Alkynes: Preparation

1. Calcium carbide with water:

The reaction of calcium carbide with water is carried out at room temperature and, for a long time,

2. Pyrolysis of methane:

At 1500 °C, methane is pyrolyzed using short reaction times. The reaction is endothermic, however, at very high temperatures it becomes exothermic

3. Elimination of 1,1- or 1,2-dihalogen alkanes:

Double elimination of 1,1- or 1,2-dihalogen alkanes with strong bases yields the corresponding compounds with triple bonds. Elimination with sodium amide in ammonia takes place at ( -33 °C).

Dihalogen alkanes are easily obtained from alkenes by halogenation and these compounds can be transferred into alkynes by double dehydrohalogenation.

4. Terminal Alkyne to Internal Alkyne:

Acetylide anion is very nucleophilic and reacts with a multitude of electrophiles in S_N²-type reactions. Using this method, terminal and internal alkynes can be synthesized.

Alkynes: Reactions

Alkynes are very reactive compounds and the triple bond participates in many electrophilic addition reactions.

1. Combustion of ethyne:

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 % ).

2. 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.

Heat of hydrogenation

Ethyne to ethene	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

3. Partial Hydrogenation of Alkyne:

By using a less active (partially poisoned) catalyst, hydrogenation can be stopped at the alkene stage. 

Lindlar catalyst (palladium on CaCO₃, poisoned with quinoline) is frequently used for this hydrogenation which stereospecifically yields cis products.

Rosenmund catalyst (palladium on BaSO₄) is also used for the same purpose and yields cis products.

4. Reduction of alkynes:

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

5. Addition of hydrogen halides to alkynes:

EAR: 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.

6. Halogenation of 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.

7. 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.

8. (Hydroboration-Oxidation:)

Overall antimarkonikov syn addition of water takes place.

This reaction yields enoles which tautomerize to the corresponding carbonyl compounds.

Ethyne yields acetaldehyde;

Terminal alkynes produce aldehydes.

Internal alkynes produce Ketones.

9. Nucleophilic addition to alkynes:

Alkynes also undergo nucleophilic addition reaction but only under harsh conditions in presence of strong electron withdrawing groups.

10. Polymerization of ethyne:

Polymerization of is initiated by carbenium ions. Subsequent chain reaction yields a long-chain molecule containing conjugated double bonds

11. Alkynes: Isomerization:

Since higher-substituted alkyl alkynes (internal alkynes) are more stable than terminal alkynes (hyperconjugation), isomerization is favored thermodynamically.

The deciding step is the tautomerization of the acetylide anion to the propargyl anion which is stabilized by mesomerism.

The triple bond migrates from the terminal position into the C-C chain.

Isomerization in the opposite direction leading to the formation of a terminal alkyne can be accomplished with strong bases, e.g. sodium amide at 150 °C, which are able to completely deprotonate terminal alkynes.

The reaction proceeds in the opposite direction because the most stable anion (acetylide) is formed under the strong basic conditions and not the more stable hydrocarbon (internal alkyne) which is formed under less basic conditions.