Acetylide refers to chemical compounds with the chemical formulas MC≡CH and MC≡CM, where M is a metal. The term is used loosely and can refer to substituted acetylides have 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 C22− 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 C22− ions in liquid ammonia. The C22− ion has a closed shell ground state of 1Σg+, making it isoelectronic to a neutral molecule N2, 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.
- RC≡CH + R"M ↔ R"H + RC≡CM
To generate acetylides from acetylene and alkynes relies on the use of organometallic or inorganic superbases in solvents which are less acidic than the terminal alkyne. In early studies liquid ammonia was employed, but etherial solvents are more common.
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.
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 in Scheme 1, the alkyne 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.
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).
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