Alkynes: Reactions and Synthesis
Alkynes are very reactive compounds and the triple bond participates in many addition reactions. Contrary to alkenes, alkynes also undergo nucleophilic additions.
Combustion of ethyne
Since the heat of combustion of ethyne is distributed over only three molecules of gases produced, the flame temperature (above 2500 ) is very high. Therefore, acetylene is frequently used for welding purposes.
- Mixtures of ethyne with oxygen are explosive over a wide range of composition (1.5 and 82 Vol % ). Therefore, they have to be handled with great care.
Hydrogenation of ethyne
During the catalytic hydrogenation of ethyne, ethene is formed first which in the next step is further reduced to ethane. In this reaction, the heat of hydrogenation of the first π bond is higher than that of the second. Internal alkynes are more stable than terminal ones as exemplified by the heat of hydrogenation of isomeric butynes to butane.
|Ethyne to ethen||= - 175.4|
|Ethene to ethane||= - 136.9|
|But-1-yne to butane||= - 292.7|
|But-2-yne to butane||= - 272.6|
By using a less active (partially poisoned) catalyst, hydrogenation can be stopped at the alkene stage. Lindlar catalyst (palladium on , poisoned with quinoline) is frequently used for this hydrogenation which stereospecifically yields cis products.
Reduction of alkynes with sodium in liquid ammonia (solvated electrons) yields trans alkenes.
Addition of hydrogen halides to alkynes
The high electron density of the triple bond favors attack by electrophiles following the Markovnikov rule. The mechanism involves protonation of the triple bond to form an alkenyl cation which subsequently is captured by a counter ion. It is difficult to limit the addition to only one HX molecule because the resulting double bond normally is more reactive than than the alkyne.
Halogen addition to alkynes
The electrophilic addition of halogens to alkynes to yield tetrahaolgen alkanes proceeds via vicinal dihalogen alkenes as intermediates. As a rule, the addition normally gives the trans product.
Hydration of alkynes
Catalyzed by mercury(II) salts, water can be added to alkynes according to the Markovnikov rule. This reaction yields enoles which tautomerize to the corresponding carbonyl compounds. Ethyne yields acetaldehyde; terminal alkynes produce methyl ketones.
Nucleophilic addition to alkynes
In contrast to non-activated alkenes, alkynes also undergo nucleophilic additions but only under harsh conditions.
Polymerization of ethyne
Polymerization of ethyne can also be initiated by carbenium ions. Subsequent chain reaction yields a long-chain molecule containing conjugated double bonds. When treated with an one-electron oxidation agent (doping), the resulting polyethyne (polyacetylene) shows electrical conductivity.