Figure 4-57. Decomposition of silver acetylide (explosion). 4.4.5. Production and Applications of Alkynes.
As mentioned above, before World War II acetylene was the most important chemical of the industry of organic compounds. The main method to produce acetylene was the hydrolysis reaction of calcium carbide, CaC2
.CaC2 + 2 H2O = HC≡CH + Ca(OH)2
Note that, in contrast with silver and copper acetylides, CaC2
is nonexplosive and reasonably safe to work with. Calcium carbide is made by the reaction of quicklime, CaO, with carbon in the form of coke. CaO + 3 C = CaC2 + CO
This reaction requires a very high temperature of around 2,000 o
C to occur and is performed in an electric arc furnace, consuming much energy and, consequently, being costly to run. While some quantities of acetylene are still made from calcium carbide, the main method to make acetylene over the last 60-70 years has been the thermal decomposition of methane (natural gas). 2 CH4 = HC≡CH + 3 H2
This process is run at a high temperature of 1,200-1,500 o
C. To spare the just formed acetylene from decomposition at such high temperatures, a flow of methane is blown through the reactor at such a velocity that it spends only a very short time in the hot zone, just 0.01 – 0.001 seconds. The gaseous mixture is then immediately cooled to a temperature that is safe for acetylene. Only 5-20% of methane is converted to acetylene in a single pass under such conditions. The acetylene is then separated from the cooled mixture and the unreacted methane is cycled back into the reactor. In one smart modification of the process, a mixture of methane and oxygen is fed into the reactor, so that some proportion of the CH4
is burnt to maintain the high temperature required for the reaction to take place. In this way, the process is self-sustainable and more economical.
The role of acetylene in the modern industry is rather modest, especially relative to what it was in the first half of the 20th
century. Acetylene currently finds applications in the industrial synthesis of some chemicals, polymers, paints, resins, and glues.
Roughly 20% of all acetylene produced is used for flame metal cutting and welding. Like other hydrocarbons, acetylene burns in air or oxygen. 2 HC≡CH + 5 O2 = 4 CO2 + 2 H2O
Due to the uncommonly large amount of energy confined in the acetylene molecule, the temperature of the flame of acetylene-oxygen mixtures is the highest a fuel gas can produce. While the maximum flame temperature for methane or propane in oxygen is around 2,800 o
C, for acetylene it approaches 3,200 o
C, which makes acetylene indispensable in flame welding and metal cutting operations. 4.4.6. Exercises.
1. Acetylene is (a) toxic because it is thermodynamically unstable; (b) conventionally stored and shipped in stainless steel cylinders under pressure of up to 150 atm; (c) used in flame welding because it is safer to work with acetylene than with methane or ethane; (d) a gas that is nontoxic, colorless, odorless, and thermodynamically unstable; (e) the most important compound of industrial organic chemistry; (f) a colorless gas with a pungent odor; (g) stored in specially designed cylinders as an acetone solution adsorbed on a porous solid under low pressure. Answer
2. Explain why there are skeletal isomers of butane and butene but not of butyne. Hint
3. Explain why cis-trans isomerism is nonexistent in alkynes. Hint
4. Using the hybridization concept, explain the linear geometry of acetylene. [Answer: See section 4.3.2]
5. The C-C bond in acetylene is (a) shorter than in ethylene but longer than in ethane; (b) longer than in ethylene and ethane; (c) shorter than in ethylene and ethane. Answer
6. Draw structural formulas for 2-hexyne; 1-octyne; 3-methyl-1-heptyne; 2-methyl-3,3-diethyl-4-decyne.
7. Name the following alkynes: