Acetylide

Acetylide

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Identifiers
CAS Number	29075-95-4 (HC≡C−) 34846-56-5 (C22−)
ChemSpider	11316391 (HC≡C−) 6113 (C22−) Y
Jmol 3D model	Interactive image
InChI InChI=1S/C2H/c1-2/h1H/q-1 Key: BLRXYTIIKIPJQL-UHFFFAOYSA-N
SMILES C#[C-]
Properties
Chemical formula	HC− 2
Molar mass	25.03 g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Acetylide is a carbanion with the chemical formula HC≡C⁻. This is the ethynyl group with one negative. Substituted acetylides, which have the general structure RC≡C− (where R is an organic substituent) can only normally be isolated in their organometallic forms (HC≡CM) and are useful reagents in organic chemistry. Some acetylides are notoriously explosive.

Acetylide also refers to the anion of the formula C²⁻
₂, which can be viewed as the double deprotonation product of acetylene C₂H₂. Its closed shell ground state of ¹Σ_g⁺ is makes it isoelectronic to the neutral molecule N₂, this affords it some stability in the gas phase but it reacts rapidly as a strong base in the condensed phase.

Structure

Although acetylides are described as anions in salts, they are not. The anion is strongly bonded to a metal cation. Thus, alkali metal acetylides adopt complex structures exhibiting extended interactions. Nonetheless, the anion names are also used for salt-like materials such as copper acetylide (Cu₂C₂), lithium hydrogenacetylide (LiC₂H), and silver methylacetylide (AgCH₃C₂). Some metal acetylides are traditionally called carbides. For example, lithium carbide and calcium carbide are really derivatives of C₂²⁻.

Preparation

Acetylides of the alkali metals can be prepared by deprotonation of acetylene in liquid ammonia. Other strong bases such as butyllithium or LiHMDS are also frequently used:

Copper(I) acetylide can be prepared by passing acetylene through a water solution of copper(I) chloride. Silver acetylide can be obtained in a similar way from silver nitrate.

Calcium carbide is prepared by heating carbon with lime CaO at approximately 2000 °C. A similar process is used to produce lithium carbide.

Reactions

Acetylide ions are very useful in organic chemistry reactions in combining carbon chains, particularly addition and substitution reactions. One type of reaction displayed by acetylides are addition reactions with ketones to form tertiary alcohols. In the reaction in scheme 1 the alkyne proton of ethyl propiolate is deprotonated by n-butyllithium at -78 °C to form lithium ethyl propiolate to which cyclopentanone is added forming a lithium alkoxide. Acetic acid is added to remove lithium and liberate the free alcohol.

Scheme 1. Reaction of ethyl propiolate with n-butyllithium to form the lithium acetylide.

Coupling reactions of alkynes like the Sonogashira coupling, the Cadiot-Chodkiewicz coupling, the Glaser coupling and the Eglinton coupling often have metal acetylides as intermediates.

Several modifications of the reaction with carbonyls are known:

• In the Arens–van Dorp Synthesis the compound ethoxyacetylene is converted to a Grignard reagent and reacted with a ketone, the reaction product is a propargyl alcohol.

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• In the Isler modification ethoxyacetylene is replaced by beta-chlorovinyl ether and lithium amide.
• In the Favorskii–Babayan synthesis ketones and acetylenic compounds react in presence of alkali.

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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

• Diatomic carbon - Neutral C₂
• Acetylenediol

References