Addition Reaction of Alkynes

The chemistry associated with carbon-carbon triple bonds can be called as alkyne chemistry. Alkynes undergo addition reactions due to the presence of loosely held pi-electrons. Due to the presence of a triple bond in alkynes, halogens, water etc.can be added to them by the process of the addition reaction. Addition products are formed via a set of steps. The stability of vinylic cations is responsible for the formation of addition products. Markovnikov’s rule is followed by asymmetric alkynes in order to undergo addition reaction. Few addition reactions of alkynes are explained below:

Halogenation: Addition of halogens

Alkynes and halogens undergo addition reaction to form halogenated alkenes which further react with halogens to give halogen substituted alkanes. The reddish orange colored solution of bromine and carbon tetrachloride gets decolorized as a result of the addition reaction. This is used as a test for unsaturation.

Addition Reaction of halogens- Alkynes

Unlike the hydrogentaion reaction mentioned above, most halogenation reactions can be stooped in the intermediate stage by lowering the temperature below \(0^{\circ}C\)..

Hydrogenation: Addition of dihydrogen

Alkynes react with dihydrogen in the presence of catalysts such as Pt/Pd/Ni in order to form alkenes. The alkenes formed, further react with dihydrogen to form alkanes. It has been observed that in most reactions triple bond is converted into double bond and double bond is then converted into a single bond due to the addition reaction with dihydrogen.

The catalyst that are involved in the case of rhodium, nickel, palladium, and platinum. Hydrogenation is a step by step process in which initially an alkene is formed. After which it undergoes further hydrogenation to form an alkane

Additiaon of dihydrogen Alkynes

Slowing down the reaction in the intermediate stage is actually quite impossible as the whole reaction is really smooth. But some alkenes are isolated with the use of poisoned catalysts. One such example of a poisoned catalyst is Lindlar catalyst.

Lindlar catalyst

Lindlar catalyst is combined form of palladium which is coated with quinoline and absorbed on calcium carbonate.

Hydration: Addition of water

Alkynes are immiscible in water.They do not react with water under normal conditions. Alkynes may react with water in the presence of dilute sulphuric acid and mercuric sulphate at a temperature of 333K.This results in the formation of carbonyl compounds.

Addition of water - Addition Reaction

Hydrohalogenation: Addition of hydrogen halides

When hydrogen halide is treated with alkynes (triple bond compounds) it results in the formation of gem halides. Gem halides are the compounds in which two halogens are attached to the same carbon atoms in a molecule.

Addition of hydrogen halides - Addition Reaction

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Practise This Question

Which one of the following can be used as a monomer in a polymerization reaction?