Alkyne

Alkynes are hydrocarbons that have at least one triple bond between carbon atoms. The simplest alkyne is ethyne (acetylene):

Alkynes, unlike alkanes are unstable and very reactive. This gives rise to the intense heat of the acetylene flame used in welding.

Alkynes also readily polymerise, like alkenes.

Obtainment of an alkyne

We start from a dihalogenized substance.

    H   H   H
    |   |   |
H - C - C - C - H
    |   |   |
    H   Cl  Cl

If we add NaOH to this, it will dissociate into Na⁺ and OH^-. Na⁺ will not react, but OH^- will pull off an electron from the substance.

• 1st possibility

2nd order elimination reaction. the 2 Cl will go to respect the tetravalent structure of the carbon.

              OH-               
             /                        Cl-
    H   Cl  H                    H         H-OH               H
    |   |   |                    |   |   |                    |     -
H - C - C - C - H   --->     H - C - C - C - H     --->   H - C - C - C - H
    |   |   |                    |   |   |                    |     -
    H   Cl  H                    H         H-OH               H  
             \\                        Cl- 
              OH-                    

• 2nd possibility

As before, 2nd order elimination reaction. the 2 Cl will go to respect the tetravalent structure of the carbon.

     OH-      OH-                  H-OH
       \\     /                            H-OH
    H   H   H                    H
    |   |   |                    |   |   |                      -
H - C - C - C - H   --->     H - C - C - C - H     --->   H - C - C - H
    |   |   |                    |   |   |                      -
    H   Cl  Cl                   H
                                            Cl- 
                                    Cl-