Structure of Alkynes, Reactions of Alkynes, Preparation of Alkynes and physical Properties of Alkynes

 Alkynes

-     Alkynes are organic compounds that contain one or more triple bond

-         They have the general formula of C_nH_2n-2

Preparation of Alkynes (Methods of Preparation of Alkynes)

1-By the reaction of alcoholic KOH with vicinal dihalide or geminal dihalide

2-Kolbe electrolysis (Kolbe Electrolysis): Electrolysis of Salts of maleic or fumaric acid or their derivatives

3-By dehalogenation of acetylenic tetrahalides:

Dehalogenation of acetylenic tetrahalides produce alkynes

Preparation of Acetylene:

The following methods may be used for preparation of acetylene:

1- Preparation of acetylene by hydrolysis of metal carbides:

This method is described as preparation of alkynes from calcium carbides

2- Preparation of acetylene by Heating haloform (iodoform) with silver powder

3- Preparation of acetylene from carbon

Physical Properties of Alkynes

• Alkynes are unsaturated nonpolar compounds
• The physical properties of alkynes are similar to the physical properties of alkanes and alkenes
• Alkynes are insoluble in water but soluble in organic solvents such as benzene
• Boiling point of alkynes are slightly higher than boiling points of alkanes and alkenes

Reactions of Alkynes

1- Addition of hydrogen to alkynes

2- Addition of bromine or chlorine to alkynes (Addition of Halogens to alkynes or Halogenation of Alkynes)

3- Addition of halogen acids according to alkynes according Markovnikov rule (Markovnikov Addition):