Alkynes are unsaturated organic compounds. In the alkyne compounds the presence of pi electrons can be seen. These electrons are loosely held and due to this reason alkynes undergo additional reactions. The principle reaction of the alkynes is an addition of atom across the carbon-carbon triple bond. In this process of addition, we can add halogens, hydrogen, water etc. to alkynes across the triple bond. Asymmetric alkynes are used to follow Markovnikov’s rule. The formula of Alkyne is (C_nH_2n-2).

These unsaturated hydrocarbons are not only more reactive but also exothermic than alkenes. It is because of the two π bonds and high π-electron density present in the alkynes. Each π bond can add two or more atoms to it, hence alkyne compounds can undergo two rounds of addition reactions.

In this article, we will discuss in detail the addition reactions of alkynes.

What is an Addition Reaction?

An addition reaction is a chemical reaction in organic chemistry where two or more substances combine together to form a single product. Atoms or groups of atoms are added to molecules through the reaction, which results in the formation of a new compound. Addition reactions involve the breaking of multiple bonds and the formation of new bonds between the reactants. This nature of addition reaction leads to the creation of a more complex molecule. These reactions play an important role in the synthesis of various compounds.

Addition Reactions of Alkynes

Alkynes are the hydrocarbons that contain the C≡C triple bond. The addition reactions of alkynes are quite similar to that of alkenes, but the alkynes are more reactive than the later. The addition reactions of alkynes are as follows:

• Hydrogenation
• Hydrohalogenation
• Halogenation
• Hydration

Addition Reactions of Alkynes with Water (Hydration)

Hydration is known to be the addition reaction of alkynes with water. The alkyne does not react with water in normal conditions. If you use 333K temperature, it reacts with water in the presence of dilute sulphuric acid and mercuric sulphate to form carbonyl compounds.

• Addition reaction of terminal alkyne with water produces aldehydes
• Addition reactions of non terminal alkynes with water produces ketons.

Addition Reactions of Alkynes with Halogens (Halogenation)

The addition reaction of alkynes with halogens is known as halogenation. This reaction leads to the formation of halogenated alkenes. It further reacts with halogens to form tetra-haloalkanes or tetra-halogenated alkanes on the carbon atoms across the C≡C bond. Unlike the hydrogenation reactions, this process can be stopped at alkene stage by reducing the temperature slightly below 0℃ . This addition reaction is an example of electrophilic addition reaction.

Addition Reaction of Alkynes with Hydrogen Halides (Hydrohalogenation)

The addition reaction of alkynes with hydrogen halides results in the formation of Gem halides. In Gem halides, two halogen atoms are attached to the same carbon atom in the molecule. This addition reaction is one of the simplest electrophilic addition reactions. It follows Markovnikov’s rule.

This reaction follows two steps. In these two steps, the addition of hydrogen and halogen atoms with carbon atoms(adjacent to the triple bond) takes place twice, forming halogenated alkene at the first step and a double halogenated alkane after the final addition reaction.

Addition Reaction of Alkynes with Hydrogen(Hydrogenation)

When alkynes react with hydrogen to form alkanes, this process is known as hydrogenation. The reaction takes place in presence of catalysts such as nickel, palladium, platinum, and rhodium. Initially, alkynes react with hydrogen to form alkenes, then the alkenes formed, further react with dihydrogen to form alkanes. In such addition reactions, a triple bond is converted into a double bond and then the double bond is converted into a single bond. Since the reaction is very smooth, It can not be easily stopped at the alkene stage. But with the use of poisoned catalyst some alkynes can be isolated. An example of catalyst is Lindlar catalyst.

• Lindlar Catalyst: A heterogeneous catalyst, poisoned by using sulfur or lead. It is named after a British chemist Herbert Lindlar. It is composed of palladium and calcium carbonate. It helps in the addition reactions of alkynes to form alkenes.