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Preparation and Properties of Acetylenes or Alkynes

The first member in the series, ethyne is commonly known as acetylene (CH CH). Acetylene is the most important member in this series.

In the laboratory, acetylene is prepared in the following ways:
  1. From calcium carbide
    Acetylene is obtained by the action of water on calcium carbide (CaC2).
    CaC2   +   2H2O CH CH   +    Ca(OH)2
    Calcium carbide is prepared by heating quicklime (CaO) with carbon at high temperature.
    CaO +   4C CaC2 +   2CO
  2. From vicinal dihalides
    Acetylene and its higher homologue can be prepared by the treatment of alcoholic alkali with vicinal dihalides.

Higher Homologues
Higher homologues are prepared starting from acetylene. When acetylene is treated with sodium in liquid ammonia, it results in the formation of sodium acetylide.
2Na   +   2NH3        2NaNH2   +   H2
H-C C-H +  NaNH2      H-C C-Na +  NH3

Sodium acetylide is treated with an alkyl halide to obtain the higher homologue of acetylene.
H-C C-Na   +   RX      H-C C-R + NaX

where R is an alkyl radical. For example, propyne can be obtained by treating with methyl iodide.
H-C C-Na +  CH3I      H-C C-CH3 +   NaI

Physical Properties of Alkynes
Alkynes have the following general physical properties.
  1. State
    Lower members in the alkyne series are gases, while higher members are liquids and solids.
  2. Colour
    Alkynes have no colour.
  3. Non polar nature
    Alkynes are non polar in nature. Therefore, alkynes are soluble in non polar (organic) solvents but insoluble in polar solvents, such as water.

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