1. Chapter 4 Alkynes Dr. Yasser Mostafa Abdallah
For Dental Students
Chapter 4
Alkynes
Dr. Yasser Mostafa Abdallah

2. General methods for the Preparation of Alkynes
1-Decomposition of calcium carbide (CaC2):
Calcium carbide is first obtained by heating limestone with coke. Calcium carbide is then reacted with water and ethyne (acetylene) is obtained.

3. 2-Dehydrohalogenation of dihalide:
Dihalides has two halogen atoms, When boiled with alcoholic potash, the molecule loses 2 molecules of hydrogen halide and the corresponding alkyne is obtained.

4. 3. From Tetrahaloalkanes
Alkynes can be obtained from tetrahaloalkanes by heating with zinc.

5. 4- By Kolbe’s electrolysis :
Electrolysis of aqueous solutions of sodium or potassium salts of dicarboxylic acids gives alkyne.
sodium maleate

6. 5-Reaction of Sodium acetylide with primary alkyl halides:
This method is used for the preparation of higher alkyne from lower one. (To increase the alkyne chain).

7. (i) Hydration of alkynes (reaction with water):
Reactions of
alkynes
(i) Hydration of alkynes (reaction with water):
Alkynes on hydration in the presence of dilute acids and mercuric sulphate gives enols (a compound containing double bond and alcohol). For example acetylene on hydration gives an enol with the following structure.
These enols are unstable compounds. They undergo rearrangement to give carbonyl compounds (ketones/ aldehyde).

8. Examples:

9. CHΞCH + HCl → CH2=CH-Cl→H3C-CHCl2
(ii) -Halogenation of alkynes:
Alkynes on halogenations give 1,2 dihalo substituted alkenes and 1,1,2,2 tetra substituted alkanes.
For example acetylene on reaction with give 1,2 dibromoethene and 1,1,2,2-tetrabromoethane.
(iii) -Addition of hydrogen halide (HX):
Alkynes add with hydrogen halides to give addition products. For example acetylene adds with 2 moles of hydrogen bromide to give 1,1 dibromo ethane.
CHΞCH + 2HBr →CH3-CH(Br)2
CHΞCH + HCl → CH2=CH-Cl→H3C-CHCl2

10. CHΞCH + CHΞCH → CH2=CH-CΞCH (vinyl acetylene)
Iv -Addition of HCN:
Alkynes reacted with HCN one time only so in the presence of excess of HCN in the reaction it give only one product.
V -Linear and cyclic polymerization:
Polymerization of alkynes can be done in linear or cyclic way. Linear polymerization of alkynes gives vinyl acetylenes and substituted vinyl acetylenes. For example acetylene on treating with zinc in ammonium chloride is polymerized to vinyl acetylene. This monomer can combine so many times to give a polymer.
CHΞCH + CHΞCH → CH2=CH-CΞCH (vinyl acetylene)

11. Similarly, cyclic polymerization of alkynes gives aromatic compounds
Similarly, cyclic polymerization of alkynes gives aromatic compounds. For example acetylene when passed through red hot tube gives benzene.

12. Convert the following:
3. CH3CH2CH3 → CH3CH=CH2

13. 5. CH3CH2CH2Br → CH3CHBrCH3
6. CH4 → CH3CH3

14. 7. CH4 → CH3CH2CH3
8. CH3CHO → CH3CH3
9. 2-iodopropane → propane