# Acetylide

Acetylide refers to chemical compounds with the chemical formulas MC≡CH and MC≡CM, where M is a metal.[1] The term is used loosely and can refer to substituted acetylides having the general structure RC≡CM (where R is an organic side chain). Acetylides are reagents in organic synthesis. The calcium acetylide commonly called calcium carbide is a major compound of commerce.

## Structure and bonding

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.[2] The C2−
2
ion has a closed shell ground state of 1Σ+
g
, making it isoelectronic to a neutral molecule N2,[3] which may afford it some stability.

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 (i.e. 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).[4]

## Preparation

Terminal alkynes are weak acids:[5]

RC≡CH + R″M ⇌ R″H + RC≡CM

To generate acetylides from acetylene and alkynes relies on the use of organometallic[6] or inorganic[7] superbases in solvents which are less acidic than the terminal alkyne. In early studies liquid ammonia was employed, but etherial solvents are more common.

Lithium amide,[5] LiHMDS,[8] or organolithium reagents, such as butyllithium,[6] are frequently used form lithium acetylides:

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

Sodium or potassium acetylides can be prepared from various inorganic reagents (e.g. sodium amide)[7] or from their elemental metals, often at room temperature and atmospheric pressure.[5]

Copper(I) acetylide can be prepared by passing acetylene through an aqueous solution of copper(I) chloride because of a low solubility equilibrium.[5] Similarly, silver acetylides can be obtained from silver nitrate.

Calcium carbide is prepared by heating carbon with lime CaO at approximately 2,000 °C. A similar process is used to produce lithium carbide.

## Reactions

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.

Illustrative is the sequence shown below, ethyl propiolate is deprotonated by n-butyllithium to give the corresponding acetylide. This acetylide adds to the carbonyl center of cyclopentanone. Hydrolytic workup liberate the alkynyl alcohol.[9]

### Coupling reactions

Acetylides are sometimes intermediates in coupling reactions. Examples include Sonogashira coupling, Cadiot-Chodkiewicz coupling, Glaser coupling and Eglinton coupling.

## Hazards

Some acetylides are notoriously explosive.[10] 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

1. ^ "IUPAC Gold Book - acetylides". goldbook.iupac.org. IUPAC. Retrieved 25 January 2019.
2. ^ 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.
3. ^ Sommerfeld, T.; Riss, U.; Meyer, H.-D.; Cederbaum, L. (August 1997). "Metastable C2−
2
Dianion". Physical Review Letters. 79 (7): 1237–1240. Bibcode:1997PhRvL..79.1237S. doi:10.1103/PhysRevLett.79.1237.
4. ^ 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: 1739–1749. doi:10.1002/chem.200400881.
5. ^ a b c d Viehe, Heinz Günter (1969). Chemistry of Acetylenes (1st ed.). New York: Marcel Dekker, inc. pp. 170–179 & 225–241. doi:10.1002/ange.19720840843.
6. ^ a b Midland, 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.
7. ^ a b Coffman, Donald D. (1940). "Dimethylethhynylcarbinol". Organic Syntheses. 40: 20. doi:10.15227/orgsyn.020.0040.
8. ^ Reich, Melanie (Aug 24, 2001). "Addition of a lithium acetylide to an aldehyde; 1-(2-pentyn-4-ol)-cyclopent-2-en-1-ol". ChemSpider Synthetic Pages: 137. doi:10.1039/SP137.
9. ^ 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.
10. ^ 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. ISSN 1574-1443.
A3 coupling reaction

The A3 coupling (also known as A3 coupling reaction or the aldehyde-alkyne-amine reaction) is a type of multicomponent reaction involving an aldehyde, an alkyne and an amine which react to give a propargyl-amine.

The reaction proceeds via direct dehydrative condensation and requires a metal catalyst, typically based on ruthenium/copper, gold or silver. Chiral catalyst can be used to give an enantioselective reaction, yielding a chiral amine. The solvent can be water. In the catalytic cycle the metal activates the alkyne to a metal acetylide, the amine and aldehyde combine to form an imine which then reacts with the acetylide in a nucleophilic addition. The reaction type was independently reported by three research groups in 2001 -2002; one report on a similar reaction dates back to 1953.

Acetylene

Acetylene (systematic name: ethyne) is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in its pure form and thus is usually handled as a solution. Pure acetylene is odorless, but commercial grades usually have a marked odor due to impurities.As an alkyne, acetylene is unsaturated because its two carbon atoms are bonded together in a triple bond. The carbon–carbon triple bond places all four atoms in the same straight line, with CCH bond angles of 180°.

Alkyne

In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and no other functional groups form a homologous series with the general chemical formula CnH2n−2. Alkynes are traditionally known as acetylenes, although the name acetylene also refers specifically to C2H2, known formally as ethyne using IUPAC nomenclature. Like other hydrocarbons, alkynes are generally hydrophobic but tend to be more reactive.

Alkynylation

Alkynylation is an addition reaction in organic synthesis where a terminal alkyne adds to a carbonyl group to form an α-alkynyl alcohol. When the acetylide is formed from acetylene, the reaction gives an α-ethynyl alcohol. This process is often referred to as ethynylation. Such process often involve metal acetylide intermediates

The Cadiot–Chodkiewicz coupling in organic chemistry is a coupling reaction between a terminal alkyne and a haloalkyne catalyzed by a copper(I) salt such as copper(I) bromide and an amine base. The reaction product is a 1,3-diyne or di-alkyne.

The reaction mechanism involves deprotonation by base of the terminal alkyne proton followed by formation of a copper(I) acetylide. A cycle of oxidative addition and reductive elimination on the copper centre then creates a new carbon-carbon bond.

Calcium carbide

Calcium carbide, also known as calcium acetylide, is a chemical compound with the chemical formula of CaC2. Its main use industrially is in the production of acetylene and calcium cyanamide.The pure material is colorless, however pieces of technical-grade calcium carbide are grey or brown and consist of about 80–85% of CaC2 (the rest is CaO (calcium oxide), Ca3P2 (calcium phosphide), CaS (calcium sulfide), Ca3N2 (calcium nitride), SiC (silicon carbide), etc.). In the presence of trace moisture, technical-grade calcium carbide emits an unpleasant odor reminiscent of garlic.Applications of calcium carbide include manufacture of acetylene gas, and for generation of acetylene in carbide lamps; manufacture of chemicals for fertilizer; and in steelmaking.

Carbanion

A carbanion is an anion in which carbon is trivalent (forms three bonds) and bears a formal negative charge in at least one significant mesomeric contributor (resonance form). Absent π delocalization, carbanions assume a trigonal pyramidal, bent, or linear geometry when the carbanionic carbon is bound to three (e.g., methyl anion), two (e.g., phenyl anion), or one (e.g., acetylide anion) substituents, respectively. Formally, a carbanion is the conjugate base of a carbon acid:

R3CH + B− → R3C− + HBwhere B stands for the base. Carbanions have a concentration of electron density at the negatively charged carbon, which, in most cases, reacts efficiently with a variety of electrophiles of varying strengths, including carbonyl groups, halogenating reagents (e.g., N-bromosuccinimide and diiodine), and proton donors. A carbanion is one of several reactive intermediates in organic chemistry. In organic synthesis, organolithium reagents and Grignard reagents are commonly regarded as carbanions. This is a convenient approximation, although these species are almost always multinuclear clusters containing polar covalent bonds rather than true carbanions.

Carbide

In chemistry, a carbide is a compound composed of carbon and a less electronegative element. Carbides can be generally classified by the chemical bonds type as follows: (i) salt-like, (ii) covalent compounds, (iii) interstitial compounds, and (iv) "intermediate" transition metal carbides. Examples include calcium carbide (CaC2), silicon carbide (SiC), tungsten carbide (WC; often called, simply, carbide when referring to machine tooling), and cementite (Fe3C), each used in key industrial applications. The naming of ionic carbides is not systematic.

Castro–Stephens coupling

The Castro–Stephens coupling is a cross coupling reaction between a copper(I) acetylide and an aryl halide in pyridine, forming a disubstituted alkyne and a copper(I) halide.

{\displaystyle {\begin{aligned}{}\\{\ce {Cu-C{\equiv }C-R'}}+{\color {Red}{\ce {R-X}}}\ &{\ce {->[{\ce {pyridine}}]CuX}}+{\color {Red}{\ce {R}}{-}}{\ce {C{\equiv }C-R'}}\\{\color {Red}{\ce {X}}}&={\color {Red}{\ce {I,Br,Cl}}}\\{\color {Red}{\ce {R}}}&={\color {Red}{\ce {Ar}}}\end{aligned}}}

The reaction was discovered in 1963 by University of California, Riverside chemists Castro and Stephens and is used as a tool in the organic synthesis of organic compounds. The reaction has similarities with the much older Rosenmund–von Braun synthesis (1914) between aryl halides and copper(I) cyanide and was itself modified in 1975 with as the Sonogashira coupling by adding a palladium catalyst and preparing the organocopper compound in situ, allowing copper to also be used catalytically.

A typical reaction is the coupling of iodobenzene with the copper acetylide of phenylacetylene in refluxing pyridine to diphenylacetylene:

Unlike the Sonogashira coupling, the Castro–Stephens coupling can produce heterocyclic compounds when a nucleophilic group is ortho to the aryl halide, although this typically requires use of dimethylformamide (DMF) as solvent.

Copper(I) acetylide

Copper(I) acetylide, or cuprous acetylide, is a chemical compound with the formula Cu2C2. Although never characterized by X-ray crystallography, the material has been claimed at least since 1856. One form is claimed to be a monohydrate with formula Cu2C2.H2O). It is a reddish solid, that easily explodes when dry.

Ethynyl

In organic chemistry, the term ethynyl designates

An ethynyl group (HC≡C–), also designated as acetylenic group (from acetylene), and referred to in IUPAC chemical nomenclature as -yne suffix. Also sometimes designed as ethinyl in compounds (ethinylestradiol, ethisterone (ethinyltestosterone)). See main page alkynes.

The ethynyl radical (HC≡C·), the compound found in interstellar medium, and transiently on earth during chemical reactions.

The ethynyl carbanion Acetylide (HC≡C−)

Favorskii reaction

The Favorskii reaction is an organic chemistry reaction between an alkyne and a carbonyl group, under a basic condition. The reaction was discovered in the early 1900s by the Russian chemist Alexei Yevgrafovich Favorskii.

When the carbonyl is an aldehyde (R"=H), a rearrangement occurs and leads to an enone. When this rearrangement is catalyzed by an acid, it is called Meyer–Schuster rearrangement.

Linear acetylenic carbon

Linear acetylenic carbon (LAC), also called carbyne, is an allotrope of carbon that has the chemical structure (−C≡C−)n as a repeating chain, with alternating single and triple bonds. It would thus be the ultimate member of the polyyne family.

This polymeric carbyne is of considerable interest to nanotechnology as its Young's modulus is 32.7 TPa – forty times that of diamond. It has also been identified in interstellar space; however, its existence in condensed phases has been contested recently, as such chains would crosslink exothermically (and perhaps explosively) if they approached each other.

Lithium carbide

Lithium carbide, Li
2
C
2
, often known as dilithium acetylide, is a chemical compound of lithium and carbon, an acetylide. It is an intermediate compound produced during radiocarbon dating procedures. Li
2
C
2
is one of an extensive range of lithium-carbon compounds which include the lithium-rich Li
4
C
, Li
6
C
2
, Li
8
C
3
, Li
6
C
3
, Li
4
C
3
, Li
4
C
5
, and the graphite intercalation compounds LiC
6
, LiC
12
, and LiC
18
.
Li
2
C
2
is the most thermodynamically-stable lithium-rich compound and the only one that can be obtained directly from the elements. It was first produced by Moissan, in 1896 who reacted coal with lithium carbonate.

${\displaystyle {\ce {Li2CO3 + 4C->Li2C2 + 3CO}}}$

The other lithium-rich compounds are produced by reacting lithium vapor with chlorinated hydrocarbons, e.g. CCl4. Lithium carbide is sometimes confused with the drug lithium carbonate, Li
2
CO
3
, because of the similarity of its name.

Organocopper compound

Organocopper compounds in organometallic chemistry contain carbon to copper chemical bonds. Organocopper chemistry is the science of organocopper compounds describing their physical properties, synthesis and reactions. They are reagents in organic chemistry.

The first organocopper compound, the explosive copper(I) acetylide Cu2C2 (Cu-C≡C-Cu), was synthesized by Rudolf Christian Böttger in 1859 by passing acetylene gas through copper(I) chloride solution:

C2H2 + 2 CuCl → Cu2C2 + 2 HCl

Rudolf Christian Böttger

Rudolf Christian Böttger (28 April 1806 – 29 April 1881) was a German inorganic chemist. He conducted most of his research at the University of Frankfurt am Main. He is credited with discovery of nitrocellulose in 1846, independently to Schönbein, and with the synthesis of the first organocopper compound copper(I) acetylide Cu2C2 in 1859.

Silver acetylide

Silver acetylide is an inorganic chemical compound with the formula Ag2C2, a metal acetylide. The compound can be regarded as a salt of the weak acid, acetylene. The salt's anion consists of two carbon atoms linked by a triple bond. The alternate name "silver carbide" is rarely used, although the analogous calcium compound CaC2 is called calcium carbide.

Pure silver acetylide is a heat- and shock-sensitive high explosive with the unusual property that on ignition it does not evolve any gas:

Ag2C2 (s) → 2 Ag (s) + 2 C (s)A common misconception about the "silver acetylide" used in commercial explosives is that it explodes without the evolution of gaseous products and that its chemical formula is Ag2C2. In reality, it is a double salt with the silver salt it was produced from, usually silver nitrate. The anion of the parent compound acts as the oxidizer in the decomposition reaction.

The detonation velocity of the mixture silver acetylide: silver nitrate is 3460 m/s. That of pure silver acetylide is 4000 m/s.

UC2

UC2 may refer to:

German submarine UC-2

SM UC-2, a German World War I submarine

German Type UC II submarine of World War II

UC2 Kraka, a Danish private diesel-electric submarine

uranium acetylide (UC2) see Uranium carbide

Uranium carbide

Uranium carbide, a carbide of uranium, is a hard refractory ceramic material. It comes in several stoichiometries (UCx), such as uranium methanide (UC, CAS number 12070-09-6), uranium sesquicarbide (U2C3, CAS number 12076-62-9),

and uranium acetylide (UC2, CAS number 12071-33-9).Like uranium dioxide and some other uranium compounds, uranium carbide can be used as a nuclear fuel for nuclear reactors, usually in the form of pellets or tablets. Uranium carbide fuel was used in late designs of nuclear thermal rockets.

Uranium carbide pellets are used as fuel kernels for the US version of pebble bed reactors; the German version uses uranium dioxide instead.

As nuclear fuel, uranium carbide can be used either on its own, or mixed with plutonium carbide (PuC and Pu2C3). The mixture is also labeled as uranium-plutonium carbide (PuC U).

Uranium carbide is also a popular target material for particle accelerators.

Ammonia synthesis from nitrogen and hydrogen is sometimes accomplished in the presence of uranium carbide acting as a catalyst.

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