Acetylide

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In chemistry, an acetylide is a compound that can be viewed as the result of replacing one or both hydrogen atoms of acetylene (ethyne) HC≡CH by metallic or other cations. The term is also used, more loosely, for any compound obtained in the same way from an acetylene derivative RC≡CH, where R is some organic side chain. [1]

Contents

An acetylide may be a salt (ionic compound) containing the anion C≡C2−, HC≡C−, or RC≡C−, as in sodium acetylide [Na+]2C≡C2− or cobalt acetylide Co2+C≡C2−. [2] Other acetylides have the metal bound to the carbon atom(s) by covalent bonds, being therefore coordination or organometallic compounds.

When both hydrogens of acetylene are replaced by metals, the compound is a special case of carbide, and may be commonly called such, as in calcium carbide Ca2+C≡C2−. When only one hydrogen atom is replaced, the anion may be called hydrogen acetylide or the prefix mono- may be attached to the metal, as in monosodium acetylide Na+HC≡C−.

Calcium carbide is an important industrial compound, which has long been used to produce acetylene for welding and illumination. Other acetylides are reagents in organic synthesis.

Structure and bonding

Acetylides of the general formula RC≡CM (where R = H or alkyl) generally show similar properties to their doubly substituted analogues.

Ionic acetylides

Alkali metal and alkaline earth metal acetylides of the general formula MC≡CM are salt-like Zintl phase compounds, containing C2−
2 ions. Evidence for this ionic character can be seen in the ready hydrolysis of these compounds to form acetylene and metal oxides, and by solubility in liquid ammonia with solvated C2−
2 ions [3] .

The C2−
2 ion has a closed shell ground state of 1Σ+
g, making it isoelectronic to a neutral molecule N2, which may afford it some gas-phase stability. [4]

Organometallic acetylides

Some acetylides, particularly of transition metals, show evidences of covalent character, e. g. for being neither dissolved nor decomposed by water and by radically different chemical reactions. That seems to be the case of silver acetylide and copper acetylide, for example.

In the absence of additional ligands, metal acetylides adopt polymeric structures wherein the acetylide groups are bridging ligands.

Preparation

Acetylene and terminal alkynes are weak acids: [9]

RC≡CH + R″M ⇌ R″H + RC≡CM

Monopotassium and monosodium acetylide can be prepared by reacting acetylene with bases like sodium amide) [10] or the elemental metals, often at room temperature and atmospheric pressure. [9]

Precipitate of copper acetylide hydrate CuC2.H2O from acetylene and copper(1) chloride. Cu2C2 precipitate.JPG
Precipitate of copper acetylide hydrate CuC2.H2O from acetylene and copper(1) chloride.

Copper(I) acetylide can be prepared by passing acetylene through an aqueous solution of copper(I) chloride because of a low solubility equilibrium. [9] Similarly, silver acetylides can be obtained from silver nitrate.

Calcium carbide is prepared by heating carbon with lime (calcium oxide) at approximately 2,000 °C. A similar process is used to produce lithium carbide.

In organic synthesis, acetylides are usually prepared by reacting acetylene and alkynes with organometallic [11] or inorganic [10] superbases in solvents which are less acidic than the terminal alkyne. The classical solvent was liquid ammonia, but ethers are now more commonly used.

Lithium amide, [9] LiHMDS, [12] or organolithium reagents, such as butyllithium (BuLi), [11] are frequently used to form lithium acetylides:

Reactions

Ionic acetylides are typically decomposed by water with evolution of acetylene:

CaC≡C + 2H2O → Ca(OH)2 + HC≡CH
RC≡CNa + H2O → RC≡CH + NaOH

Acetylides of the type RC2M are widely used in alkynylations in organic chemistry. They are nucleophiles that add to a variety of electrophilic and unsaturated substrates.

A classic application is the Favorskii reaction, such as in the sequence shown below. Here ethyl propiolate is deprotonated by n-butyllithium to give the corresponding lithium acetylide. This acetylide adds to the carbonyl center of cyclopentanone. Hydrolysis liberates the alkynyl alcohol. [13]

Reaction of ethyl propiolate with n-butyllithium to form the lithium acetylide. Acetylide carbonyl addition.png
Reaction of ethyl propiolate with n-butyllithium to form the lithium acetylide.

The dimerization of acetylene to vinylacetylene proceeds by insertion of acetylene into a copper(I) acetylide complex. [14]

Coupling reactions

Acetylides are sometimes used as intermediates in coupling reactions. Examples include Sonogashira coupling, Cadiot-Chodkiewicz coupling, Glaser coupling and Eglinton coupling.

Hazards

Some acetylides are notoriously explosive. [15] Formation of acetylides poses a risk in handling of gaseous acetylene in presence of metals such as mercury, silver or copper, or alloys with their high content (brass, bronze, silver solder).

See also

Related Research Articles

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Acetylene is the chemical compound with the formula C2H2 and structure H−C≡C−H. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in its pure form and thus is usually handled as a solution. Pure acetylene is odorless, but commercial grades usually have a marked odor due to impurities such as divinyl sulfide and phosphine.

<span class="mw-page-title-main">Alkyne</span> Hydrocarbon compound containing one or more C≡C bonds

In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and no other functional groups form a homologous series with the general chemical formula CnH2n−2. Alkynes are traditionally known as acetylenes, although the name acetylene also refers specifically to C2H2, known formally as ethyne using IUPAC nomenclature. Like other hydrocarbons, alkynes are generally hydrophobic.

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<span class="mw-page-title-main">Copper(II) acetate</span> Chemical compound

Copper(II) acetate, also referred to as cupric acetate, is the chemical compound with the formula Cu(OAc)2 where AcO− is acetate (CH
3CO−
2). The hydrated derivative, Cu2(OAc)4(H2O)2, which contains one molecule of water for each copper atom, is available commercially. Anhydrous copper(II) acetate is a dark green crystalline solid, whereas Cu2(OAc)4(H2O)2 is more bluish-green. Since ancient times, copper acetates of some form have been used as fungicides and green pigments. Today, copper acetates are used as reagents for the synthesis of various inorganic and organic compounds. Copper acetate, like all copper compounds, emits a blue-green glow in a flame.

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<span class="mw-page-title-main">Organocopper chemistry</span> Compound with carbon to copper bonds

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2C
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<i>ortho</i>-Carborane Chemical compound

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<span class="mw-page-title-main">Hexa(tert-butoxy)ditungsten(III)</span> Chemical compound

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References

  1. ↑ IUPAC , Compendium of Chemical Terminology , 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) " acetylides ". doi : 10.1351/goldbook.A00067
  2. ↑ Junichi Nishijo, Kentaroh Kosugi, Hiroshi Sawa, Chie Okabe, Ken Judai, Nobuyuki Nishi (2005): "Water-induced ferromagnetism in cobalt acetylide CoC2 nanoparticles". Polyhedron, volume 24, issues 16–17, pages 2148-2152. doi : 10.1016/j.poly.2005.03.032
  3. ↑ Hamberger, Markus; Liebig, Stefan; Friedrich, Ute; Korber, Nikolaus; Ruschewitz, Uwe (21 December 2012). "Evidence of Solubility of the Acetylide Ion C2−
    2    : Syntheses and Crystal Structures of K2C2·2 NH3, Rb2C2·2 NH3, and Cs2C2·7 NH3". Angewandte Chemie International Edition. 51 (52): 13006–13010. doi:10.1002/anie.201206349. PMID   23161511.
  4. ↑ Sommerfeld, T.; Riss, U.; Meyer, H.-D.; Cederbaum, L. (August 1997). "Metastable C2−
    2     Dianion". Physical Review Letters. 79 (7): 1237–1240. Bibcode:1997PhRvL..79.1237S. doi:10.1103/PhysRevLett.79.1237.
  5. ↑ Klöss, Karl-Heinz; Hinz-Hübner, Dirk; Ruschewitz, Uwe (2002). "Über eine neue Modifikation des Na 2 C 2". Zeitschrift für Anorganische und Allgemeine Chemie. 628 (12): 2701–2704. doi:10.1002/1521-3749(200212)628:12<2701::AID-ZAAC2701>3.0.CO;2-#.
  6. ↑ S. Hemmersbach, B. Zibrowius, U. Ruschewitz (1999): "Na2C2 und K2C2: Synthese, Kristallstruktur und spektroskopische Eigenschaften". Zeitschrift für anorganische und allgemeine Chemie, volume 625, issue 9, pages 1440-1446. doi : 10.1002/(SICI)1521-3749(199909)625:9<1440::AID-ZAAC1440>3.0.CO;2-R
  7. ↑ Chui, Stephen S. Y.; Ng, Miro F. Y.; Che, Chi-Ming (2005). "Structure Determination of Homoleptic AuI, AgI, and CuI Aryl/Alkylethynyl Coordination Polymers by X-ray Powder Diffraction". Chemistry: A European Journal. 11 (6): 1739–1749. doi:10.1002/chem.200400881. PMID   15669067.
  8. ↑ Schubert, Bernd; Weiss, Erwin (1983). "(PHCCLi)4(tmhda)2, A Polymeric Organolithium Compound with Cubic Li4C4 Structural Units". Angewandte Chemie International Edition in English. 22 (6): 496–497. doi:10.1002/anie.198304961.
  9. 1 2 3 4 Viehe, Heinz Günter (1969). "Chemistry of Acetylenes". Angewandte Chemie. 84 (8) (1st ed.). New York: Marcel Dekker: 170–179 & 225–241. doi:10.1002/ange.19720840843.
  10. 1 2 Coffman, Donald D. (1940). "Dimethylethhynylcarbinol". Organic Syntheses. 40: 20. doi:10.15227/orgsyn.020.0040.
  11. 1 2 Midland, M. M.; McLoughlin, J. I.; Werley, Ralph T. Jr. (1990). "Preparation and Use of Lithium Acetylide: 1-Methyl-2-ethynyl-endo-3,3-dimethyl-2-norbornanol". Organic Syntheses. 68: 14. doi:10.15227/orgsyn.068.0014.
  12. ↑ Reich, Melanie (August 24, 2001). "Addition of a lithium acetylide to an aldehyde; 1-(2-pentyn-4-ol)-cyclopent-2-en-1-ol". ChemSpider Synthetic Pages (Data Set): 137. doi:10.1039/SP137.
  13. ↑ Midland, M. Mark; Tramontano, Alfonso; Cable, John R. (1980). "Synthesis of alkyl 4-hydroxy-2-alkynoates". The Journal of Organic Chemistry. 45 (1): 28–29. doi:10.1021/jo01289a006.
  14. ↑ Trotuş, Ioan-Teodor; Zimmermann, Tobias; Schüth, Ferdi (2014). "Catalytic Reactions of Acetylene: A Feedstock for the Chemical Industry Revisited". Chemical Reviews. 114 (3): 1761–1782. doi: 10.1021/cr400357r . PMID   24228942.
  15. ↑ Cataldo, Franco; Casari, Carlo S. (2007). "Synthesis, Structure and Thermal Properties of Copper and Silver Polyynides and Acetylides". Journal of Inorganic and Organometallic Polymers and Materials. 17 (4): 641–651. doi:10.1007/s10904-007-9150-3. ISSN   1574-1443. S2CID   96278932.
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