Hydrocarbons: Physical and Chemical Properties of Alkynes, Uses of Alkynes (For CBSE, ICSE, IAS, NET, NRA 2022)

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Physical Properties of Alkynes

  • First three members of alkynes are gases, the next eight members are liquids and members having more than twelve carbon atoms are solids.
  • They are colorless and odorless, except ethyne which has a garlic odour.
  • The melting points, boiling points and densities of alkynes increase with the increasing molar mass.
  • Alkynes are very slightly soluble in water and soluble in acetone.

Chemical Properties of Alkynes

Addition Reactions

Addition of Hydrogen

Addition of hydrogen to alkynes takes place in the presence of a catalyst like , or .

Addition of Halogens

When halogens are added to alkynes, they form 1,2-dihaloalkenes and 1,1, 2,2-tetrahaloalkanes.

Addition of Halogen Acids (HX)

Addition of Water

Addition of water takes place in the presence of mineral acids like and in the presence of as the catalyst.

Addition of Water

Addition of

Oxidation

Oxidation with

The colour of alkaline solution of is discharged on reaction with alkyne.

Combustion

Combustion of ethyne in excess of oxygen or air gives carbon dioxide and water.

Ozonolysis

On ozonolysis, alkynes give dicarbonyl compounds at the position of without breaking the chain of carbon atoms.

Oxidation: Ozonolysis

Formation of Acetylides

Ethyne forms precipitates of copper and silver acetylides when passed through ammoniacal solution of cuprous chloride and ammoniacal silver nitrate.

Formation of Acetylides

Acidic Nature of Alkyne

  • The acidic nature of hydrocarbons can be determined with the help of the percentage of s-character of the hydrocarbon.
  • The greater the percentage of s-character of a hydrocarbon, the more will be its acidic nature.
Acidic Nature of Alkynes
  • Alkynes have 50 % s- character, they are the most acidic in nature.
  • The reaction of ethyne with sodium and sodamide confirm its acidic nature.
Acidic Nature of Alkynes

Uses of Alkynes

  • Ethyne (acetylene) is used for producing oxyacetylene flame (2800ºC) which is used for welding and cutting of iron and steel.
  • It is used for artificial ripening of fruits and vegetables.
  • It used in the production of a number of other organic compounds such as ethanal, ethanoic acid, ethanol, synthetic rubbers.
Uses of Alkynes

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