 |
 |
| Calcium carbide miner's lamp. | Combustion of acetylene. Click image for a video of this reaction. |
Acetylene (ethyne, C2H2) is the simplest alkyne. Acetylene burns in air to produce a hot, luminous flame. Acetylene is commonly used in oxacetylene welding. It was used in carbide miner's lamps but the flames were a safety hazard in mines where methane was present. These lamps were replaced by battery powered lamps.
|
|
| Calcium carbide miner's lamp. | Combustion of acetylene. Click image for a video of this reaction. |
The combustion of one mole of acetylene in air requires 2.5 moles of oxygen according to the equation.
The acetylene oxygen flame can reach a temperature of about 3000oC. The oxygen-acetylene flame is luminous because acetylene decomposes at high temperature or under pressure to carbon and hydrogen. The hot carbon particles are heated to glowing which makes the acetylene flame a good light source. The decomposition of acetylene under pressure requires special handling for acetylene gas. Cylinders of acetylene contain the gas dissolved in acetone or adsorbed onto a solid support for stabilization.
The two sp carbon atoms in acetylene are linked by a carbon-carbon triple bond and have a bond angle of 180o. In acetylene two pi bonds are perpendicular to each other. As a result, electron density surrounds the sigma framework of the molecule.
Line bond structure of acetylene and Chime animation.
Molecular orbital representations of acetylene showing overlap of pi orbitals.
Electron density shown as a mesh surrounding the acetylene molecule.
In 1862 Friedrich Wöhler discovered calcium carbide by heating calcium oxide with carbon in the form of coke. When calcium carbide was reacted with water acetylene was formed.
Acetylene may also be produced commercially by the catalytic cracking of ethylene.
Commercial application of acetylene:
Substituted acetylenes are described as terminal acetylenes if they are monosubstituted (1-alkynes). An important property of acetylene and 1-alkynes is their acidity. Strong bases such as butyl lithium (C4H9Li) or sodium amide (NaNH2) react with 1-alkynes to form carbanions. These carbanions can react as nucleophiles in substitution reactions with alkyl halides or undergo addition to carbonyl groups.
Acetylenes also undergo addition to carbonyl groups to produce ethynyl alcohols. Steroid hormones are active in fertility control and calcium metabolism in humans. The natural hormones cannot be given orally because they are rapidly converted to water-soluble, inactive metabolites by microorganisms in the intestinal tract. These transformations usually involve the secondary alcohol at the 17 position. Treatment of the corresponding ketone with acetylide anion gives the 17a-ethynyl hormone. Examples include ethynylestradiol, an estrogen used in commercial birth control pills, and norethindrone, a contraceptive progesterone analog.
Like alkenes, alkynes undergo electrophilic addition to the double bond. The initial product of addition is an alkene. The alkene can undergo addition of a second mole of reagent to form an alkane.
One of the major commercial applications of acetylene is the production of vinyl chloride, the starting material for production of the polymer polyvinylchloride.
Addition of a second mole of HCl gives 1,1-dichloroethane.
Bromine and chlorine add to carbon-carbon triple bonds to first give dihaloalkenes and finally, to give tetrahaloalkanes. For example, acetylene reacts with bromine to give trans-1,2-dibromoethene which reacts with a second mole of bromine to give 1,1,2,2-tetrabromoethane.
Generation of acetylene in the presence of chlorine gas results in a highly exothermic reaction producing flames and soot. Click on the image below for a video clip of this reaction.
Catalytic hydrogenation of alkynes initially gives alkenes which may be further reduced to alkanes. The reaction may be stopped at the alkene stage using special catalysts that are partially deactivated (poisoned). Lindlar catalyst, palladium treated with lead acetate, is commonly used to prepare alkenes from alkynes. An important aspect of this reaction is its high stereoselectivity. The less stable cis-alkenes are the predominant products.
Sodium metal in liquid ammonia reduces alkynes to trans-alkenes.
Sodium dissolves in liquid ammonia (bp -33oC) and ionizes to form sodium ions and solvated electrons. These electrons may react with organic functional groups. The reduction of alkynes is believed to proceed through a one electron reduction to give a radical anion followed by protonation with ammonia to give a free radical. A second one electron reduction produces an anion which gives the alkene after protonation. The trans- stereochemistry is attributed to the higher stability of the E-radical relative to the Z-isomer.
For example, 4-octyne is reduced to cis-4-octene under these conditions.
The addition of water to acetylenes is catalyzed by a mixture of sulfuric acid and mercury(II) acetate. The initial product of addition is an enol which rearranges to the more stable keto form. Acetaldehyde is produced by hydration of acetylene under these conditions.
