# Alkynes: Reactions and Synthesis

## Alkynes: Reactions

Alkynes are very reactive compounds and the triple bond participates in many . Contrary to alkenes, alkynes also undergo nucleophilic additions.

### Combustion of ethyne

Fig.1
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.

Caution
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

Fig.2
Hydrogenation of ethyne to ethene

During the catalytic 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.

Fig.3
Hydrogenation of ethene to ethane
Tab.1
Heat of hydrogenation
Ethyne to ethen $ΔH°$ = - 175.4 $kJmol-1$ $ΔH°$ = - 136.9 $kJmol-1$ $ΔH°$ = - 292.7 $kJmol-1$ $ΔH°$ = - 272.6 $kJmol-1$

By using a less active (partially poisoned) , 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.

Fig.4
Hydrogenation with Lindlar catalyst
Fig.5
Reduction with sodium in liquid ammonia

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

### Addition of hydrogen halides to alkynes

Fig.6

The high of the triple bond favors attack by electrophiles following the . The mechanism involves 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.

Fig.7

The electrophilic addition of halogens to alkynes to yield tetrahaolgen alkanes proceeds via dihalogen alkenes as intermediates. As a rule, the addition normally gives the trans product.

Fig.8

### Hydration of alkynes

Fig.9
Hydration of alkynes

Catalyzed by mercury(II) salts, water can be added to alkynes according to the Markovnikov rule. This reaction yields which tautomerize to the corresponding carbonyl compounds. Ethyne yields acetaldehyde; terminal alkynes produce methyl ketones.

Fig.10