Alkenes and Alkynes

 

Methods of Preparation of Alkynes

(i)        By dehydrohalogenation of dihalides :

                           dehydrohalogenation-of-dihalides

            Vinyl halide being less reactive & thus to get better yield a stronger base sodalime NaNH2 is used in II step.

(ii)        By dehalogenation of tetrahalides :

                        CHX2

                        |           dehalogenation-of-tetrahalides     CH≡CH

                        CHX2                      

(iii)       By haloform : only for acetylene

                       CHX3 + 6Ag + X3HC heat CH≡CH + 6AgX

                                                 Powder

(iv)      By Kolbe's electrolytic method :

                       kolbes-electrolytic-method

(v)       Laboratory method : Acetylene is prepared in laboratory by the action of water on calcium carbide.

                        CaC2 + 2H2O → Ca(OH)2 + C2H2 (Wohler reaction)

 

        The air of the flask in which acetylene is to be prepared is displaced with oil gas as acetylene forms an explosive mixture with air. Acetylene evolved is collected over water. The acetylene so prepared is contaminated with small amounts of impurities such as PH3, H2S, AsH3 and NH3 which are removed by passing the mixture through CuSO4 solution before its collection over water.

(vi)      Manufacture :

           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

(vii)     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

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