Acetylide

In chemistry, an acetylide is a compound that can be viewed as the result of replacing one or both hydrogen atoms of acetylene (ethyne) by metallic or other cations. Calcium carbide is an important industrial compound, which has long been used to produce acetylene for welding and illumination. It is also a major precursor to vinyl chloride. Other acetylides are reagents in organic synthesis.

Nomenclature

The term acetylide is used loosely. It apply to an acetylene , where R = H or a side chain that is usually organic. The nomenclature can be ambiguous with regards to the distinction between compounds of the type and . When both hydrogens of acetylene are replaced by metals, the compound can also be called carbide, e.g. calcium carbide , which is calcium acetylide. 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 or sodium hydrogen acetylide, . An acetylide may be a salt (ionic compound) containing the anion , , or , as in disodium acetylide or cobalt(II) acetylide .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. Other acetylides have the metal bound to the carbon atom(s) by covalent bonds, being therefore coordination or organometallic compounds.

Alkali and alkaline earth acetylides

Alkali metal and alkaline earth metal acetylides have the general formula (M = Li, Na, K, etc) and (M' = Mg, Ca, etc).They are sometimes represented as and , but they are covalent compounds. Rather than salt-like, they can be considered Zintl phase compounds, containing ions, with a triple bond between the two carbon atoms. They undergo ready hydrolysis to form acetylene and metal oxides. Their solutions in ammonia are proposed to contain solvated ions.

The ion has a closed shell ground state of ¹Σ, making it isoelectronic to a neutral molecule , which may afford it some gas-phase stability.

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.

File:Na2C2structure.jpg, Structure of sodium acetylide . Color code: gray = C, blue = Na.
File:K2C2 unit cell.png , Structure and unit cell of potassium acetylide .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.
File:XAPGIF2.png , Portion of the structure of the polymer copper(I) phenylacetylide .

File:CAMNEJ.svg, Structure of the cluster formed from complexed to N,N,N′,N′-tetramethyl-1,6-diaminohexane (methylene groups omitted for clarity). Color key: turquoise = Li, blue = N.

Preparation

Of the type

Acetylene and terminal alkynes are weak acids:
:

Monopotassium and monosodium acetylide can be prepared by reacting acetylene with bases like sodium amide or with the elemental metals, often at room temperature and atmospheric pressure.
Copper(I) acetylide can be prepared by passing acetylene through an aqueous solution of copper(I) chloride because of a low solubility equilibrium. Similarly, silver acetylides can be obtained from silver nitrate.

In organic synthesis, acetylides are usually prepared by treating acetylene and alkynes with organometallic or inorganic Classically, liquid ammonia was used for deprotonations, but ethers are now more commonly used.

Lithium amide, LiHMDS, or organolithium reagents, such as butyllithium (), are frequently used to form lithium acetylides:

:

Of the type and

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

Dilithium acetylide, , competes with the preparation of the monolithium derivative .

Reactions

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

:

:

Acetylides of the type 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.

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

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. 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

* Sodium hydrogen acetylide
* Calcium acetylide
* Strontium acetylide
* Barium acetylide
* Copper(I) acetylide
* Silver(I) acetylide
* Ethynyl
* Ethynyl radical
* Diatomic carbon (neutral C₂)
* Acetylenediol

References

{{Alkynes

Anions
Functional groups