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 CnH2n-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.
CaC2(s) + 2H2O(l) → Ca(OH)2(aq) + C2H2(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
- Lower members (C2 to C4) are gases; middle one (C5 to C12) are liquids; higher are solids.
- The boiling and melting point of alkynes show a regular increase with increase in molecular weight; however less volatile than alkene.
- All are colourless & pure ethyne is a sweet smelling gas, however C2H2 has garlic odour due to the impurities of PH3, H2S etc.
- Soluble in organic solvents like acetone, alcohol & sparingly soluble in water.
- 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