Alkynes – Hydrocarbons – Class 11
- 1 Alkynes or Acetylene
- 2 Isomerism in Alkynes
- 3 Classification of Alkynes
- 4 Methods of preparation of Alkynes
- 5 Physical properties of Alkynes
- 6 Reactivity of alkynes versus Alkenes
- 7 Chemical Reaction Of alkynes Acidic character of Alkynes
- 8 Electrophilic Addition Reactions
- 9 Reduction of Alkynes
- 10 Oxidation Reaction of Alkynes
- 11 Uses of Alkynes
Alkynes or Acetylene
Each carbon atom of ethyne is sp-hybridized and hence has two sp-hybridized orbitals. One sp-hybridized orbital of each carbon undergoes head on overlap with sp-hybridized orbital of another carbon to form a sp-sp, C-C, σ – bond.
The second sp-hybridized orbital of each carbon overlaps along the internuclear axis with 1s-orbital of each of the two hydrogen atoms forming two sp-s, C-H, σ-bonds. Each carbon is now left with two unhybridized p-orbitals (2px and 2py) which are perpendicular to each other.
The two 2px-orbitals, one on each carbon, are parallel to each other and hence overlap sideways to form a π bond.Similar overlap between 2py orbitals, one on each carbon, results in the formation of a second π bond.
Since a p-orbital has two lobes, the electron cloud of a π-bond has two halves. If the two halves of one π bond are considered to lie in the plane of the paper, then one of the two halves of the second π-bond would lie above the plane of the paper and the other below the plane of the paper.
The four halves of the electron clouds of two π-bonds do not stay as such but merge together to form a single electron cloud which has cylindrical symmetry about the internuclear axis. It is because of this cylindrical symmetry of the electron cloud between two carbon atoms that ethyne is a linear molecule with H-C-C bond angle of 180°.
Carbon carbon triple bond consists of one strong σ – bond and two weak π-bonds. Due to smaller size of sp-orbitals and sideways overlap of p-orbitals, the carbon-carbon bond length in ethyne is shorter than those of C = C and C -C .
Alkynes are less reactive than alkenes towards addition reactions. Alkynes unlike alkenes do not exhibit geometrical isomerism due to their linear structure.
Isomerism in Alkynes
Alkynes having or five or more carbon atoms show isomerism due to different structures of the carbon chain.
Alkynes are functional isomers of dienes i.e. compounds containing two double bonds.
Ring chain isomerism
Alkynes show ring chain isomerism with cycloalkenes.