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Alkynes: Reactions and Synthesis

Alkynes: Reactions

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

Combustion of ethyne (acetylene)

Since the heat of combustion of ethyne is distributed over only three molecules of gases produced, the flame temperature (above 2500 °C) is very high. Therefore, acetylene is frequently used for welding purposes.

Movie of welding with an acetylene flame

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

Hydrogenation of ethyne to ethene

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.

Hydrogenation of ethene to ethane
Heat of hydrogenation
Ethyne to ethen ΔH° = - 175.4 kJmol-1
Ethene to ethane ΔH° = - 136.9 kJmol-1
But-1-yne to butane ΔH° = - 292.7 kJmol-1
But-2-yne to butane ΔH° = - 272.6 kJmol-1

By using a less active (partially poisoned) catalyst, hydrogenation can be stopped at the alkene stage. Lindlar catalyst (palladium on BaSO4, poisoned with quinoline) is frequently used for this hydrogenation which stereospecifically yields cis products.

Hydrogenation with Lindlar catalyst
Reduction with sodium in liquid ammonia

Reduction of alkynes with sodium in liquid ammonia (solvated electrons) yields trans alkenes.

Addition of hydrogen halides to alkynes

Addition of hydrogen halide

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

Addition of halogens

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

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

Nucleophilic addition to alkynes

In contrast to non-activated alkenes, alkynes also undergo nucleophilic additions but only under harsh conditions.

Polymerization of ethyne

Cationic polymerization

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.

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