The simplest alkyne is acetylene, with molecular formula . Alkynes are also called acetylenes because of the name of the first member of the series is acetylene. Molecular, condensed, structural and dot and cross formulae of a few alkynes are given in Table 12.2.
| Name | Molecular formula | Condensed formula | Structural formula | Cross and dot formula |
|---|---|---|---|---|
| (i) Acetylene (ethyne) | C2H2 | HC ≡ CH | H - C ≡ C - H | H·C·C·C·C·H |
| (ii) Methyl Acetylene (propylene) | C3H4 | H3C - C ≡ CH | H - C - C ≡ C - H | H·C·C·C·C·C·H |
| (iii) Dimethyl Acetylene (butyne) | C4H6 | H3C - C ≡ C - CH3 | H - C - C ≡ C - C - H | H·C·C·C·C·C·H |
Occurrence:
Acetylene does not occur free in nature. Traces of acetylene are present in coal gas (about ).
12.3.1 Preparation of Alkynes
Alkynes are prepared by the following methods.
12.3.1.1 Dehydrohalogenation of Vicinal Dihalides
When a vicinal dihalide is heated with alcoholic KOH, two hydrogen atoms along with two halogen atoms are removed from two adjacent carbon atoms with the formation of a triple bond between the adjacent carbons:
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12.3.1.2. Dehalogenation of Tetrahaliades
When alkyl tetrahaliades are heated with Zinc dust, the elimination of halogen atoms takes place to form ethyne.
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Physical Properties
1. Alkynes also form a series of compounds. Its first member is acetylene. It is a colourless gas with faint garlic odour.
2. Acetylene is slightly soluble in water but soluble in organic solvents such a benzene, alcohol, acetone, ether, etc.