bonds and two C–C π bonds. The strength of C ≡ C bond (bond enthalpy kJ mol - ) is more than those of C=C bond (bond enthalpy kJ mol – ) and C–C bond (bond enthalpy kJ mol – ). The C ≡ C bond length is shorter ( pm) than those of C=C ( pm) and C–C ( pm). Electron cloud between two carbon atoms is cylindrically symmetrical about the internuclear axis.
Thus, ethyne is a linear molecule. . . Preparation .
From calcium carbide: On industrial scale, ethyne is prepared by treating calcium carbide with water. Calcium carbide is prepared by heating quick lime with coke. Quick lime can be obtained by heating limestone as shown in the following reactions: CaCO ∆ CaO O ( . ) Fig.
. Orbital picture of ethyne showing (a) sigma overlaps (b) pi overlaps. CaO + 3C CaC CO ( . ) Calcium carbide CaC + 2H O Ca(OH) + C H ( .
) . From vicinal dihalides : Vicinal dihalides on treatment with alcoholic potassium hydroxide undergo dehydrohalogenation. One molecule of hydrogen halide is eliminated to form alkenyl halide which on treatment with sodamide gives alkyne. .
. Properties Physical properties Physical properties of alkynes follow the same trend of alkenes and alkanes. First three members are gases, the next eight are liquids and the higher ones are solids. All alkynes are colourless.
Ethyene has characteristic odour. Other members are odourless. Alkynes are weakly polar in nature. They are lighter than water and immiscible with water but soluble in organic solvents like ethers, carbon tetrachloride and benzene.
Their melting point, boiling point and density increase with increase in molar mass. Chemical properties Alkynes show acidic nature, addition reactions and polymerisation reactions as follows : A. Acidic character of alkyne: Sodium metal and sodamide (NaNH ) are strong bases. They react with ethyne to form sodium acetylide with the liberation of dihydrogen gas.
These reactions have not been observed in case of ethene and ethane thus indicating that ethyne is acidic in nature in comparison