Organic compounds containing a metal bound to a triple-bonded carbon
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]
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 carbideCa2+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 acetylideNa+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 shellground 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.
Structure of sodium acetylide [Na+]2C≡C2−.[5] Color code: gray = C, blue = Na.
Structure and unit cell of potassium acetylide [K+]2C≡C2−.[6]
Portion of the structure of the polymer copper phenylacetylide (CuC2C6H5).[7]
Structure of the cluster formed from PhC2Li complexed to N,N,N′,N′-tetramethyl-1,6-diaminohexane (methylene groups omitted for clarity). Color key: turquoise = Li, blue = N.[8]
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]
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.
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.
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).
^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. PMID23161511.
^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
^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. PMID15669067.
^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.
^ abMidland, 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.
^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.
^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.
^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. ISSN1574-1443. S2CID96278932.