Acetylide

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, . Metal acetylide may be described as salts, but that description rarely comports with crystallographic analysis.

Alkali and alkaline earth acetylides

In the absence of additional ligands, metal acetylides adopt polymeric structures wherein the acetylide groups are bridging ligands. Alkali metal acetylides have the general formula (M = Li, Na, K, etc).They are sometimes represented as but the C---M bonding might also be described as polar covalent. They dissolve without decomposition in ammonia. Such solutions are proposed to contain solvated ions.

Alkali metal and alkaline earth metal acetylides have the general formula (M' = Mg, Ca, etc). 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: :

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. 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 .

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.

Transition metal acetylides

Acetylides of the transition metals, show evidence of covalent character, e. g. for being neither dissolved nor decomposed by water and by radically different chemical reactions. The inventory of such complexes numbers in the hundreds. Even the stoichiometrically simple silver acetylide and copper acetylide appear highly covalent..

File:XAPGIF2.png | Portion of the structure of the polymer copper(I) phenylacetylide .

Preparation

Of the type {{chem2|MC\tCR}}

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

:

Of the type {{chem2|MC\tCM}} and {{chem2|CaC2}}

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

The copper-catalyzed Click reaction of terminal alkynes and azides proceeds via copper(I) acetylide intermediates.

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).

References

References

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