ALKYNES

An alkyne is a molecule made up entirely of carbon and hydrogen where on or more carbon atoms are connected by triple bonds. The general formula for an alkyne is C_nH_2n-2 where n is the number of carbon atoms in the molecule. Alkynes are named by adding the the -yne suffix to the prefix associated with the number of carbon atoms present in the molecule.

The first three are:

C₂H₂ – Ethyne

C₃H₄ – Propyne

C₄H₆ – Butyne

Ethyne is the first member of the alkyne series with a molecular formular C₂H₂ – and  a structural formular CH≡CH.

Laboratory Preparation of Ethyne (Acetylene)

Acetylene is prepared in laboratory by the action of cold water on calcium carbide. The gas evolved is passed through an acidified coppe(II) tetraoxosulphate (vi) solution to remove and phosphine present as an impurity.

CaC_2(s) + 2H₂O_(l) → Ca(OH)_2(aq) + C₂H_2(g)

The air of the flask in which acetylene is to be prepared is displaced with oil gas as acetylene forms an explosive mixture with air. Acetylene evolved is collected over water. The acetylene so prepared is contaminated with small amounts of impurities such as PH₃, H₂S, AsH₃ and NH₃ which are removed by passing the mixture through CuSO₄ solution before its collection over water.

Physical Properties of Alkynes

1. Lower members (C₂ to C₄) are gases; middle one (C₅ to C₁₂) are liquids; higher are solids.
2. The boiling and melting point of alkynes show a regular increase with increase in molecular weight; however less volatile than alkene.
3. All are colourless & pure ethyne is a sweet smelling gas, however C₂H₂ has garlic odour due to the impurities of PH₃, H₂S etc.
4. Soluble in organic solvents like acetone, alcohol & sparingly soluble in water.
5. It is slightly lighter than air

 Chemical Properties of Ethyne

   COMBUSTION REACTION: Ethyne burns in air to give a very smoky flame and luminous flame owing to its hih carbon content. A mixture of ethyne in air or oxygen may explode violently upon ignition.

2C₂H₂ + 5O₂ 4CO₂ + 2H₂O + heat

ADDITION REACTIONS: Ethyne is highly unsaturated, containing carbon carbon triple bond in its structure. It can readily form addition products combining with a maximum of four univalent atoms or radicals per molecule. These reactions take place in 2 stages:

• the first stage yields a product with a carbon carbon double bond.
• the second stage converts this into a fully saturated compound with only carbon carbon single bond

Read more below-

SS2 Chemistry Third Term: Unsaturated Hydrocarbons – Alkynes