Methods of Preparation of Alkynes

Acetylene itself is formed from the reaction of the inorganic compound calcium carbide with water. 

CaC2 + 2 H2O → Ca(OH)2 + HCCH

This method was once an important industrial process for the manufacture of acetylene. However, the method has now been replaced by a process in which methane is pyrolyzed in a flow system with short contact time.

This reaction is endothermic at ordinary temperatures, but is thermodynamically favored at high temperatures.

  • Laboratory Methods of Preparation of Alkynes:

1.   Dehydrohalogenation of vic-Dihalides or gem-Dihalides

2.   Dehalogenation of vic-Tetrahalogen Compounds

This reaction has the drawback that the halogen compound is itself prepared by halogen addition to alkynes.

3.   Alkyl Substitution in Acetylene; Acidity of  \equiv C-H

There is a fair amount of variety possible using this method. Acetylene itself may be alkylated either once to make a terminal alkyne or twice to make an internal alkyne. 

Since acetylide ions are highly basic, competing elimination is a common side reaction.

The products of such an elimination reaction are an alkene (from the alkyl halide) and an alkyne.

In practice, the alkylation of acetylene or another terminal alkyne is only a good method for the syntheiss of alkynes when applied to primary halides that do not have branches close to the reaction centre. With secondary halides, and even with primary halides that have branches close to the reaction center, elimination is usually the major reaction.

Refer to the following video for synthasis of alkynes:

 

  • Industrial Method for preparation of alkynes: 

old method :              2C + H2electric-arc C2H2  (Bertholots reaction)

 Modern method :        6CH4 + O2 150-degree 2CH≡CH + 2CO + 10H2

 or                            2CH4 electric-arc  CH≡CH + 3H2

Preparation of higher alkynes : Higher alkynes may be obtained b acetylene.

a) CH≡CH + Na   liq-nh3  CH≡CNa +  1/2H2

CH≡CNa + RX → CH≡CR + NaX


                                            RX
b) CH≡CH + RMgX → XMg-C≡CH → R-C≡CH + MgX2

  • Some Other General  Methods of Preparation  of Alkynes:  

1. By Haloform : Only for acetylene

 CHX3 + 6Ag + X3HC heat CH≡CH + 6AgX

                                                 Powder

2. By Kolbe's electrolytic method :

   kolbes-electrolytic-method

Refer the below mentioned links to get an immediate solution to all queries on Organic chemistry:

JEE Organic Chemistry Syllabus

Organic Chemistry Revision Notes

Reference books of Organic Chemistry