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) 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.

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⁻. 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.

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.

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⁻. Color code: gray = C, blue = Na.
• 
  Structure and unit cell of potassium acetylide [K⁺]₂C≡C²⁻.
• 
  Portion of the structure of the polymer copper phenylacetylide (CuC₂C₆H₅).
• 
  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.

Preparation

Acetylene and terminal alkynes are weak acids:

RC≡CH + R″M ⇌ R″H + RC≡CM

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

Precipitate of copper acetylide hydrate CuC_2.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. 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 or inorganic 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, LiHMDS, or organolithium reagents, such as butyllithium (BuLi), are frequently used to form lithium acetylides:

${\ce {{H-C{\equiv }C-H}+BuLi->[{\ce {THF}}][-78^{\circ }{\ce {C}}]{Li-\!{\equiv }\!-H}+BuH}}$

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.

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.

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

• Ethynyl
• Ethynyl radical
• Diatomic carbon (neutral C₂)
• Acetylenediol