Alkyne:
An alkyne contains two hydrogen atoms less than the corresponding alkene. It is characterized by the presence of a triple bond between two carbon atoms. A triple bond consists of one sigma (σ) and two Pi (π) bonds. The general formula of alkyne is Cn H2n – 2, where n = 2, 3, 4, 5, etc. The first member of the series is acetylene (C2H2).
Structure of Ethyne (C2H2):
Let us consider acetylene (HC≡CH) for illustrating the orbital make up of alkynes. In acetylene, the carbon atoms are sp-hybridized. They are attached to each other by a σ-bond and two π bonds. The σ-bond results from the overlap of two sp-hydrid orbitals. The π bonds are formed from the separate overlap of the two p-orbitals from the two adjacent carbon atoms as shown below.
Methods of Preparation of alkyne:
Alkynes are prepared by the following methods-
1) Dehydrohalogenation of 1, 2-Dihalides: Alkyne is prepared by heating ethylene dibromide with alcoholic KOH.
Br—CH2—CH2—Br + Alco. KOH —–> CH ≡ CH + KBr + H2O
1, 2-Dibromoethane Acetylene
2) Dehalogenation of tetrahalide: When 1, 1, 2, 2-tetrahalide are heated with zinc dust in presence of alcohol, alkynes are formed.
Br2—CH—CH—Br2 + 2Zn (dust) ——> CH ≡ CH + ZnBr2
1, 1, 2, 2-Dibromoethane Acetylene
3) From calcium carbide: Calcium carbide reacts with water to yield acetylene.
CaC2 + H2O ——> CH ≡ CH + Ca(OH)2