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.[2]Structure of Na2C2.[3] Color code: gray = C, blue = Na.
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, there is also some evidence for the solubility of C2− 2 ions in liquid ammonia.[4] The C2− 2 ion has a closed shellground stateof1Σ+ g, making it isoelectronic to a neutral molecule N2, which may afford it some gas-phase stability.[5]
Analogous acetylides prepared from other metals, particularly transition metals, show covalent character and are invariably associated with their metal centers. This can be seen in their general stability to water (such as silver acetylide, copper acetylide) and radically different chemical applications.
Acetylides of the general formula RC≡CM (where R = H or alkyl) generally show similar properties to their doubly substituted analogues. In the absence of additional ligands, metal acetylides adopt polymeric structures wherein the acetylide groups are bridging ligands.
Portion of the structure of the polymer copper phenylacetylide (CuC2C6H5).[6]
To generate acetylides from acetylene and alkynes relies on the use of organometallic[8]orinorganic[9]superbases in solvents which are less acidic than the terminal alkyne. In early studies liquid ammonia was employed, but ethereal solvents are more common.
Monopotassium and monosodium acetylide can be prepared from various inorganic reagents (such as sodium amide)[9] or from their elemental metals, often at room temperature and atmospheric pressure.[7]
Illustrative is the sequence shown below, ethyl propiolate is deprotonated by n-butyllithium to give the corresponding lithium acetylide. This acetylide adds to the carbonyl center of cyclopentanone. Hydrolytic workup liberate the alkynyl alcohol.[11]
Reaction of ethyl propiolate with n-butyllithium to form the lithium acetylide.
Some acetylides are notoriously explosive.[13] Formation of acetylides poses a risk in handling of gaseous acetylene in presence of metals such as mercury, silverorcopper, 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.
^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.
^ 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.
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