HYDROCARBONS!!! ALKYNE EDITION!!!
STRUCTURE
The strength of C≡C bond (bond enthalpy 823 kJ mol-1) is more than those of C=C bond (bond enthalpy 681 kJ mol–1) and C–C bond (bond enthalpy 348 kJ mol–1).
The C≡C bond length is shorter (120 pm) than those of C=C (133 pm) and C–C (154 pm). Electron cloud between two carbon atoms is cylindrically symmetrical about the internuclear axis.
PREPARATION
from calcium carbide
from vicinal dihalides: react them with alcoholic KOH, this will give alkene, and then NaNH2 (Sodamide) which will give alkyne.
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. Ethyne 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
acidity Due to the maximum percentage of s character (50%), the sp hybridised orbitals of carbon atoms in ethyne molecules have highest electronegativity. Thus in ethyne, hydrogen atoms can be liberated as protons more easily as compared to ethene and ethane. Hence, hydrogen atoms of ethyne attached to triply bonded carbon atom are acidic in nature. Reaction of alkynes with NaNH2 is used to differentiate them from alkenes and alkanes.
addition reactions: - H2 - X2 (Reddish orange colour of the solution of bromine in carbon tetrachloride is decolourised. This is used as a test for unsaturation.) - HX : give GEMENAL dihalides - H2O: Like alkanes and alkenes, alkynes are also immiscible and do not react with water. Give carbonyl compounds.
linear and cyclic polymerisation
