Acetylene

 

Acetylene

Chemical formula	C2H2
Index of refraction	1.00045
Molar Mass(g/mol)	26
Density(g/cc)	0.0011
Fuel Value(MJ/kg)	48.466
Tad(O2)(°C)	3480
Solubility in water(g/L)	1.45
NFPA 704	4 1 3

Uses

Justification Questioned

Other

• Lighting/Welding Fuel
• Feedstock for benzene

Hazards

• Acetylene is toxic, flammable, explosive, and moderately reactive.
• Under pressure over 25atm, physical shocks or rapid temperature changes can result in spontaneous explosions.

The key safety benchmark is to avoid any substantial volume of pressurized acetylene in a rigid container.

Character

Unlike other gasses that can be kept in pressurized metal containers, acetylene is prone to explode and decompose directly into hydrogen and carbon under any pressure over 25atm. Because of this the approach to storing acetylene is substantially different, as described in this patent

When the pressure within the ganged cylinders reaches approximately 25 bar (about 360 psia), filling is halted because, from a safety point of view, this is considered to be about the highest pressure to which acetylene should normally be compressed.
 The partially filled cylinders are then left standing to cool. After a sufficient period of cooling time, the cylinders stabilize in temperature, and can then be further charged with cetylene to complete the filling procedure. The filled cylinders are then disconnected from the filling apparatus and individually weighed to make certain that they contain, within certain tolerances, the prescribed quantity of acetylene. Any cylinders showing excess weight are slightly emptied. Any cylinders which have been insufficiently filled are given an additional filling. In temperate climates, about seven hours is a normal time for the initial filling of a solvent-charged cylinder with acetylene gas, followed by about a twelve hour pause for cooling, whereafter a final filling with acetylene usually requires about an additional two hours. In hotter climates these filling and cooling times are con siderably longer. In order to diminish filling time, it has been proposed to cool acetylene cylinders during filling by spraying their outer walls with a liquid coolant.

Synthesis

Acetylene has a long history and a myriad methods of synthesis.

From ethanol

Dehydrate ethanol in the vapor phase over aluminum oxide:

2 CH₃CH₂OH(v) + O₂

{Al₂O₃

→

heat}

2 C₂H₂ + 4 H₂O

From dihalogenated ethane

Reflux dibromoethane in ethanol with potassium hydroxide, freeing a chlorine from one carbon and a hydrogen from the other, twice. This produces acetylene as well as potassium bromide.

BrCH₂-CH₂Br + KOH

{CH₃CH₂OH

→

80°C}

CH₂-CHBr + H₂O + KBr

CH₂-CHBr + KOH

{CH₃CH₂OH

→

80°C}

CH≡CH + H₂O + KBr

From carbides

• The carbides of alkali metals and alkali earth metals produce acetylene when exposed to water:

CaC₂ + 2 H₂O → Ca(OH)₂ + C₂H₂

Na₂C₂ + H₂O → Na₂O + C₂H₂

Production of carbides

Most of these methods are roughly interchangeable with which metal they work upon.

• Calcium carbide is the most common example, formed in an arc lamp from calcium oxide

CaO + 3 C

{

→

2000°C}

CaC₂ + CO

• Lithium carbide can be formed by reacting carbon dioxide and molten metal

2 CO₂ + 10 Li

{

→

730°C}

Li₂C₂ + 4 Li₂O

• Sodium carbide and producer gas are formed by reacting lye with an excess of carbon. The hydrogen should be drawn off as rapidly as possible

2 NaOH + 3 C

{

→

700-875°C}

Na₂C₂ + 2 CO + H₂

Partial oxidation of methane

Carbon monoxide and methane are passed through a red-hot tube, producing acetylene and water:

CH₄ + CO → C₂H₂ + H₂O

From Trichloromethane=

Heat trichloromethane in the presence of silver powder, producing acetylene

2 CHCl₃ + 6 Ag

{

→

heat}

6 AgCl + 2 C₂H₂

Testing

Acetylene will rapidly decolorize solutions of aqueous bromine and acidified potassium permanganate.

Storage

The best storage medium for acetylene (under pressure) is dissolving it in propanone with lots of porous ceramic (unreactive) material, which will provide lots of nucleation sites to prevent large bubbles of acetylene from forming. Any space above the surface of the propanone liquid is broken up with inert porous material, preventing a single volume of acetylene from forming above the propanone level.

At 1atm, approximately 1 mol of acetylene (27g) will dissolve in 1L of propanone.