Alkynes are unsaturated hydrocarbons. Like alkenes characteristic reactions of alkynes are electrophilic addition reactions. However, alkynes are less reactive than alkenes towards electrophilic addition reactions.
1. Addition of Hydrogen
Alkynes react readily with hydrogen in the presence of finely-divided nickel, platinum or palladium as catalysts. The process is known as catalytic hydrogenation. The ultimate products of this reaction are alkanes
The hydrogenation of alkynes can be restricted to alkenes by employing suitable reaction conditions. For example, use of Lindlar’s catalyst (Pd supported over CaCO3 which is partially poisoned by lead acetate) can restrict the hydrogenation to alkene stage.
Reduction of alkynes to alkene stage can also be carried out with sodium or lithium in liquid ammonia.
In case of non-terminal alkynes, the reduction to alkene stage can result into cis or trans alkene. It is found that the reduction using Lindlar’s catalyst results into cis-alkene and the reduction using Na/liq. NH3 leads to formation of predominantly trans-alkene
2. Addition of Halogens
Halogens, especially chlorine and bromine add on alkynes readily producing a tetra-halogen derivatives. The reaction is carried out in inert solvent like carbon tetrachloride.
Due to the presence of two n-bonds, each molecule of the alkyne can react with two molecules of the halogen. For example, the addition of bromine to ethyne can be controlled to give 1, 2-dibromoethene or 1, 1, 2, 2-tetrabromoethane
During the addition of bromine to alkynes, the reddish brown colour of bromine disappears.
3. Addition of Halogen Acids
In alkynes two molecules of halogen acid add across triple bond to form dihalogen derivatives as products
4. Addition of Water (Hydration)
Alkynes undergo hydration in the presence of 60% H2SO4 and mercury(m tetraoxosulphate (VI) as catalyst at about 330 K. The products are carbonyl compounds. In case of ethyne, ethanal is produced. The initially formed vinyl alcohol is unstable and rearranges to more stable ethanal
Similarly, propyne reacts with water under similar conditions to give propanone.
Alkynes burn with a luminous and sooty flame when heated in the presence of air to form carbon dioxide and water. The reaction is highly exothermic in nature.
2C2H2 + 5O2 à 4CO2 + 2H2O + Heat
The heat evolved during the reaction is employed as oxyacetylene flame for welding purposes. Ethyne burns with a sooty flame because it has very high percentage of carbon. Oxygen in the air is unable to oxidise all the carbon of ethyne to carbon dioxide. Due to unburnt carbon particles, the flame becomes sooty.
6 Oxidation with Alkaline KMnO4 Solution
Alkynes are oxidised with alkaline KMnO4 to a variety of products depending upon the structure. For example, ethyne is oxidized to oxalic acid.
7. Polymerisation in Alkynes
When acetylene is passed through red hot tube of iron or quartz it trimerises to form benzene
This is the best route for entering from aliphatic to aromatic compounds.
This is an example of cyclic trimerisation.
Four molecules of acetylene polymerise under high pressure in presence of nickel cyanide.
It is an example of cyclic tetramerisation.
Linear polymerisation of ethyne, under suitable conditions, yields polyethyne
Polyethyne is a high molecular mass polyene. Under special conditions this polymer conducts electricity. It can be used for making electrodes in batteries. Polyethyne electrodes are lighter and cheaper than metal electrodes.
In the presence of CuCI /NH4Cl, acetylene gives vinyl acetylene and finally divinyl acetylene
But-1-en-3-yne reacts with one mole of HCl to form chloroprene which is the starting material for the manufacture of neoprene rubber.
8. Acidic Character of Alkynes
Terminal alkynes ( 1-alkynes) behave as very weak acids. They react with strong bases like NaNH2 to form acetylides. The acetylenic hydrogen atom is removed as a proton by the strong base leading to the formation of a stable acetylide
1-Alkynes also react with sodium metal to produce hydrogen gas.
Acetylides react with alkyl halides to produce acetylene homologues.