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Chemical Properties of Alkynes

Alkynes are unsaturated compounds. Therefore, like alkenes, these are quite reactive. The most common type of reactions of alkynes are addition reactions. On addition, alkynes ultimately give a saturated compound. Alkynes also undergo oxidation and polymerization.

Addition Reactions of alkynes
Halogen acids, hydrogen, etc. add onto alkynes ultimately to give a saturated compound. Under suitable conditions, alkynes also undergo hydration reactions.
  1. Hydrogenation
    In the presence of a catalyst, hydrogen adds onto alkyne to ultimately give alkane.
    H-C C-H +  H2
        alkyne                                   alkene                         alkane
  2. Halogen acid
    This adds onto alkynes to give a dihalide. This addition follows Markownikoff's rule,

The addition of halogen acid (HX) to alkynes is similar to that of alkenes. Halogen acid is polarized due to high electronegativity of the halogens followed by an electrophilic attack of H+ on alkyne.

  1. Halogen
    Halogens adds on to alkynes to give halogen-substituted alkanes.

  1. Hydration
    When acetylene is bubbled through 40% sulphuric acid in the presence of mercuric sulphate (HgSO4), acetaldehyde is obtained. This reaction can be considered as the addition of water to acetylene.


Alkynes can be oxidized to different products by using different reagents and conditions of oxidation.
  1. Burning in air
    Acetylene burns in air with a sooty flame, emitting a yellow light. For this reason, it is used for illumination.
  2. Burning in excess of air
    Acetylene burns with a blue flame when burnt in an excess supply of air or oxygen. A very high temperature (3000 K) is obtained by this method. Therefore, in the form of oxy-acetylene flame it is used for welding and cutting of metals.
    2H-C C-H +  5O2    4CO2 + 2H2O +  heat
  3. Hot, alkaline KMnO4
    Alkynes are oxidized to carboxylic acid by hot alkaline KMnO4.

  1. Ozone
    Alkynes react with ozone to give ozonide. Ozonide yields a dicarbonyl compound on reduction with zinc/water.

The dicarbonyl compounds on oxidation with hydrogen peroxide yields carboxylic acids. By identifying carboxylic acids, it is possible to fix the position of the triple bond in an alkyne molecule. For example

The carboxylic acids formed on ozonolysis of a pentyne contain 2 and 3 carbon atoms, respectively. Therefore, the triple bond in pentyne must be between carbon atoms 2 and 3.


Alkynes polymerize to give compounds having higher molecular masses.
  1. Benzene is obtained when acetylene is passed through a red-hot copper tube.
    3 H-C C-H
  2. Cyclotetraene is formed when a solution of acetylene in tetrahydrofuran (a solvent) is heated in the presence of nickel cyanide, Ni(CN)2 (catalyst).

Acidic nature of Alkyne

Acetylene forms salt-like compounds because hydrogen atoms are slightly acidic. Therefore, the hydrogen atoms directly attached to carbon atoms linked by a triple bond can be replaced by highly electropositive metals, such as sodium, silver, copper, etc. Salts of acetylenes are known as acetylides.
  1. Sodium acetylide is formed when acetylene is passed over molten sodium
H-C C-H + Na     H-C C-Na + H
Sodium acetylide is also formed when acetylene is treated with sodamide (NaNH2). Sodamide is obtained by dissolving sodium metal in liquid ammonia.
Na   +   NH3  NaNH2     +    H2

         (liquid)        sodamide
NaNH2   +   H-C C-H Na-C C-H     +    NH3
NaNH2   +   Na-C C-H   Na-C C-Na   +   NH3
Other alkynes can also react in a similar manner.
NaNH2    +   CH3-C C-H      CH3-C C-Na    +   NH3
  1. Silver acetylide is obtained as a white precipitate when acetylene is passed through ammoniacal silver nitrate (Tollens reagent).
H-C C-H   +  2AgNO3 +  2NH4OH     Ag-C C-Ag    +   2NH4NO3 +  2H2O
                                                            silver acetylide
                                                         (white precipitate)
  1. Copper acetylide is obtained as a red precipitate when acetylene is passed through an ammoniacal solution of cuprous chloride.
H-C C-H   +  Cu2Cl2 +  2NH4OH       Cu-C C-Cu   +   2NH4Cl +  2H2O
                                                                          cuprous acetylide 
                                                                           (red precipitate)

Alkynes form insoluble silver acetylide when (alkynes having acidic hydrogen) passed through ammoniacal silver nitrate (Tollens reagent),
R-C C-H   +    Ag+    R-C C-Ag      +      H+
                           Tollens     white precipitate

Copper acetylide is formed when acetylene is passed through ammoniacal solution of cuprous chloride (Cu2Cl2).
R-C C-H +  Cu+     R-CC-Cu      +     H+
                                      red precipitate
An alkyne having no hydrogen atom attached to the carbon atoms, linked by a triple bond, does not form acetylide, e.g. CH3-C C-CH3 (butyne-2) does not form acetylide as it lacks an acidic hydrogen (acetylenic hydrogen). Thus, salt formation by alkyne is only shown by those alkynes which contain a triple bond at the end of the molecule (i.e. when the triple bond is a terminal group). Alkynes having a nonterminal triple bond do not yield acetylides. Therefore, this reaction is used for distinguishing terminal alkynes from non terminal alkynes.

Cause of Acidic Nature

Hydrogen attached to carbon atoms by sp hybrid orbital is slightly acidic, the reason for the acidic nature is that sp hybrid orbital has greater s character than sp2 or sp3 hybrid orbitals (The s character in sp3,sp2 and sp hybrid orbital are 25,33 and 50% respectively). An s orbital tends to keep electrons closer to the nucleus than a p orbital. It means that greater the share of the s orbital in the hybrid orbital, the nearer will be the shared pair of electrons to the nucleus of carbon atom. This makes the sp hybrid carbon more electronegative than an sp2 or sp3 hybridized carbon atom. Thus the hydrogen attached through sp hybrid orbital acquires a slightly positive charge. Therefore, it can be replaced by highly electropositive metals (such as sodium, copper, silver, etc.).

The acidic character of hydrocarbons varies as: alkyne, alkene < alkane. However, alkynes are extremely weak acids. Compared to carboxylic acids like acetic acid (ethanoic acid) ethyne is 1020 times less acidic. Ethane is 1040 times less acidic than acetic acid.

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