preparation from ethyl alcohol distilled from wine.[13] This contrasts with allylic halides, in which the halogen attaches to a sp³-hybridized carbon adjacent to, but not part of, the C=C double bond, such as in .[14] Vinyl chloride () holds particular industrial significance as the precursor to polyvinyl chloride (PVC), a widely used polymer.[5]Molecular Structure and Bonding
Vinyl halides consist of a halogen atom directly attached to a carbon-carbon double bond, exemplified by compounds such as vinyl chloride (CH₂=CHCl). The carbon atoms in the vinyl group are sp² hybridized, leading to a trigonal planar geometry with bond angles of approximately 120° around each carbon involved in the double bond. This hybridization arises from the overlap of one s and two p orbitals to form three sp² hybrid orbitals, with the remaining p orbital on each carbon forming the π bond of the C=C double bond. The planar arrangement restricts rotation around the C=C bond, contributing to the overall rigidity of the molecule.[15] The C-X bond (where X is the halogen) in vinyl halides exhibits partial double-bond character due to resonance, resulting in shorter bond lengths and greater strengths compared to those in alkyl halides. For instance, the C-Cl bond in vinyl chloride is shorter than the typical C-Cl bond in saturated alkyl chlorides like ethyl chloride, reflecting increased s-character in the sp²-hybridized carbon orbital and resonance stabilization.[16] This resonance involves delocalization of the halogen's lone-pair electrons into the π system of the C=C bond, as depicted in the contributing structures: the primary form CH₂=CH–X and the resonance form ⁻CH₂–CH=X⁺, where the p orbital on the halogen overlaps with the adjacent carbon's p orbital to distribute electron density. Such overlap leads to electron delocalization, enhancing bond strength and reducing polarity relative to alkyl halides.[16] In disubstituted vinyl halides, where each carbon of the double bond bears two different substituents, geometric stereoisomerism occurs, designated as E or Z configurations based on Cahn-Ingold-Prelog priority rules. For example, in 1-bromo-2-chloroethene (BrHC=CHCl), the (E) isomer has the higher-priority Br and Cl on opposite sides of the double bond, while the (Z) isomer has them on the same side; this isomerism stems from the restricted rotation imposed by the π bond's partial double-bond character. Spectroscopic methods confirm these structural features: infrared (IR) spectroscopy shows the C=C stretching vibration at 1600–1680 cm⁻¹, shifted slightly lower due to conjugation with the halogen, and C-X stretches around 700–800 cm⁻¹ for C-Cl or 500–600 cm⁻¹ for C-Br.[17] In ¹H NMR, vinylic protons typically resonate at 5–6 ppm, influenced by the deshielding effect of the double bond and nearby electronegative halogen.[18]Properties
Physical Properties
Vinyl halides, compounds of the general formula CH₂=CHX where X is a halogen (F, Cl, Br, or I), are typically colorless gases or low-boiling liquids at room temperature, exhibiting physical properties that vary systematically with the atomic mass and polarizability of the halogen substituent.[2][19] The lighter members, such as vinyl fluoride and vinyl chloride, exist as gases under standard conditions, while heavier analogs like vinyl bromide and vinyl iodide are liquids near ambient temperatures due to increased intermolecular forces from higher molecular weights.[20][21] The melting and boiling points of vinyl halides increase progressively across the halogen series, reflecting rising molecular weights and dipole moments that enhance van der Waals interactions and polarizability. For instance, vinyl fluoride has a melting point of -160.5°C and a boiling point of -72.2°C, vinyl chloride melts at -153.8°C and boils at -13.4°C, vinyl bromide at -139.5°C and 15.6°C, and vinyl iodide boils at 56°C.[22][23][24] This trend underscores the lowest boiling point for vinyl fluoride, attributed to its minimal polarizability compared to the other halides.[5]| Compound | Melting Point (°C) | Boiling Point (°C) | Liquid Density (g/cm³) |
|---|---|---|---|
| Vinyl fluoride | -160.5 | -72.2 | 0.71 (at 0°C) |
| Vinyl chloride | -153.8 | -13.4 | 0.969 (at -14.2°C) |
| Vinyl bromide | -139.5 | 15.6 | 1.51 (at boiling point) |
| Vinyl iodide | Not well-documented | 56 | 2.08 (at 20°C) |
Chemical Properties
Vinyl halides exhibit a high degree of stability in their carbon-halogen (C-X) bonds compared to alkyl halides, primarily due to the sp² hybridization of the carbon atom involved, which results in shorter and stronger bonds with greater s-character (approximately 33% s-character versus 25% in sp³ hybrids).[33] This enhanced bond strength renders vinyl halides significantly less reactive toward nucleophilic substitution reactions than their alkyl counterparts, where the C-X bond is weaker and more susceptible to attack.[33] The partial double-bond character arising from resonance between the halogen's lone pairs and the π-system further contributes to this stability, increasing the energy barrier for bond cleavage.[34] This inherent stability manifests in strong resistance to hydrolysis and nucleophilic substitution mechanisms such as SN1 and SN2. In the vinylic position, the departure of the halide leaving group is disfavored because it would require forming an unstable vinyl carbocation lacking effective resonance stabilization, unlike the more stable alkyl carbocations in SN1 pathways.[33] For SN2 reactions, the planar geometry of the sp² carbon imposes severe steric hindrance, preventing effective backside attack by nucleophiles and reducing reaction rates by orders of magnitude relative to alkyl halides.[33] Consequently, vinyl halides remain largely inert under conditions that readily hydrolyze alkyl halides. The vinylic hydrogens in vinyl halides display moderate acidity, with a pKa value around 44-45, making them slightly more acidic than the hydrogens in alkanes (pKa ≈ 50) but far less acidic than those in terminal alkynes (pKa ≈ 25).[35] This enhanced acidity relative to alkanes stems from the sp² hybridization, which increases the s-character of the C-H bond and better stabilizes the resulting vinyl anion through resonance with the adjacent π-bond. Vinyl halides, particularly vinyl chloride, show sensitivity to light and heat, undergoing photochemical decomposition that involves the elimination of hydrogen chloride (HCl).[36] Upon absorption of ultraviolet light around 193-210 nm, vinyl chloride dissociates primarily via HCl elimination, forming acetylene and chlorine atoms, a process driven by excitation to repulsive potential energy surfaces.[36] Thermal decomposition follows a similar pathway at elevated temperatures, highlighting the compound's instability under energetic conditions despite overall bond robustness. Stability trends among vinyl halides follow the order fluorides > chlorides > bromides > iodides, reflecting the decreasing bond dissociation energies of the C-X bonds (C-F ≈ 485 kJ/mol, C-Cl ≈ 385 kJ/mol, C-Br ≈ 285 kJ/mol, C-I ≈ 240 kJ/mol in vinyl systems).[37] Vinyl fluorides are the most stable due to the strong, short C-F bond and high electronegativity of fluorine, while vinyl iodides are the least stable, prone to easier cleavage. This trend influences their dipole moments, with vinyl chloride exhibiting a value of 1.4 D, arising from the electronegativity difference and molecular geometry.[38]Synthesis
Laboratory Methods
Laboratory methods for synthesizing vinyl halides in research settings prioritize flexibility, enabling the preparation of structurally diverse and stereochemically defined compounds on a small scale. These approaches leverage elimination reactions, organometallic additions, and phosphorus-based olefinations, often under mild conditions that allow precise control over regiochemistry and geometry. Unlike industrial processes focused on commodity production, laboratory techniques emphasize adaptability for complex substrates and functional group compatibility. Dehydrohalogenation of vicinal dihalides represents a classical and straightforward route to vinyl halides, typically employing a strong base to facilitate E2 elimination. This method is particularly effective for terminal vinyl halides, where the vicinal dihalide is treated with alcoholic potassium hydroxide (KOH) to remove hydrogen bromide (HBr). For example, 1,2-dibromoethane reacts to form vinyl bromide:
The reaction proceeds via anti-periplanar geometry in the E2 mechanism, ensuring stereospecificity when applied to cyclic or substituted systems.[39]
Elimination reactions from geminal dihalides provide an alternative entry to vinyl halides, again using base-induced dehydrohalogenation to generate the alkene. Geminal dihalides, where both halogens reside on the same carbon, undergo E2 elimination with a base such as sodium ethoxide, abstracting an adjacent proton and expelling one halide. A representative example is the conversion of 1,1-dihaloethane to vinyl halide:
This approach is valuable for preparing monosubstituted vinyl halides from readily available gem-dihalides, with the reaction favoring the less substituted alkene due to the base strength and solvent choice.[39]
A common laboratory method for preparing vinyl halides is the electrophilic addition of hydrogen halides (HX) to alkynes, following Markovnikov's rule. For terminal alkynes such as acetylene, one equivalent of HX adds to yield the 2-haloalkene. For example:
The reaction can be controlled for monoaddition using catalytic amounts of mercury(II) salts or peroxides for regioselectivity, preventing further addition to geminal dihalides.[40]
Carbometalation of alkynes offers a stereoselective pathway to substituted vinyl halides, involving the syn addition of an organozinc reagent across the triple bond followed by electrophilic quenching. For terminal alkynes like acetylene (HC≡CH), organozinc halides (RZnX) add in a regioselective manner, with the zinc attaching to the terminal carbon, yielding a vinylzinc intermediate. Subsequent quenching with a halogen source, such as iodine or bromine, affords the corresponding vinyl halide with high (E)- or (Z)-selectivity depending on the reagent and conditions. This method excels in constructing functionalized vinyl halides for further synthetic elaboration.[41]
Olefination techniques, particularly variants of the Wittig reaction, enable the synthesis of substituted vinyl halides from carbonyl compounds, providing control over substitution patterns. In the Wittig process, a halo-substituted phosphonium ylide reacts with an aldehyde to form the alkene, often with incorporation of the halide via in situ halogenation. For instance, β-oxidophosphonium ylides generated from aldehydes and Wittig reagents can be trapped with electrophilic halogens to yield (E)-β-aryl vinyl halides in a one-pot manner. Complementary to this, the Stork-Zhao olefination utilizes iodomethylene phosphorane (generated from CHI₃ and Ph₃P) with aldehydes to produce (Z)-vinyl iodides stereoselectively under mild conditions, ideal for sensitive substrates. These methods are prized for their compatibility with diverse functional groups and tunable stereochemistry.[42][43]
Industrial Production
The industrial production of vinyl halides primarily focuses on vinyl chloride monomer (VCM), which accounts for the vast majority of global output due to its role as the precursor for polyvinyl chloride (PVC). Historically, VCM was produced via the hydrochlorination of acetylene derived from coal or calcium carbide, a process dominant before the 1950s. Post-World War II, the availability of inexpensive petroleum-derived ethylene led to a rapid shift to ethylene-based routes, which are more economical and scalable, rendering acetylene processes obsolete in most regions by the early 1960s. This transition was driven by lower feedstock costs and improved process efficiency, with ethylene-based production now comprising over 95% of global VCM capacity.[44] The dominant method for VCM production is the balanced ethylene process, which integrates direct chlorination, oxychlorination, and thermal pyrolysis to achieve high chlorine utilization and minimize waste. In direct chlorination, ethylene reacts with chlorine gas in a liquid-phase reactor at 40–70°C using ferric chloride (FeCl₃) as a catalyst to form 1,2-dichloroethane (EDC):
This exothermic step occurs in a water-cooled reactor to control temperature. The EDC is then purified and subjected to pyrolysis in a furnace at approximately 500°C and low pressure (1–3 atm), cracking it to VCM and hydrogen chloride:
The HCl byproduct is recycled into the oxychlorination step, where it reacts with additional ethylene and oxygen (or air) in a fluidized-bed reactor at 200–250°C, typically using a copper chloride (CuCl₂)-based catalyst supported on alumina:
This closes the chlorine loop, with the overall balanced process yielding a net reaction of:
[45]
Global VCM production reached approximately 47 million metric tons in 2022, with major producers in China, the United States, and Europe operating integrated facilities that consume significant energy—typically 10–15 GJ per ton of VCM—for the high-temperature pyrolysis and compression steps.[46] Fluidized-bed reactors in oxychlorination enhance selectivity (>95% to EDC) but require robust catalyst regeneration to handle coke formation and maintain yields.[45] These processes are highly integrated with chlorine production from electrolysis, emphasizing process engineering for energy efficiency and byproduct management. VCM is primarily used in PVC polymerization, linking production directly to downstream plastics manufacturing.
Vinyl fluoride, a less common vinyl halide, is produced industrially via the catalytic hydrofluorination of acetylene with hydrogen fluoride at moderate temperatures (around 40–100°C), using mercury(II) chloride on activated carbon or aluminum-based catalysts:
This vapor-phase reaction achieves high selectivity (>90%) under controlled conditions to avoid over-fluorination to 1,1-difluoroethane.[47] Unlike VCM, vinyl fluoride production remains acetylene-based due to the specificity of the fluorination chemistry and smaller market scale, primarily for polyvinyl fluoride applications.[47]