Why is ethane less reactive than ethene
to directory mode
Alkynes are very reactive compounds. The triple bond enters into a variety of addition reactions. In contrast to the alkenes, alkynes also undergo nucleophilic additions.
When ethyne is burned, the heat of combustion is distributed over only three molecules of the product gas, which means that the flame temperature is very high (over 2500). Acetylene is therefore often used as a welding gas.
Film for welding with an acetylene flame
- Mixtures of ethyne with oxygen are explosive over a wide range (1.5 - 82 vol%). Appropriate care should be taken when handling.
Hydrogenation of ethyne
In the catalytic hydrogenation of ethyne, ethene is formed first and then ethane in a further step. The heat of hydrogenation of the first π bond is higher than that of the second. Internal alkynes are more stable than terminal ones. This is illustrated by the heats of hydrogenation of the isomeric butynes to butane.
- Tab. 1
- Heat of hydrogenation
|Ethine to ethene||= - 175,4|
|Ethene to ethane||= - 136,9|
|But-1-yne to butane||= - 292,7|
|But-2-yne to butane||= - 272,6|
The hydrogenation can be stopped at the alkene stage if a less active (partially poisoned) catalyst is used. The so-called Lindlar catalyst (on palladium, poisoned with quinoline) is often used. The hydrogenation is stereospecific cis.
The reduction of alkynes with sodium in liquid ammonia (solvated electrons) leads to trans-Alkenes.
Addition of hydrogen halides to alkynes
Since the triple bond has a high electron density, it is easily attacked by electrophiles according to Markovnikov's rule. The mechanism involves protonation of the triple bond with the formation of an alkenyl cation, which is then trapped by the counterion. It is difficult to limit the addition to just one molecule of HX, since the resulting double bond is usually more reactive than the alkyne.
Halogen addition to alkynes
The electrophilic addition of halogens to alkynes proceeds via vicinal dihaloalkenes to give the tetrahaloalkanes. The dihaloalkenes can usually be isolated. The addition usually goes to trans-Product.
Hydration of alkynes
Under catalysis by mercury (II) salts, water can be added to alkynes according to the Markovnikov rule. Enols are formed which tautomerize into the corresponding carbonyl compounds. Ethyne gives acetaldehyde, terminal alkynes give methyl ketones.
Nucleophilic addition to alkynes
In contrast to non-activated alkenes, alkynes also undergo nucleophilic additions, but only under drastic conditions.
Polymerization of ethyne
The polymerization of ethyne can also be triggered by carbenium ions. The subsequent chain reaction leads to a long-chain molecule with conjugated double bonds. The resulting polyethine (polyacetylene) has electrical conductivity when it has been treated with a one-electron oxidizing agent (doping).
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